Jean-Luc Wybo

The Role of Simulation Exercises in the Assessment of Robustness and Resilience of Private or Public Organizations

Keywords: Emergency management, simulation of accidents, organizational learning, resilience, robustness

This paper deals with the organization of simulation exercises to prepare organizations to face emergencies. The original objective of such simulations is to train people to emergency procedures and devices; we raise the question of training people to face potential crisis situations: are simulations fitted to that objective? Through the observation of a number of exercises organized by private companies and rescue services, we can answer that naïve interpretation of simulation results limits their benefits to the correction of gaps between prescribed and observed actions, without addressing complex organizational behaviour. We introduce a method to organize simulations that gives access to this complexity and to the resilience and robustness capacities of the organization by giving specific roles to observers. This method uses a model of the organization seen as a combination of three levels: structures, relations and meaning.

Introduction

With the growing complexity of technological and organizational systems, companies and public bodies have developed the use of simulations, exercises and drills in order to train their staff to face hazardous situations.

These simulations are related to emergency plans: dangerous situations or critical phenomena that have been identified as potential threats are played out and analyzed with the aim to define or check prevention and protection measures and to validate intervention plans. Exercise scenarios are designed to give people opportunities to "play the game" in realistic conditions, practice plans and test the use of technological devices.

Our observations and analysis of a series of simulations (toxic leak in a refinery, fire and toxic gas in road tunnels, terrorist attack in the metro, etc.) have shown the benefits but also the limitations of such simulations in terms of lessons (behaviors, decision making) learned by participants.

Among the benefits, the setting up of the exercise is in itself a good opportunity to gather the many stakeholders and to discuss "who is in charge of what"; this facilitates mutual knowledge among technical staff and shared values in the different organizations. The second kind of benefit is the test in realistic conditions of the technological devices and means (medical tents, fire extinction means, communications, etc.) and their use.

Among the limitations, the poor level of realism of the simulation is often an obstacle to the commitment of participants, who don't react as they would in a real, stressful situation. Another limitation lies in the evaluation method; people are evaluated for their strict application of plans, compared to "official" plans and procedures. Any difference in behavior is seen as a violation of rules and sanctioned or at least pointed to as an error, which strongly reduces people's willingness to innovate.

From this analysis, it can be concluded that simulations are efficient in creating opportunities for people to work together and to improve their practice of anticipated situations. But do simulations improve the ability of organizations to avoid or manage crises? Based on different non-participant observations and in-depth interviews, our answer is "no", to the extent that their analysis is limited to the identification and correction of deviations from the prescribed tasks.

We define a crisis as a situation in which an organization is overwhelmed and destabilized, as compared with emergency management, a situation in which the organization remains in control, applying known plans and procedures [START_REF] Wybo | Mastering risks of damage and risks of crisis : the role of organizational learning[END_REF]. In this context, resilience corresponds to the ability of the organization (at any level) to keep achieving its tasks by adapting its functioning to hazardous situations, uncertainty, time pressure and threats. Robustness corresponds to the ability of the organization to survive and stay under control by the emergence of new organizational patterns.

In observing simulations, we have identified individual and collective reactions to difficulties that were neither part of the scenario nor foreseen in the emergency plans. If the difficulties are relatively small, individuals and groups use their skills and experience to adapt plans, procedures and behaviors, in order to achieve their tasks in those degraded contexts. By doing so, they contribute to the organization's capacities of resilience. When faced with unanticipated and threatening events or situations, some people (generally the most experienced) show capacities of robustness: they emerge from the group, commit themselves to "do something" and find innovations to cope with the real situation. In some of those situations, new organizational patterns emerge, for example the creation of different communication flows or an alternate distribution of tasks. With such actions, those individuals and groups demonstrate their capacity to contribute to a higher level of robustness in the organization.

In order to go further in this reflection, we have designed a method to observe and analyze simulation exercises, allowing the identification of such behaviors, the context in which they appear and the lessons that can be learned from this analysis in terms of resilience and robustness capacities [START_REF] Wybo | Using simulation of accidents to assess resilience capacities of organizations[END_REF].

The role of simulations

EMERGENCY MANAGEMENT AND RISK PERCEPTION

It is now widely accepted that safety management inside the organization has to consider, in addition to the physical characteristics of dangers in the working place, the way risks are perceived by those involved.

The study of risk p e r c e p t i o n i n s i d e o r g a n i z a t i o n s d e a l s w i t h t h e comprehension of tasks and the definition of working situations when these may constitute a threat for the physical integrity of personnel, installations security, and even the safety and health of populations outside the organization.

Taking risk perception into account can be considered as one remedy for "normal accidents" as suggested by [START_REF] Weick | Interpretive sources of high reliability: Remedies for normal accidents[END_REF] who shows that systems vulnerable to normal accidents [START_REF] Perrow | Normal accidents[END_REF] are at the same time contexts where individuals try to understand and manage complexity. Weick suggests preventive and learning actions through which individuals and groups become able to build elaborated interpretations of what they do and experience, leading them to analyze events other than those strictly limited to technical matters. [START_REF] Jacques | From post crisis to preventive learning[END_REF] have shown, using cognitive mapping, the evolution of learning over time by using preventive learning technique.

The challenge is thus for the organization to integrate safety management in two manners: first by taking into account human and organizational factors and not only technical factors of safety, and second, through involving fully all the actors inside the organization. When the risk of accident concerns outside populations, a wide array of stakeholders can be involved.

Safety integration can be achieved through training and preventive actions. A second way is through simulation. The specific advantage of simulation as a source of lessons is that it puts forward many aspects of what people prefer not to think about in their everyday working life situation: accidents and crises. In terms of perception, a simulated accident makes manifest events which otherwise might remain unknown, unseen and unheard of.

The simulation of a large accident involves many different players whose role is to intervene in case of accident. In normal times, these people do not meet nor are part of the high reliability organization (they are generally not part of the organization in charge of the routine activities). When the accident occurs, they must understand the situation, switch from the routine work to emergency management, cooperate and coordinate themselves in order to mitigate the accident and keep the situation under control. This very fact is in itself a complex organizational achievement, costly in terms of material and human resource mobilization.

Under normal operations, it is the occurrence of the accident that triggers a wide array of events, ranging from the focus of attention of personnel to the calling together of many outside players. The accident has the upper hand. In the case of simulation, the human minds of scenarists will take the lead. What difference does this make? This is what we aim to distinguish here, principally in terms of learning, which is chief among the assessment of resilience and robustness capacities of organizations.

Put simply, one can say that simulation creates for the organization an intermediate state between normal operations and crisis situations. While some of the preparedness required to face potential crisis situations can be acquired during normal times, other levels of learning can be acquired only outside normal situations, i.e. during crisis situations or simulations.

We focus in this paper on learning during simulation situations. To understand the dynamics of this type of learning, we have to consider some aspects of safety management in ordinary times. How safety is recovered during and after a crisis can then be examined on that basis.

SAFETY MANAGEMENT IN NORMAL TIMES AND DURING CRISIS

When a safety issue is raised, one may observe that everyone in the organization has an opinion on the question. While the starting point of this opinion is the shared objective to avoid accidents and prevent operational errors, reasoning and references, often implicit, may rapidly diverge. This can occur according to the position and role of the actor, his knowledge of risks and operations, possible defensive mechanisms that are in use in his working environment (Argeris & Schön, 1996), (Dejours, 2000), orientations set by the hierarchy and the top management, etc.

In addition, the perceived contradiction between the a priori shared goal of a high level of safety and secondary divergences can lead to misunderstandings or stalled situations, which in turn are counterproductive for safety. The extent and efficiency of preventive actions, as well as communication with stakeholders, can thus suffer.

Organizations are bound by many levels of requirement or obligation in regard to safety assurance [START_REF] Poumadère | Perception des risques et gouvernance de la sécurité industrielle[END_REF]. The first level is individual obligation, which includes to follow consigns and basic rules, such as wearing the mandatory individual protections. Obedience is a must, along with the notion of self protection and, in some cases, that of survival. In most settings and most of the time, this level of obligation can appear disconnected from the apparent requirements of tasks. However, during accident and crisis situations, this basic level plays its full role and it is expected to be automatically integrated in behaviors. This context may restrain individuals to commit themselves to adapt procedures or innovate, as this behavior will be perceived as violations of rules.

