119530771
Acceleration in the linear non-scaling fixed-field alternating-gradient accelerator EMMA
In a fixed-field alternating-gradient (FFAG) accelerator, eliminating pulsed magnet operation permits rapid acceleration to synchrotron energies, but with a much higher beam-pulse repetition rate. Conceived in the 1950s, FFAGs are enjoying renewed interest, fuelled by the need to rapidly accelerate unstable muons for future high-energy physics colliders. Until now a ‘scaling’ principle has been applied to avoid beam blow-up and loss. Removing this restriction produces a new breed of FFAG, a non-scaling variant, allowing powerful advances in machine characteristics. We report on the first non-scaling FFAG, in which orbits are compacted to within 10 mm in radius over an electron momentum range of 12–18 MeV/c. In this strictly linear-gradient FFAG, unstable beam regions are crossed, but acceleration via a novel serpentine channel is so rapid that no significant beam disruption is observed. This result has significant implications for future particle accelerators, particularly muon and high-intensity proton accelerators.
55801859
PULSE - A High-Repetition-Rate Linac Driver for X-ray FELs
We describe a staged concept for a linac-based freeelectron laser providing coherent tuneable VUV and soft X-ray output with pulse lengths down to 10 fs or less. Use of 1.3 GHz cryomodules operated with CW RF will allow flexible pulse patterns at potentially very high repetition rates up to 1 MHz. We consider the options for FELs that may be included in a facility of this type.
114544071
DESIGN OF POST LINAC BEAM TRANSPORT FOR THE UK NEW LIGHT SOURCE PROJECT
The design of free electron laser (FEL) driver needs careful beam transport design to pass very short bunches through the switchyard/spreader to switch the beam to different FEL lines. The spreader design which allows flexibility in operation has been adapted following the LBNL design [1]. In order to measure the slice properties of the bunches two beam diagnostics lines are proposed, a straight one for beam commissioning purposes and a branch of the spreader similar to the FEL lines to measure the adverse effects that may arise due to passing the short bunches through the kicker and septum magnets. As a part of machine protection, post linac collimation system collimates the halo particles in transverse and energy planes. The design of the collimation, beam spreader and beam diagnostics lines is discussed.
119203536
A Modular Path Length Corrector for Recirculating Linacs
Abstract We present a novel modular magnetic system that can introduce a large and continuously variable path length difference without simultaneous variation of the longitudinal dispersion. This is achieved by using a combination of an electrically adjustable magnetic chicane and a mechanically adjustable focusing chicane. We describe how such a system may be made either isochronous or with a given longitudinal dispersion, and show that the nonlinear terms in such a system are relatively small.
55291027
DESIGN OF THE RECIRCULATING LINAC OPTION FOR THE UK NEW LIGHT SOURCE
We present progress in the design of the recirculating linac option for the UK New Light Source (NLS). Improvements in all accelerator sections have been made such that the output meets the required specifications to drive the seeded NLS free-electron lasers (FELs). Full start-to-end simulations are presented.
112783302
Design of the MAX IV Ring Injector and SPF/FEL Driver
16217538
Three routes to jet collimation by the Balbus–Hawley magnetorotational instability
Three completely different lines of work have recently led to the conclusion that the magnetorotational instability (MRI) may create a hoop stress that collimates jets. One argument is based upon consideration that magnetohydrodynamic (MHD) turbulence, in general, and turbulence driven by the MRI, in particular, is more nearly viscoelastic than it is viscous. Another argument is based upon the dispersion relation for the MRI in the context of 1 D simulations of core collapse. Yet a third argument rests in the results of direct numerical MHD simulations of collapsars and thick accretion flows. I elaborate on my previous work regarding the first argument above and I briefly discuss how these three sets of results are all related. I also discuss the different roles played by the magnetic tension and the magnetic pressure within the context of this work. I point out that this leads to consideration of the normal stress difference between the hoop stress and the radial stress, in preference to a focus on just the hoop stress itself. Additionally, I argue that simulations of thick accretion flows and collapsars are not self-consistent if they include a phenomenological model for an MRI-induced viscous stress but disregard these other MRI-induced stress components. I comment briefly on the RHESSI observation of polarization in the gamma-ray burst GRB0212206.1 argue that this polarization is consistent with a tangled field, and does not require a large-scale organized field. Finally, I suggest that the role of magnetic fields in creating jets, as described here, should be understood not to work within the confines of magnetocentrifugal models of jets, but rather as an alternative to them.
7486677
The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI
The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.
2857756
Comment regarding the functional form of the Schmidt law
Abstract Star formation rates on the galactic scale are described phenomenologically by two distinct relationships, as emphasized recently by Elmegreen [Elmegreen, B.G., 2002. ApJ, 577, 206. astro-ph/0207114 .]. The first of these is the Schmidt law, which is a power-law relation between the star formation rate SFR and the column density Σ. The other relationship is that there is a cutoff in the gas density below which star formation shuts off. The purpose of this paper is to argue that: (1) these two relationships can be accommodated by a single functional form of the Schmidt law, (2) this functional form is motivated by the hypothesis that star formation is a critical phenomenon, and that as a corollary, (3) the existence of a sharp cutoff may thus be an emergent property of galaxies, as was argued by Seiden [Seiden, P.E.,1983. ApJ, 266, 555.], as opposed to the classical view that this cutoff is due to an instability criterion.
14148736
Viscoelastic Analogy for the Acceleration and Collimation of Astrophysical Jets
Jets are ubiquitous in astronomy. It has been conjectured that the existence of jets is intimately connected with the spin of the central object and the viscous angular momentum transport of the inner disk. Bipolar jet-like structures propelled by the viscous torque on a spinning central object are also known in a completely different context, namely the flow in the laboratory of a viscoelastic fluid. On the basis of an analogy of the tangled magnetic field lines of magnetohydrodynamic (MHD) turbulence to the tangled polymers of viscoelastic polymer solutions, we propose a viscoelastic description of the dynamics of highly turbulent conductive fluid. We argue that the same mechanism that forms jets in viscoelastic fluids in the laboratory may be responsible for collimating and powering astrophysical jets by the angular momentum of the central object.