R. Carr
The FATE project is a free electron laser (FEL) research effort designed to support the Linear Coherent Light Source (LCLS). When the design wavelength of radiation from a free electron laser is decreased into the x-ray regime, at least two issues must be addressed. The first is that a laser oscillator becomes infeasible because there are no efficient mirrors to form an optical cavity. Therefore, the laser must be a single-pass amplifier. The second is that it is not possible to pre-bunch the electron beam with another sort of laser, as one can do at longer wavelengths. Electron beam bunching is required for FEL action, but for an x-ray FEL there is no `seed' radiation source; its electromagnetic wave must start from spontaneous emission or `noise'. The FATE experiment is designed to demonstrate FEL exponential gain starting from self-amplified spontaneous emission (SASE). We are hoping, in addition, to observe superradiance, where the phenomenon of slippage leads to a clean-up of the noise and a square law dependence of FEL power on electron current. These effects are predicted in theory, but have not been studied experimentally, and a firm understanding of these effects is essential for the development of the LCLS. FATE will operate at infrared (1-3 micron) wavelengths, however, because these effects can be seen at these wavelengths most conveniently given the 100 MeV limitation on the energy of our injector linac.
FATE is a joint effort led by Max Cornacchia and Rodolfo Bonifacio. Rodolfo Bonifacio and Lucia DeSalvo (University of Milan and INFN) have contributed most of the development of the FEL theory. The University of Milan has also contributed to the funding of the FATE experiment. The concept is to use the SSRL injector s-band linac as a 100 MeV electron source that will send electrons though a 6-meter-long, 30 mm period FEL undulator built near the booster synchrotron. The electrons will come from the existing thermionic RF electron gun and be formed into a string of 3-5 pulses by the existing alpha magnet and chopper.
With computational help from Bill Fawley (LBL) and Ted Scharlemann (LLNL), Heinz-Dieter Nuhn and Soon-Kwon Nam (Kangwon National University, Korea) have performed extensive computer modeling of FATE, mostly with Bill Fawley's GINGER code on a Cray computer at LLNL. The essential results of this modeling are the output power levels that can be achieved from electron pulses with various currents and emittance; we believe that SASE, exponential gain, and superradiance from FATE can be observed from 150 A peak current, 20 mm-mrad normalized emittance pulses.
Bob Hettel and Heinz-Dieter Nuhn have devoted an intense effort to study the peak current of the injector linac electron pulses and the emittance of the beam. High peak current (charge/pulse length) and low emittance are crucial for FEL action. Values of these parameters observed on Helmut Wiedemann's nominally identical SUNSHINE 30 MeV accelerator on the Stanford campus lead us to believe that pulses of adequate peak current and emittance can be obtained at SSRL, though we have only observed 30 A, 50 mm-mrad pulses so far. These observations, however, have been tremendously instructive, and most of the effort has been devoted to learning diagnostic and control techniques. Heinz-Dieter Nuhn has also been designing the electron beam transport optics between the injector linac and the undulator. The beam line will include an isochronous, achromatic bend and will be designed to match the electron beam size and divergence to FATE requirements.
Roger Carr and Steve Lidia (UC Davis) are working on the design of the FATE undulator, presently conceived as a fixed-gap pure permanent magnet device built in segments. An FEL undulator must have tighter control of magnetic errors than a conventional storage ring x-ray source. A further complication arises because the FATE beam must not be allowed to grow in size; either the undulator must itself focus the beam both horizontally and vertically or a separate focusing system must be designed. Yen-Chieh Huang (Stanford) has contributed to the undulator specifications and has also been researching possible FATE detectors. We will need to verify that the FEL radiation has enough power to indicate amplifier action. We also wish to verify that the `noise' that started the FEL action has been cleaned up in the final output and shows a characteristic superradiant spike. Measuring power should be easy, but verifying clean-up appears very challenging. In a recent exciting development, it appears that Helmut Wiedemann and his students may have already observed far-infrared SASE using just a few period undulator on SUNSHINE.
Bob Hettel and Jim Weaver are implementing modifications and upgrades to the SSRL electron gun and beam transport system and diagnostic equipment. With these improvements, we are optimistic that the peak current and emittance issues will be resolved with experiments to be conducted at the beginning of the 1995-96 experimental run. We will attempt to resolve the undulator focusing and radiation detection issues in the same time period. When these problems are solved, construction, which is already partially funded, can begin. The construction layout, with its many options and cost constraints, has been studied by Richard Boyce, who leads the engineering and construction efforts.