Another level is that of economic obligation; simple economic rationality favors investments in prevention so as to avoid the very high direct and indirect costs of accidents. This level is most often disregarded during accident and crisis, as urgency prevails and exceptional expenditures may be undertaken. Even so, adaptations and innovations achieved on the spot are often done with very few resources, but without a clear evaluation of costs that may result later on from these actions (either in reduction or aggravation of damage).

The managerial obligation resides in the fact that safety figures among collective performance factors and must be managed to that effect, in itself and alongside other performance factors with which it is in interaction. During accidents or crisis, this managerial coordination of decisions is likely to be crucial, as long as the system remains under control of the organization. When the situation goes beyond the limits of routine, this obligation becomes less and less crucial compared to the willingness of players to do something useful at their own scale. In resilient organizations, the management will decentralize control and give to their staff more and more degrees of freedom in action as the situation escapes from control.

The legal obligation refers to the existence of the organization within a state of law, applied to all organizations which have to conform to established norms and prescriptions. Often this level prevails in the organization when safety is considered; i.e., safety actions and investments may be framed as responses to minimum legal requirements. However, during accident and crisis situations, the need to invent ad-hoc solutions and organizational patterns may make that level less predominant. In a similar way than individual obligation, legal obligation reduces the willingness for adaptation and innovation, as people may be sanctioned for violating rules, especially if there are casualties and financial damage.

The professional level of obligation corresponds to the best possible application of scientific and technical knowledge by Safety experts; in some contexts all members of the organization are invited to integrate safety within their professional identity. During degraded situations, experts tend to consider adaptations and innovations as a "normal way" to use their skills and experience when they have to cope with a risky situation. By doing so, they will be reluctant to talk about (either because they think that every professional would have done the same or because they want to keep secret what gives them the status of expert) and this "expertise in action" will not be shared and learned by other players.

The reciprocity obligation corresponds to interdependency and solidarity, which exist de facto within working relationships and make each of us responsible for others' safety, which can extend to relations with the environment of the organization and its protection. This obligation plays its role during degraded and crisis situations and contribute to organizational resilience as it governs interactions among players.

The moral obligation puts forward the value of human life, primordial within our cultural ethics, and for which better safety for all is a goal. This obligation plays an important role in the commitment of people when they perceive threats for other people.

These levels of obligation influence perceptions and role playing at all levels of the organization and thereby influence safety practices in routine activities and during emergencies. As shown above, obligations change in meaning and salience according to whether the organization is in a normal, degraded or crisis situation. It is thus useful in each context to assess safety obligation levels, both in ordinary times and for accident or crisis situations. The bridges built between these two different instants in organizational life can contribute to resilience and robustness if it is possible to observe and analyze them. This is our main objective during the simulation exercises.

INTERPRETATIONS OF SIMULATIONS' ANALYSIS

In the usual way of organizing exercises, one uses a rational model to measure the gaps between prescribed and observed actions as the scenario unfolds, and to explain in an objective way the causes of those deviations. This approach can be called "naïve", as it doesn't reach the complexity of the organization during emergencies and crisis. It can be associated with "simple-loop learning": measuring gaps and correcting them.

In this paper, we present a method to observe an organization during a simulation based on the plurality of specialized observers' points of view. Taking their observations together allows us to address the complexity of organizational behavior, to generate meaning and learning from the postsimulation analysis and thereby to go beyond the simple diagnosis of deviations from standard practice. The method is framed by a three dimensional model of organization: structures, relations and meaning:

"that of structures in which actors' games take place, that of relations between actors who set structures in motion, changing them through their games, giving rise to new organisational forms. This dimension also takes into account the principles of (sense making) legitimacy through which actors justify their games and their constructed orders." [START_REF] Jacques | Cognition towards crisis: the blind man held a handful of snow … and concluded that white was cold[END_REF]

Structures

Structures consist in what is prescribed by the organization, objectivable and measurable: the product of division of work, tasks, the means to achieve tasks, formal rules, procedures, technology, coordination tools and artifacts, etc.; what Minsberg (1993) calls the hierarchy line, technostructure and support services. This first layer allows the organization to deal with its routine tasks in a safe way: the "normal situation" is under control. It is associated with hierarchy: control is achieved by the management layer.

Relations

Relations are often represented as the roles played by the different actors. Following [START_REF] Crozier | Travail : usure mentale[END_REF], this dimension takes into account the fact that the structure is a context for action, which includes relations and interactions among people.

Each actor provides his resources, stakes, interests and power. This dimension can be observed at the micro/local level. It concerns verbal and non verbal, formal and non formal communication. It raises questions about the group's dynamics. This second layer gives the organization its flexibility to deal with deviations. By interacting, people adjust their activities to cope with changes from the routine conditions in order to proceed with their missions and put the system back into its normal state. This layer corresponds to the resilience of the organization. It is associated with networking: control is achieved by a series of adjustments and interactions at different levels of the hierarchy.

Sense

Sense making is what people use to justify their actions; it is related to the notions of legitimacy, ethics, interests and values. The actor's representation of the situation influences his behavior. People belonging to different "worlds" have difficulties to act together for a common task (Boltansky and Thevenot, 1991), [START_REF] Weick | Sense making in organizations: small structures with large consequences[END_REF].

Organizations that are able to make sense of ambiguous and uncertain situations demonstrate their plasticity to deal with unforeseen situations, pressure of events and and to avoid chaos and crises. This layer corresponds to the robustness of the organization. It is associated to emergence: control is achieved by people at any hierarchical level that invent ad-hoc solutions where and when they are needed to ensure the survival of the organization.

Organization of simulation exercises

In order to be prepared for emergency management activities, risk-prone companies and rescue services organize on a regular basis exercises that simulate accidents and catastrophes. In France for instance, a national regulation requests that all dangerous industrial sites organize once a year a large exercise with rescue services and local authorities.

The purpose of such exercises is to train people to apply procedures and plans, to become familiar with technical systems and locations, and to evaluate the efficiency and appropriateness of procedures. Their objective is also to give opportunities to the different organizations to communicate and act together. These practical sessions have one more advantage: they can be organized more frequently than staff turns over.

In this way, teams become accustomed to work together and if some need for improvement is identified, progress can be assessed during the next exercise, as it will be carried out by the same people in comparable conditions. This advantage is especially great when the exercise concerns very rare events and/or when the stakeholders have a high rate of turn-over. By running exercises at an appropriate frequency, the organization increases its capacities to face "known events".

PREVENTION OF CRISIS: THE ROLE OF SIMULATIONS

When dealing with the management of crisis situations, the interest of such exercises in terms of training can be questioned. Crises are situations in which plans and procedures are not appropriate, so how exercises can provide experience for such situations? How may exercises increase the resilience and robustness of organizations?

By placing observers in appropriate locations with precise missions during emergency exercises, we observed that people playing their roles in the exercise sometimes go beyond the procedures describing their tasks: when facing various types of difficulties (included or not in the scenario), they develop communication and coordination activities with other people (inside and outside their organization) and they adapt their activity to the real context in which they are. If the debriefing of the exercise is focused on the strict application of plans and procedures, such deviations and ad-hoc solutions are evaluated negatively by the management and so, they are generally hidden or minimized by the participants.

"Given the emphasis on plans, even those that are impossible to execute, it is not surprising that departing from them is often cited as evidence of a failure. Disasters, however, break the rules that guide the ordinary conduct of business and government, at least for a period of time. Disasters create new environments that must be explored, assessed, and comprehended, change the physical and social landscape, and therefore require a period of exploration, learning, and the development of new approaches", [START_REF] Kendra | Creativity in Emergency Response after the World Trade Center Attack[END_REF].

These deviations from the standard procedures are indicators of the ability of people to adapt to difficulties and by that, they reveal on one hand the need for adaptation of procedures and on the other hand the resilience and robustness capacities of the organization: the ability of people and groups to be flexible and innovative to avoid destabilization and crises.

