A 3D model of the 4GLS VUV-FEL conceptual design including improved modelling of the optical cavity
The Conceptual Design Report for the 4th Generation Light Source (4GLS) at Daresbury Laboratory in the UK was published in Spring 2006. The proposal includes a low-Q cavity (also called a regenerative amplifier) FEL to generate variably-polarised, temporally-coherent radiation in the photon energy range 3-10eV. A new simulation code has been developed that incorporates the 3D FEL code Genesis 1.3 and which simulates in 3D the optical components and radiation propagation within the non-amplifying sections of an optical cavity*. This code is used to estimate the optimum low-Q cavity design and characterise the output from the 4GLS VUV-FEL.
REGENERATIVE AMPLIFIER FEL
We present experimental results of saturation of the Regenerative Amplifier Free-Electron Laser (RAFEL) at Los Alamos. RAFEL is a multi-pass self-seeded selfamplified spontaneous emission (SASE) FEL that uses a low-Q optical cavity to re-inject a small fraction (less than 10%) of the optical power into a high-gain undulator. The low-Q optical cavity consists of two imaging paraboloids and two annular mirrors that out-couple more than half of the generated radiation. The output radiation achieves saturation within a few passes through the undulator and the saturated optical power exceeds the single-pass SASE power by six orders of magnitude. Under normal operating conditions, the Regenerative Amplifier FEL produces between 1 and 2 J of infrared energy in each macropulse. The maximum output energy is 2.1 J over a 15 μs macropulse containing 1600 micropulses. We deduce each micropulse has an optical energy of 1.3 mJ over a nominal 16 ps pulse length. The optically measured extraction efficiency is 1.8% whereas the electron beam energy deceleration as measured with the electron spectrometer is 3.5%, indicating that only half of the generated optical power is detected. In addition, the optical spectra exhibit spectral spikes characteristic of the high-gain SASE FEL. 1 REGENERATIVE AMPLIFIER FEL The SASE FEL has evolved from a laboratory curiosity to the technology of choice for the fourth generation light source [1]. In a SASE FEL, a high-current electron beam traversing a long undulator first emits radiation and then develops density modulations that grow with distance. The density modulation amplifies the generated radiation exponentially with distance to a very high power level in a single pass through the undulator. Large single-pass gains have been demonstrated in the mm-wave and infrared regions of the spectrum. Recent SASE experiments have produced high gains in the visible and the vacuum ultraviolet. Future plans are to achieve saturation by increasing the undulator length, and to shift the SASE wavelength to the x-ray regions. An alternative approach to saturation without substantially increasing the undulator length is to use a low-Q optical cavity to re-inject a small fraction of the optical power exiting the undulator to restart the amplification process. Starting the optical amplification from coherent optical power instead of the spontaneous noise allows the FEL to reach saturation in a few passes. Using a low-Q optical cavity with a large out-coupling allows most of the optical power to exit the cavity, thereby reducing the risk of optical damage and increasing the amount of light that exits the cavity as useful power. In the one-dimensional treatment of high-gain FEL, the optical power as a function of undulator length is given by
Is match ethics an oxymoron?
BACKGROUND
Graduate medical education has undergone economic and structural changes-changes that have placed increasing pressure on medical students and programs to match effectively. This study documents the experiences, perceptions, and ethical dilemmas of medical students with the 1998 match process.


METHODS
Surveyed were 437 senior students from three medical schools. Students were questioned about: interviewing practices, communication with programs, commitments made, ethical dilemmas faced, and the match process itself. Data were analyzed using Student's t test, the chi-square test, and descriptive statistics.


RESULTS
In all, 314 (72%) students responded. Programs expect postinterview communication from students (57%). Students perceive that programs are making "informal" commitments (43%), lying to them (33%), and encouraging their unethical behavior in order to match (21%). Ethical dilemmas are related to requests for informal commitments.


CONCLUSIONS
The NRMP's ruling that denounces prematch commitments is being broken by students and programs alike, resulting in the promotion of unprofessional behavior and gamesmanship.