ANALYSIS OF EXERCISES

Simulating accident situations and collecting lessons learned is a challenge both for authorities and for other stakeholders involved. Accident is a sensitive situation including technical malfunctions, human errors and organizational flaws as root causes. Accident investigation in general is often closely focused on finding causes of the accident and less effort is put in studying organizational factors that influence the effectiveness of emergency management.

Beyond these difficulties, exercises provide an important source of information about emergency management in accident situations. The major advantage of simulated cases is that it is easier to collect lessons learned and to identify difficulties, if precautions are taken not to put too much pressure on participants about responsibility. Tackling the question of guilt can form an obstacle to the collection of relevant information about errors and organizational drawbacks from participants.

Analysis of simulation exercises is based on a method which main objective is to develop organizational learning from accidents and crisis. This method was originally designed for the analysis of real accident situations in industrial plants, public transport [START_REF] Wybo | A methodology for sharing experiences in incident management[END_REF], floods and oil spills.

The method associates people who have been acting at different levels of hierarchy and in different organisms along the development of the situation. It is based on collecting individual stories from those who have been involved in the management of an accident situation and sharing these individual experiences among them in order to develop an organizational learning process.

In the case of real accidents, we select representative people that participated in the management of the accident at different levels of the hierarchy and from the different organizations (company, rescue services, officials, etc.).

In the case of simulation exercises, we introduce two categories of people in the process: a set of people that played the simulation and the group of observers. By this way, we get a chance to access the insights of the organization at work during the exercise and to identify aspects concerning the three levels: structure, interaction and sense.

Each interview starts with the interviewee telling his "own story". From that narration, the researcher points out the key moments in the story and asks questions like "why did you do that?", "how did you do that?", "what else could have been done?". By this way, relevant information about explicit knowledge (context, events, actions and decisions) can be identified from the story, along with some tacit knowledge: perceptions, motivations and alternatives. [START_REF] Wybo | Gestion des dangers et systèmes d'aide à la gestion[END_REF].

This knowledge is formalized as a set of "particles of experience". These particles of experience constitute the meaningful pieces of memories of each person having experienced a stressful situation. They represent either the person's reaction to an event or his actions to cope with a change in the current situation. An overall picture of the development of the situation is then drawn by the researchers, by merging information from individual stories into a collective story. Each particle of experience is divided into four phases:

• Context: the main aspects of the current situation;

• Analysis: how people perceived (on the spot) the situation and its evolution, and the hypothesis that were considered;

• Action: decisions made and actions carried out;

• Effect: a posteriori evaluation of effects of actions on the development of the situation.

This common story is then discussed and validated during a "mirror meeting" gathering all participants, in order to reach an agreement among them and to identify lessons to learn from the management of that exercise.

The collective processing of individual perceptions, suggestions and experiences favors the commitment of participants to the conclusions reached during the process, which is very useful to promote learning in the organization and to apply the lessons learned in the future.

OBSERVATION OF EXERCISES

Based on this method, the introduction of specialized observers was tested in several exercises, in order to get the full picture of the organization at work and to identify these deviations from the prescribed world, from the combination of the points of view of the participants and the points of view of the different observers.

Three kinds of observers were defined: those who observe the activity of key people (information they receive and emit, people with whom they collaborate, decisions they make, etc.), those who observe a specific task (how it is achieved, difficulties encountered, who participates, what resources are used, etc.) and those who observe a specific place (who is there, what is done, how this place is perceived by people, etc.).

Using this combination of points of view (people playing roles and specialized observers), we build the "full picture" of the simulation that we present to the players during the debriefing session. This analysis results in the identification of a number of deviations, in particular the emergence of organizational patterns and communication flows among stakeholders, some of them proving efficient to prevent the situation from turning into crisis. Studying the character and value of these deviations makes it possible to capitalize on them to improve emergency procedures and plans, and to increase the mutual knowledge and efficient cooperation of stakeholders [START_REF] Wybo | Improving resilience of organizations by increasing mutual knowledge of stakeholders[END_REF].

"The reliability of learning of an organization is if it develops common understandings of its experience and makes its interpretation public, stable and shared." [START_REF] March | Learning from samples of one or fewer[END_REF]

Case 1: assessment of resilience capacities

We present here an example of t h i s m e t h o d t o t h e s i m u l a t i o n o f a n emergency in a road tunnel. The scenario was: a truck carrying a toxic gas tank stops in the tunnel as its tank is leaking; the driver goes to the nearest emergency shelter and calls the tunnel control room; four cars and a bus are in the tunnel at that time and stop at a walking distance from the truck. This exercise was set to test the management of such accident and to understand the behavior of passengers before the rescue services reach the site. More than 200 rescue people participated in the exercise, plus passengers of vehicles, tunnel operators, policemen and officials in the control room. 30 observers participated in the instrumentation of the exercise.

During the exercise, the tunnel management software partially failed (this was not in the scenario) and it was no more possible for the operator to answer calls from rescue shelters in the tunnel with the usual hardware (PC display, mouse and audio helmet). He remembered that there was a secondary rack that permitted to answer those calls, but this rack was difficult to access (in a dark part of the room, near the ground) and the indication of the calling shelter number was difficult to read (small digits, not illuminated). Anyway, the operator took with him a paper map of the tunnel with indication of shelter phone numbers & locations and began to answer calls.

By the way, he succeeded to get essential information from the people calling, reassure them and give them simple indications to be safe while rescue forces were on their way. This is an example of the robustness capacities of this operator, which allowed him to keep people safe in the tunnel and to give precious indications to rescue services (how many people, in which locations: cars, bus and shelters).

Case 2: assessment of a lack of robustness

On the other side, it is also possible to observe and understand some drawbacks that would ruin the efficiency of emergency management in a real situation.

In the simulation of another accident in a road tunnel, two observers were placed in the control room of the tunnel: one observing the manager on duty and the other observing the activity in the control room. At the same time, other observers were inside and outside the tunnel (15 observers participated in this simulation). When the simulated accident occurred (collision of a car with a truck, putting fire to both vehicles), some of the car passengers, shocked but not wounded, were wandering on foot in the tunnel, looking for an escape route, and the rescue services took a long time to localize and shelter them. In real conditions, those people would probably die from smoke heat and toxicity.

By combining observers' data, we identified the organizational cause of this difficulty. The control room operator was facing a set of video screens in which it was possible to observe these people wandering in the tunnel. At the same time, in a corner of the room, an operator from the rescue team had established a radio terminal to communicate with their colleagues in the tunnel and they were listening to the conversations over the radio. Finally, the manager on duty was trying to assess the situation and prepared to answer the requests that he could receive from the chief of rescue forces (located at one of the exits of the tunnel) or other stakeholders.

But the operator had not been told to report to the manager what he saw on the videos (he was trained to answer questions, not to be proactive) and the operator m a n a g i n g t h e r a d i o t e r m i n a l w a s n o t t r a i n e d t o i d e n t i f y problems that his colleagues encountered in their actions from what he heard over the radio, so none of them communicated to each other or to the tunnel manager.

When the analysis of the different observations (in the control room, in the tunnel, at the rescue headquarters, etc.) was carried out, this drawback appeared as a lack of capacities of sense making in this group of people: none of them took an initiative to invent a form of cooperation that was not defined in the emergency plan.

From this analysis, it was possible for the tunnel managers and the rescue officers to share these results with their staff and to set up improved emergency procedures. This analysis also pointed out the importance of interactions among people from different organisms and proactive behaviors to build resilience and robustness capacities; otherwise even the best structure (cameras and video screens, wireless communications, procedures, etc.) is useless in such degraded situations.

Conclusion

Simulations are one of the most efficient tools that can be used to train people to emergency situations, especially for situations with a low frequency of occurrence and a high potential of damage.

Emergency services and risk-prone companies have gained significant experience in the setting up of exercises and this practice contributes to the capacity of their organizations to face "planned" emergencies (reinforcement of the structure layer).

In order to face situations that may turn into crisis, because of surprise, speed of development, uncertainty, lack of resources or difficulties in communications among stakeholders, organizations need to assess and develop their resilience and robustness capacities. In this paper we have shown that exercises can contribute to that on the condition that their analysis goes beyond naïve interpretations and gives access to the complexity of organizational behavior, specially the levels of interaction and sense making.

The method presented here provides means to reach this objective with only minor changes in the simulation set up. By defining specific missions

for observers based on a model of organizational behavior, it is possible to identify more precisely the organization's resilience and robustness capacities and handicaps.


Markus Drescher
email: markus.drescher@desy.de

Ulrike Frühling

Maria Krikunova

Theophilos Maltezopoulos

Marek Wieland

Time-diagnostics for improved dynamics experiments at XUV FELs

come

Introduction

The new generation of linear-accelerator-based extreme-ultraviolet (XUV) and X-ray free electron lasers (FEL) represent extremely promising light sources for studying the dynamics of electronic and nuclear rearrangements in atoms, molecules, on surfaces, and in bulk material. Short wavelengths guarantee high spatial resolution in diffraction experiments [START_REF] Chapman | Femtosecond diffractive imaging with a soft-X-ray free-electron laser[END_REF][START_REF] Gutt | Resonant magnetic scattering with soft x-ray pulses from a free-electron laser operating at 1.59 nm[END_REF] and facilitate deep penetration of the radiation into the electron shells, thus providing chemical specificity [START_REF] Ch | Ultrafast X-ray Absorption Spectroscopy[END_REF]. High flux yields sensitivity down to the singlemolecule level [START_REF] Neutze | Potential for biomolecular imaging with femtosecond X-ray pulses[END_REF] and high peak intensities enable remarkable steps towards nonlinear optics in the X-ray range [START_REF] Sorokin | Photoelectric effect at ultrahigh intensities[END_REF][START_REF] Nagler | Turning solid aluminium transparent by intense soft X-ray photoionisation[END_REF]. Finally and most significantly, the short pulse duration opens the door for utilizing well-developed X-ray tools for exploring the response of matter to an optical stimulus on the relevant time-scale.

The full exploitation of this potential not only requires an optimization of these properties but also relies strongly on the capability to deal with the issue of reproducibility: It turns out, that the currently operated and commissioned FEL sources based on self-amplified spontaneous emission (SASE) are subject to pronounced shot-toshot variations of basically all radiation properties, including pulse energy and spectrum, as well as the spatial and temporal profile. In the long-term, significant progress is expected from advanced concepts like injection seeding of the undulators [START_REF] Garzella | Using VUV high-order harmonics generated in gas as a seed for single pass FEL[END_REF], but on a more temporary scale much improvement can be achieved by utilizing a tagging concept. If the properties of individual FEL pulses are measured independently, the data of simultaneously performed experiments can be filtered or sorted according to these attributes. For a correction of pulse energy variations this concept is readily available to users of FLASH by virtue of an energy monitor with single-shot capability [START_REF] Richter | Measurement of gigawatt radiation pulses from a vacuum and extreme ultraviolet free-electron laser[END_REF], after the current major shutdown also spectral measurements will be available for each FEL pulse. This article reports on recent progress in obtaining single-shot information on i) the duration / temporal profile of individual pulses from FLASH and ii) their relative timing with respect to laser pulses delivered by the facility.

In contrast to lasers in the visible range, FELs operated in the soft and hard X-ray regime are currently lacking optical resonators that would provide a selection of defined cavity modes. Instead, quantum noise is picked-up at the undulator entrance and is further amplified in the SASE avalanche [START_REF] Dohlus | Ultraviolet and Soft X-Ray Free-Electron Lasers: Introduction to Physical Principles, Experimental Results, Technological Challenges[END_REF]. As a result, an unpredictable pattern of modes is formed within the spectral window ∆ω of the FEL bandwidth. While in modelocked laser oscillators the pulse duration ∆t laser corresponds almost to the Fourier-limit given by the available spectrum, i. e. ∆ω × ∆t laser ≈ 1, pulses from SASE FELs will usually exhibit several temporal modes within an envelope dictated by the duration of the lasing part of the electron bunch. Since 2005, FLASH has been operated in a mode where the electron charge banks up within < 50 fs at the leading edge of the electron bunch [START_REF] Ackermann | Operation of a free-electron laser from the extreme ultraviolet to the water window[END_REF]. On the other hand, its 0.5 eV bandwidth at 100 eV photon energy corresponds to a minimum (Fourier-limited) duration of each temporal spike of about 4 fs. Thus, depending on the number of sub-pulses and their temporal distribution the total FEL pulse length will be somewhere between 4 -50 fs and will fluctuate erratically between subsequent pulses. The number of sub-pulses is expected to be small (< 5) for FLASH [START_REF] Ackermann | Operation of a free-electron laser from the extreme ultraviolet to the water window[END_REF] but can reach large numbers for hard-X-ray FELs.

In time-resolved experiments using the pump-probe technique for sampling the (linear) response of a system to a pulsed optical excitation with a delayed XUV pulse, it is the envelope width σ FEL that enters into the achievable time-resolution. As the result is usually composed from an average over many shots, the effective resolution is determined by the average pulse envelope σFEL . On the other hand, the outcome of experiments probing the nonlinear response of matter to an intense XUV pulse [START_REF] Sorokin | Photoelectric effect at ultrahigh intensities[END_REF], may critically depend on the distribution of energy within the pulse envelope, because the process will be dominated by the peak-rather than the average intensity. For the former case, the tagging concept will help improving time-resolution by confining the analysis to data obtained with FEL pulses below a selectable threshold of measured pulse durations. For the latter case, sorting of the data according to measured durations and pulse-structure will significantly enhance the results expressiveness.

The formation of ultra-short FEL pulses involves the synchronization of the accelerating radio frequency field with the laser-driven electron gun and a second laser delivering femtosecond pulses to the experimental end stations. Stabilization of the timing between XUV-and laser pulses to a level where the resolution of time-resolved experiments would merely be limited by the FELs pulse duration will require locking of the phase of the acceleration field to the laser pulses with a precision of ∼ 2 * 10 -5 rad. XUV / laser cross-correlation experiments averaging over many shots have revealed a resolution limit of 250 fs rms [START_REF] Radcliffe | Single-shot characterization of independent femtosecond extreme ultraviolet free electron and infrared laser pulses[END_REF] that can be attributed to arrival time jitter at the endstation. A single-shot metrology for the timing between XUV and laser pulses will aid in the identification and reduction of jitter sources and allows an immediate improvement of the resolution by jitter compensation.

In section 2 a technique for obtaining single-shot information on the duration of XUV pulses from FLASH will be reviewed. The instrument, a light-field driven streak camera, will be demonstrated to unveil the expected sub-structure in the temporal pulse profile, varying erratically from shot-to-shot. Section 3 reports on the singleshot determination of the FEL/laser timing, utilizing an optical cross-correlation on a solid surface in a non-collinear geometry. The improvement in temporal resolution achieved by applying the tagging concept will be reported in section 4, demonstrating a significant advance in the capability to study linear as well as non-linear electronic relaxation processes with FEL radiation.

Single-shot measurement of XUV pulse duration

The strongly modulated temporal profile of SASE-FEL pulses is usually also reflected in a modulation of their spectrum. Unfortunately, a straightforward measurement of the power spectrum will generally not reveal the desired temporal profile due to a lack of phase information. Streak cameras are established devices for a direct sampling of the time-profile of visible [START_REF] Breadley | Direct linear measurement of ultrashort light pulses with a picosecond streak camera[END_REF] as well as X-ray pulses [START_REF] Feng | An x-ray streak camera with high spatio-temporal resolution[END_REF][START_REF] Shakya | Achieving 280 fs resolution with streak camera by reducing deflection dispersion[END_REF]. A photocathode transforms the incoming light pulse into an electron bunch that initially represents an exact temporal copy of the light distribution. As depicted in Figure 1(a) the electron packet is then accelerated and subsequently deflected in a transient transverse electric field. Since the transverse momentum an electron gains depends on the strength of the field at the deflector entrance, the temporal information is projected onto a spatial position on a fluorescence screen. Main factors limiting the temporal resolution of these devices are the finite rise time of electro-optical switches creating the transient field and the spread of the propagating particle wave packet introduced by Coulomb repulsion and dispersion. These constraints are removed by utilizing a variant of the streak camera principle, where the solid photocathode is substituted by an atomic gas and the deflecting field slope is replaced by the electric component of an oscillating electromagnetic field (Figure 1(b)). Much higher temporal gradients of the streaking field boost the resolution together with an elimination of the dispersive wave packet spread. As the photoemitting atoms are embedded into the streaking field, the electron wave is instantaneously sampled during its formation, thus prohibiting a temporal broadening. In contrast to a conventional streak camera, no directed electron beam is formed, but the electrons initially escape from the photoemitter in all directions according to a partial wave composition determined by the initial state and the photon energy and polarization of the ionizing radiation. Correspondingly, the streaking action of the electric field is not observed as a steering of a particular electron trajectory but rather in the form of a momentum transfer in the field direction. Thus, for this lightfield streak camera the temporal information is projected onto a change of the detected kinetic electron energy. The time-resolution achievable with this type of instrument is ultimately limited only by the speed of the photoemission process [15] and is therefore used in attosecond physics for the characterization of XUV photon pulses with durations of about 100 as [START_REF] Goulielmakis | Single-Cycle Nonlinear Optics[END_REF][START_REF] Sansone | Isolated Single-Cycle Attosecond Pulses[END_REF]. In those studies a near-infrared field with an oscillation period of only ∼ 2.5 fs (800 nm wavelength) was employed. Even at moderate laser intensities the rapid change of the streak field guaranteed an extreme streaking speed above 10 eV/fs. It is determined by the rate of change of the electron kinetic energy with the delay between photon pulse and streaking field. For the characterization of FEL pulses with an expected duration of up to 50 fs the streaking electromagnetic radiation must obey a monotonically rising electric field within this interval. This implies a lower wavelengthlimit of 30 µm, i.e. radiation in the terahertz (THz) regime. Targeting a streaking field induced energy shift ∆W of twice the spectral width σ W of the photolines requires a field strength E streak according to

∆W = e E streak λ streak W 0 2π 2 m e c 2 > 2 σ W (1)
with the initial kinetic electron energy W 0 = IP -hν x , given by the ionisation potential IP of the atomic medium at the photon energy hν x , and with e and m e c 2 the electron charge and rest energy, respectively [START_REF] Itatani | Attosecond Streak Camera[END_REF]. For W 0 = 100 eV, σ W = 1 eV, and a wavelength λ streak = 100 µm for example, the electric field strength shall exceed 6 MV/m. Note that the scaling with λ streak reduces the required light intensities I ≈ E 2 by many orders of magnitude upon changing streaking wavelengths from the near-infrared to the far-infrared (= THz) regime.

In the THz field driven streak camera experiment at FLASH 13.5 nm pulses are collinearly overlapped with intense linearly polarised THz-pulses from the FLASH THz undulator [START_REF] Gensch | New infrared undulator beamline at FLASH Infr[END_REF][START_REF] Borisov | Simulation of Electromagnetic undulator for far infrared coherent source of TTF at Desy Proceedings of[END_REF] in a gas target (krypton or xenon). The gas atoms are ionised by the XUV pulses. The kinetic energies of the resulting photoelectrons are measured with two time-of-flight (TOF) spectrometers, one parallel and one perpendicular to the THz-polarisation plane. The photoelectrons are accelerated by the THz field in the direction of its polarization, thereby the temporal structure of the XUV pulses is mapped onto changes of kinetic energies of photoelectrons with a final drift velocity parallel to the THz polarization. On the other hand, the THz-field-induced energy shift of photoelectrons with a final drift velocity perpendicular to the THz polarization is negligible (since U P << W 0 ) and their energy distribution represents the spectrum of the XUV pulses shifted by the binding energy of the electrons. Thus, it is possible to measure simultaneously the streaked photoelectron spectra and the XUV spectra with the two electron spectrometers. For details of the setup see [START_REF] Frühling | Single-shot terahertz-field-driven X-ray streak camera[END_REF].

Mapping of the THz-field

For a measurement of the XUV pulse duration by THz streaking a precise knowledge of the time-dependent streaking field is essential. It can be sampled by measuring the field induced energy shift of the photoelectrons upon scanning the time delay between the THz and XUV pulses. The energy shift directly represents the vector potential

A THz (t) = ∞ t E THz (t )dt (2)
of the THz pulse [START_REF] Goulielmakis | Direct Measurement of Light Waves[END_REF], where E THz is the THz electric field. Figure 2 shows a series of kinetic energy spectra of Xe 5p photoelectrons detached by 13.5 nm FLASH pulses in the presence of intense THz pulses. The delay between both pulses was varied in steps of 33 fs and each spectrum represents an average over 25 shots. The spectra in Figure 2(a) were measured without any THz filters. The ten oscillations of the field correspond to the ten periods of the THz undulator [START_REF] Feldhaus | The infrared undulator project at the VUV-FEL[END_REF]. A Fourier transformation of the THz vector potential yields the THz spectrum consisting mainly of the fundamental at 92 µm with a bandwidth of 10% and small contributions of the second and third harmonic which is consistent with simulations [START_REF] Feldhaus | The infrared undulator project at the VUV-FEL[END_REF] and spectral measurements.

For the measurement of the XUV pulse duration a smooth THz field is essential. Therefore, a THz bandpass filter (QMC Instruments) transmitting between 82 and 107 µm was used during these measurements. This way the spectral content of higher harmonics is removed and the field is smoothened (Figure 2(b)). From the observed oscillation a wave period of 306 fs can be extracted. This corresponds to a wavelength of 92 µm and a frequency of 3.3 THz. The maximal field strength was 50 MV/m and the maximal streaking speed was typically in the order of 0.1 eV/fs. At the FLASH facility, the same electron bunches are used to produce both the XUV and the THz pulses, therefore a good synchronization of the pulses is expected. If the delay between the XUV and THz pulses is set close to a zero transition of the THz vector potential a small timing jitter between both pulses translates into energy fluctuations of the streaked photoelectrons. By analyzing the fluctuation of the mean energy of single-shot photoelectron spectra measured with and without the THz pulses, respectively, a jitter of 5 fs root mean square (rms) was determined.

Measurement of the XUV pulse properties

To determine the duration and the linear chirp of the FLASH XUV pulses the delay between the XUV and THz pulses was set to zero transitions of the THz vector potential and streaked and non-streaked single shot photoelectron spectra were measured simultaneously. The applied streaking field leads to a time dependent energy shift of the electron wave packet, with a sign depending on the sign of the slope of A THz .

If the XUV pulse carries a linear frequency chirp, the resulting total chirp and thus the width of the streaked spectra is enlarged or reduced for consecutive zero transitions of A THz respectively [START_REF] Itatani | Attosecond Streak Camera[END_REF]. Based on such analysis, the chirp of attosecond XUV pulses has previously been determined [START_REF] Kienberger | Atomic transient recorder[END_REF]. Under the assumption of a Gaussian pulse shape and a constant linear chirp the pulse durations of the individual XUV pulses were deduced (Figure 3). The average pulse duration was 35 fs FWHM with a width of the distribution (standard deviation) of 9 fs. A small percentage of the pulses has pulse durations of less than 7 fs FWHM. The employed gas target does not significantly alter the XUV pulses, so it is possible to measure the XUV pulse duration simultaneously to other time-resolved experiments. By sorting the measured data according to the XUV pulse duration a significant increase of the temporal resolution of XUV-pump/X-ray-probe experiments can be expected. For a fast reconstruction of the pulse duration of many shots a Gaussian pulse shape was assumed. In addition, for some pulses it is possible not only to reconstruct the pulse duration or envelope of the pulse but also a temporal sub-structure consisting of separated spikes. Figure 4 shows an example of a streak spectrum with two clearly resolved peaks (b) and the corresponding non-streaked photoelectron spectrum (a). To reconstruct the temporal structure of the XUV pulse two Gaussian sub-pulses were assumed and the corresponding streaked and non-streaked spectra were calculated. The central energies, amplitudes, widths and the temporal separation of the sub-pulses were optimized to fit the measured spectra. The optimized spectra are plotted as lines in Figure 4. The corresponding temporal structure is plotted as a bold line in Figure 4(c).

For the analyzed spectra about 10% showed two or more well separated features, whereas 90% had a compact envelope.

3. Single-shot measurement of Laser / XUV timing

Concepts of XUV / laser cross-correlation measurements

Since in a single-shot instrument a scan of the delay between both pulses in subsequent shots is not possible, the timing information must be encoded in a measurable parameter. Spatial encoding utilized here requires the superposition of both beams on the target under a certain angle. This way, the pulse-fronts intersect each other at different locations with different relative timing, thus mapping the temporal delay onto a spatial coordinate.

Arrival time fluctuations of the XUV pulses from FLASH relative to the laser pulses were measured with two different cross-correlation techniques at the experimental end-station. Any cross-correlation between two light waves relies on the mediation by a target that delivers an observable which depends simultaneously on both radiation fields. The XUV and laser pulses were non-collinearly superimposed in space and time in a noble gas target and on a solid surface, respectively. For the noble gas crosscorrelator the observable is the electron sideband formation, where the XUV generated photoelectrons are shifted to spectral sidebands by the laser field [START_REF] Glover | Observation of Laser Assisted Photoelectric Effect and Femtosecond High Order Harmonic Radiation[END_REF]. For the solid surface cross-correlator the observable is a reflectivity modulation of the surface. Here the illumination of the surface with XUV pulses from FLASH significantly changes the crystal's reflectivity for laser light [START_REF] Gahl | A femtosecond X-ray/optical cross-correlator[END_REF].

The experimental setup of the cross-correlator using sideband formation is shown in Figure 5(a). The unfocused XUV pulse enters the target chamber through an entrance slit of 0.5 mm height and 0.15 mm width. In this time-to-space mapping geometry, the XUV pulse height of 0.5 mm corresponds to a time window of 1.7 ps. A krypton gas target for the generation of photoelectrons by the XUV beam is supplied by a gas nozzle. The laser pulse is focused perpendicularly to the FEL in front of the entrance aperture of the electron optical system to achieve intensities of ∼ 10 13 W/cm 2 required for the formation of the spectral sidebands. Both beams are linearly polarised parallel to the axis of the electron optics and both pulses are overlapped in the interaction region in space and time. Photoelectrons from this region are energetically shifted to spectral sidebands due to the interaction with the laser light field [START_REF] Glover | Observation of Laser Assisted Photoelectric Effect and Femtosecond High Order Harmonic Radiation[END_REF]. The photoelectron energy spectrum is separated from the unperturbed photoline by multiple integers of the laser photon energy. Time-to-space mapping in this setup is achieved by imaging the interaction region with an electron lens system onto a MCP/phosphor assembly. A grid in the electron path realizes a high-pass energy filter, which separates the energetically up-shifted sideband electrons from the unperturbed photoelectrons by applying a retarding voltage. A detailed description of this experiment can be found in [START_REF] Cunovic | Time-to-space mapping in a gas medium for the temporal characterization of vacuum-ultraviolet pulses[END_REF].

The experimental setup of the cross-correlator on a solid surface is shown in Figure 5(b). Here, the unfocused and spatially filtered laser pulse illuminates the entire area of a GaAs or Si 3 N 4 sample and the reflected light is imaged with a lens system onto a CCD camera. The horizontally propagating focused XUV pulse is absorbed in Time-diagnostics for improved dynamics experiments at XUV FELs the sample, creating a highly excited electron distribution with significantly changed optical properties for the laser pulse. The sign of a resulting change in reflectivity for visible light depends on the choice of material and was found to be negative for GaAs at ∼ 44 eV exciting photon energy and positive for Si 3 N 4 at ∼ 90 eV. In the chosen non-collinear geometry, the spatial position of the reflectivity change is a direct measure for the relative delay between XUV and laser pulse, again mapping time-into spaceinformation. A detailed description of this experiment can be found in [START_REF] Th | Single-shot timing measurement of extreme-ultraviolet free-electron laser pulses[END_REF] and [START_REF] Krikunova | Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser New[END_REF].

Cross-correlation based on sideband signal from rare gas photoionization

The cross-correlation signal of photoelectrons in krypton excited by 32 nm radiation is shown in Figure 6. In (a) an electron optical image obtained at temporal overlap after summation over several macro pulses is presented. The photoelectrons from spatial and temporal overlap, which are spectrally up-shifted and form spectral sidebands, are separated from the main photo line with the high-pass energy filter. It should be noted, however, that the signal of a single shot image is far too low. In order to get an evaluable image it requires a summation over many individual pulses. A delay scan reveals additional signal from these electrons at distinct delay settings, which is shown in Figure 6(b). Every data point corresponds to one macro pulse containing 30 micro pulses. The Gaussian fit shows a temporal width of 1.9 ps FWHM. This value is the result of a convolution between the experimental temporal window (1.7 ps), the jitter (∼ 1 ps), and the pulse widths of the XUV (35 fs) and laser pulses (120 fs). The comparably high jitter value results from the machine performance in the beginning of the user operation of FLASH. Meanwhile, significant progress has been achieved in short and long term timing stability (see Figure 8). While this set-up in principle facilitates single-shot XUV arrival time measurements, the present signal level requires averaging over many pulses in order to obtain sufficient signal.

An enhancement of the sideband signal can be achieved by either increasing XUV pulse energy or laser intensity in the interaction region. However, no electron image formation could be achieved with increased XUV pulse energies due to space charge effects. So far, only 0.015 mJ laser pulses could be applied with this technique. Meanwhile, laser pulses with 2 mJ pulse energy are available at the experimental endstation of FLASH. With two orders of magnitude stronger laser intensities a more efficient sideband formation could enable single-shot data acquisition with this technique in the future.

Single-shot arrival time measurements using the solid surface cross-correlation

Image distorting space charge effects are completely avoided in the photon-in / photonout cross-correlation set-up sketched in Figure 5(b). A typical image of a single-shot cross-correlation using a Si 3 N 4 substrate is shown in Figure 7(a). The area, where the XUV pulse (13 nm) has excited the Si 3 N 4 sample prior to the arrival of the laser pulse, appears brighter. An average of horizontal signal pixels within the region of interest (ROI) marked in Figure 7 of the laser pulse duration (120 fs). The inflection point (marked as dashed line) is used as arrival-time marker for the XUV pulse. The developed timing tool can be operated with a fraction of the XUV fluence delivered by FLASH, needs very little energy from the laser pulse and does not require sample-scanning since the underlying physical process is completely reversible. In this manner, the XUV arrival-time is measured directly at the experimental end-station and, thus, takes all possible jitter sources into account. Figure 8(a) shows a sequence of 18 single-shot cross-correlation images. Due to arrivaltime fluctuations of the XUV pulses, the region of reflectivity change fluctuates from shot-to-shot. A histogramm of arrival times, which are deduced out of 515 single-shot images, is plotted in Figure 8(b). The Gaussian fit shows a FWHM of 408 fs. Thus, without corrections a multi-shot pump-probe experiment using XUV and laser pulses will be limited to this temporal resolution. The single-shot arrival-time measurement demonstrated here is a way to improve the temporal resolution by sorting the pumpprobe spectra pulse-by-pulse according to the measured delay as will be demonstrated in the next chapter.

Jitter-compensated pump-probe experiments

In the previous section the XUV/laser cross-correlator for the single-shot arrival time measurement of FLASH pulses was introduced. In the following the improvement obtained by the simultaneous operation of the cross-correlator with a XUV pump/visible probe experiment is demonstrated. For the first proof-of-principle, ion-charge-state spectrometry for studies of the time-resolved electron dynamics was selected [START_REF] Krikunova | Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser New[END_REF]. In this approach, an inner-shell vacancy created in an atom by the XUV (pump) pulse decays preferably via radiation-less electron relaxation resulting in the ejection of one or more (Auger) electrons. Depending on the intermediate states involved in the deexcitation process several multiple-charged ionic states are created. The population of the intermediate excited ionic states is probed with a delayed optical laser pulse via multi-photon ionisation (or tunneling). Different ionic states created in the target volume are detected by a time-of-flight (TOF) ion mass-to-charge spectrometer by changing the time delay between pump and probe pulses. From the shape of the transient ion yield the lifetime of excited states can be extracted. An implementation of ion-charge-state spectrometry at intense photon-sources like FLASH has several advantages compared to photo-electron spectroscopy. Ions are less sensitive to space charge effects than electrons making the measurements feasible at much higher XUV fluence. Furthermore, the ion yield is less influenced by the spectral fluctuation of FEL pulses than the kinetic energy distribution of the photo-lines. Combined with a high detection probability ion-charge-state spectrometry allows singleshot data analysis. The shape of the transient pump-probe signal is generally described by the convolution integral over a response from the sample and an instrument response function. The sample response is defined by the lifetime of the intermediate excited states involved. The instrument response determines the experimental time resolution and can be approximated by a Gaussian with a FWHM ∆t:

∆t = ∆t 2 laser + ∆t 2 XUV + ∆t 2 jitter , (3)
Thus, selecting the system with well known dynamics as a reference the experimental time resolution can be extracted from the data. Currently, the pulse duration ∆t laser = 120 fs (FWHM) of the optical laser system provided by the FLASH user facility dominates against the much shorter XUV pulses ∆t XUV = 35 fs (FWHM). A substantial arrival time jitter ∆t jitter of the XUV pulses with respect to an external laser will further diminish a temporal resolution.

Exploring the intrinsic temporal resolution of FLASH with the time-resolved ion-charge-state spectrometry on xenon

A scheme of the experimental geometry with the pump-probe vacuum chamber installed in front of the cross-correlator setup is shown in Figure 9. An optical delay line available at FLASH scans the arrival time of the optical laser with respect to the XUV pulse from negative values (optical probe first) to positive values (XUV pump first) within a several ns time window. Another delay stage introduced before the cross-correlator setup compensates for the optical pathway difference between both experiments. The delay time zero corresponds to the coincidence of the peaks of the XUV and laser pulses and is determined subsequently from fitting to the experimental data (see [START_REF] Krikunova | Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser New[END_REF] for details).

As a reference for an assessment of the achieved jitter compensation a relaxation process in xenon atoms following a 4d core excitation within the 4d → f giant resonance was selected. The relaxation pathways of the created vacancy via single (A1) and double (A2) Auger decay involving several intermediate states were directly observed by detecting 4d and two Auger electrons in coincidence [START_REF] Penet | Multielectron spectroscopy: The xenon 4d hole double Auger decay[END_REF]. Time constants for the Auger decay of τ A1 = 6.0 ± 0.7 fs and τ A2 = 30.8 ± 1.4 fs leading to Xe 2+ and Xe 3+ final states with two or three vacancies in the outer 5p shell, respectively, are known with high precision from time-domain studies [START_REF] Uiberacker | Attosecond real-time observation of electron tunnelling in atoms[END_REF] and energy-resolved measurements [START_REF] Jurvansuu | Inherent lifetime widths of Ar 2p -1 , Kr 3d -1 , Xe 3d -1 , and Xe 4d -1 states[END_REF]. Averaged Xe 3+ and Xe 4+ ion yields (grey opened circles) as a function of the time delay are plotted in Figure 10(a) and (b) together with a simplified scheme of energy levels and transitions (c) and (d). The data set was collected by scanning an optical delay line with 10 second acquisition time (50 shots) per 200 fs step, without any compensation for the XUV arrival time jitter. The shape of the transient Xe 3+ and Xe 4+ profiles manifests the fast rising edge characterized by the lifetime of the 4d hole (τ A1 ). The laser-enhanced Xe 3+ ion yield shows no decrease for positive delays in correspondence with a long life time of the intermediate states as compared to the experimental time scale (Figure 10(c) and [START_REF] Uiberacker | Attosecond real-time observation of electron tunnelling in atoms[END_REF]). The electron dynamics leading to the transient Xe 4+ ion yield is governed by short-lived intermediate excited states (Figure 10(d) and τ A2 in [START_REF] Uiberacker | Attosecond real-time observation of electron tunnelling in atoms[END_REF]) leading to a fast decrease for positive time delays. Transient changes of the ion yields are observed within a noticeably broad time window (Figure 10 The power of the introduced data tagging approach is especially underlined by the observation of a new electron relaxation pathway manifested in the Xe 5+ transient ion yield. The threshold for the Xe 5+ formation is about 168 eV, i.e. significantly higher than the 92 eV exciting photon energy. Based on the nonlinear behaviour of this previously not observed decay channel it was attributed to a double photoionisation of the 4d core electron shell [START_REF] Krikunova | Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser New[END_REF]. Figure 11 shows energy levels and transitions involved in the formation of the transient Xe 5+ ion yield. A double ionisation of the 4d shell by a two-photon absorption leads to a 4d -2 → 5p -4 decay, while additional energy introduced by an intense laser pulse activates a 4d -2 5p -1 → 5p -5 channel. The results indicate a particular role of the 4d → f giant resonance in increasing the double ionisation probability in Xe compared to other rare gases in accordance with [START_REF] Richter | Extreme ultraviolet laser excites atomic giant resonance[END_REF]. An observation of multi-photon processes is generally hampered by high intensity fluctuations.

Summary and Conclusion

Tagging proves to be a powerful concept for dealing with the intrinsically unstable character of XUV or X-ray pulses from FELs operated in a SASE mode. For the particularly demanding measurement of the temporal properties of individual pulses, solutions have now been found, where the time-information, being inaccessible to even the fastest direct detectors, is encoded in a spectral or spatial coordinate, respectively, thus making it available to slow detection schemes.

A light-field streak-camera, driven by a strong electromagnetic THz field demonstrates a temporal resolution better than 10 fs, significantly beyond the capabilities of conventional X-ray streak cameras. The deduced mean duration of (35 ± 9) fs coincides well with the value of (29 ± 5) fs obtained with an XUV/XUV autocorrelation, which has successfully been demonstrated at FLASH in an averaging mode [START_REF] Mitzner | Direct autocorrelation of soft-x-ray free-electron-laser pulses by time-resolved two-photon double ionisation of He[END_REF]. In addition to the single-shot capability, as a matter of principle the THz/XUV cross-correlation mechanism can yield more detailed information than an autocorrelation. The streaking data allow an extraction of spectral phase information to an extent that is determined by the number of simultaneously acquired non-redundant samples of a single streaking event and the associated signal-to-noise-ratio. In the current implementation, observation in two orthogonal directions suffices to asses the complexity of a pulse, to allocate a temporal width to the ones with a compact shape, and to resolve the sub-structure of pulses with a low complexity.

The jitter issue between XUV pulses from FLASH and external laser pulses has been successfully attacked by realization of a single-shot XUV/laser cross-correlator. Compared to a temporal correlation between the laser and the accelerated electron bunch [START_REF] Azima | Time-resolved pump-probe experiments beyond the jitter limitations at FLASH[END_REF], the optical/optical cross-correlation removes jitter sources associated with the SASE process itself and the transport of the laser beam to the experimental endstation. A direct comparison [START_REF] Krikunova | Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser New[END_REF] suggests that the all-optical method yields superior results if a resolution below 100 fs is desired. The degree of improvement obtained by removing jitter upon tagging individual pulses with measured delays was tested in XUVpump/laser-probe experiments on an atomic sample. A remarkable reduction of the rise time of averaged data from ∼ 400 fs to ∼ 100 fs could be demonstrated. This figure is currently limited by the laser pulse duration; as soon as shorter laser pulses become available further advance is to be expected to a level where the temporal resolution approaches the XUV pulse duration and the information of the time structure of each FLASH pulse extracted from the single-shot THz-field-driven X-ray streak camera will become important.

So far the methods discussed in this article, have been tested using pulses in the soft-X-ray range below 100 eV photon energy. Extension to harder X-rays is straightforward in the case of THz streaking. As the streaking amplitude ∆W scales with √ W 0 (Equation 1) and the FEL bandwidth σ W with W 0 , their ratio ∆W/σ W scales with 1/ √ W 0 . For preserving contrast the streaking field strength E THz must be increased according to √ W 0 , i.e. the square root of the photon energy. This weak scaling should facilitate the analysis of single X-ray pulses extending well into the keV range. An assessment of the scalability of jitter compensation towards harder radiation is more demanding because it relies on the specific properties of the used surface material and on the corresponding physical processes evolving after excitation with X-rays. As a general rule, the material should be chosen to contain elements that exhibit reasonable absorption at the desired wavelength, i.e. above an absorption edge.

Combining the now available techniques for the characterization of varying temporal, spectral, energetic, and spatial properties of individual X-ray pulses from SASE FELs many of the constraints connected with those fluctuations can be removed or relaxed. Still, as studies of nonlinear processes or more generally of dynamics that depends critically on the evolution of the instantaneous intensity, spectrum, and phase will require the optimal definition of the exciting radiation, this passive approach cannot replace further efforts to actively improve the control over the pulsed radiation delivered by linear-accelerator-based X-ray sources.

Figure 1 .

 1 Figure 1. Schematic of streak camera principles. (a) Conventional streak camera: The XUV pulses hit a photocathode where electron bunches with temporal structures identical to those of the XUV pulses are created. The electrons are accelerated and deflected by a fast varying electrical field. The deflection depends on the time the electron enters the deflector. Thus, the temporal structure of the XUV pulses is mapped onto spatial positions of electrons on a screen. (b) Light-field driven streak camera: The XUV pulses hit a gas target, the resulting photoelectrons are accelerated by the electric field of a superimposed infrared or THz light pulse. The momentum change of the electrons depends on the state of the field at the ionisation time τ . Thus, the temporal structure of the XUV pulses is mapped onto kinetic energy modulations of photoelectrons.

Figure 2 .

 2 Figure 2. Sampled THz vector potential. Series of kinetic energy spectra of 5p electrons detached from xenon atoms by a 13.5 nm XUV pulse in the presence of an intense pulsed THz field. The energy shift of the electrons versus the XUV/THz delay directly represents the vector potential A THz . The less intense electron signal at lower energies stems from stronger bound xenon 5s electrons. The spectra shown in (a) were measured without any THz filters. For the measurement of the spectra in (b) a bandpass filter was used to narrow and smooth the THz spectrum.

Figure 3 .Figure 4 .

 34 Figure 3. Reconstructed pulse durations. FWHM durations of 960 consecutive 13.5 nm FLASH pulses reconstructed by an analysis considering a Gaussian pulse profile.

Figure 5 .

 5 Figure 5. Different cross-correlation schemes (a) Unfocused XUV and focussed laser pulses are superimposed in a krypton gas target. The electrons in spectral sidebands from the region of temporal and spatial overlap are imaged by an electron lens system with a high-pass energy filter. (b) An unfocussed laser pulse is reflected from a solid substrate onto a CCD camera. The focused XUV pulse is absorbed in the substrate and changes the reflectivity for the laser pulse. The boundary of reflectivity change is used as arrival-time marker for the XUV pulse.

Figure 6 .

 6 Figure 6. Averaged electron cross-correlation signal in a krypton gas target (a) Sideband-photoelectron image at temporal overlap (fixed delay). XUV and laser paths are marked with arrows. The image is generated by summation over 1200 macropulses with 30 micropulses and is background subtracted. (b) Integrated sideband-photoelectron signal plotted as a function of XUV/laser delay. Spectral sideband signal appears only at temporal overlap of both pulses. Every data point corresponds to one macropulse with 30 micropulses. .

Figure 7 .

 7 Figure 7. Single-shot arrival time measurement by reflectivity change on Si 3 N 4 (a) Areas where the XUV pulse hits the substrate before the laser pulse is reflected appear brighter. For the time axis t (shown on the left) the zero point is set arbitrarily to the top of the image. (b) Horizontally average signal of pixels within the ROI marked in (a). One pixel corresponds to 17 fs. The inflection point marked as dashed line is used as an XUV arrival-time marker.

Figure 8 .

 8 Figure 8. Series of XUV/laser single-shot cross-correlation images (a) XUV arrival times with respect to the laser for 18 arbitrarily chosen single-shots. (b) Arrival time distribution for the complete data-set of 515 shots with 17 fs binning.

Figure 9 .

 9 Figure 9. Scheme of the experimental geometry (top view) Pump-probe experiment installed before the XUV/laser cross-correlator setup shares the same XUV beam. The 800 nm optical laser pulses are frequency doubled and overlapped at a small angle in a Xe gas target of the pump-probe experiment. A TOF ion massto-charge spectrometer detects ions created in the interaction region. The crosscorrelator is operated with the remaining spatial frequency filtered 800 nm light. A delay stage before the setup compensates for the optical pathway difference between both experiments.

Time-diagnostics for improved dynamics experiments at XUV FELs 15 -

 15 Figure X+2

  (a) and (b)). The extracted time resolution ∆t ≈ 450 fs is clearly dominated by the XUV arrival time jitter ∆t jitter estimated from the histogram of XUV arrival times measured with the XUV/laser cross-correlator (Figure 8(b)). Within a second run (black closed circles in Figure 10(a) and (b)) the data set was collected at a fixed delay stage position and simultaneously tagged with XUV arrival time information from the XUV/laser cross-correlator. The data were accordingly arranged in 30 fs time bins. The time window of the observed transient changes in the Xe 3+ and Xe 4+ ion yields is considerably contracted (Figure 10(a) and (b)), demonstrating a substantial improvement in the temporal resolution of the transient signal. The degree of improvement is further corroborated by the extracted time resolution ∆t = 125±5 fs (standard deviation) in good correspondence with the expected 125 fs by assuming a complete compensation of the arrival time jitter ∆t jitter . The established temporal resolution is now dominated by the optical laser pulse duration. Using shorter laser pulses the precision of the introduced tagging technique carries the potential of pushing the temporal resolution towards the intrinsic dictated by the XUV pulse duration.4.2. Multi-parameter data tagging revealing nonlinear dynamics of the XUV-excited Xe 4d -2 core holes

  Figure 11(b) demonstrates the sensitivity of the Xe 5+ ion yield to the XUV pulse energy. No time dependence of the Xe 5+ signal (grey closed circles) can be resolved below 1 µJ of XUV pulse energy. The relatively low statistics at negative time delays is explained by the difficulty to determine an inflection point at the top of the time window of the cross-correlator at low XUV fluencies. Black open circles in Figure11are filtered from the same experimental run according to the XUV intensities higher than 3 µJ. Transient changes in the Xe 5+ ion yield similar to the Xe 4+ ion yield are now clearly resolved.

Figure 11 .

 11 Figure 11. (a) Transitions in Xe excited by XUV pulses (adopted from [34]) and probed by visible laser leading to a transient formation of Xe 5+ signal. (b) Single-shot Xe 5+ ion yield as a function of the time delay: Data (open circles) measured at XUV energies higher than 3 µJ with corresponding fit (solid line) and data obtained at pulse energies below 1 µJ (closed circles). The ion yield was scaled in order to superimpose both data sets. Data are tagged according to the XUV arrival time measured by the XUV/laser cross-correlator.

Acknowledgments

The work presented in this review has been realised part in the framework of the "Forschungsschwerpunkt 309" of the German Ministry for Education and Science (Project 05KS4PB1/0). Further funding by the European Union (EU-JRA2), the Graduate School GrK1355, the DFG Sonderforschungsbereich SFB 668, the HGF virtual institute "accelarator metrology" as well as from the Joachim-Hertz-Stiftung is greatfully acknowledged. Finally, the authors would like to thank all the people of the FLASH team, who made this work possible at all.