The temporal structure and phase evolutions of a high-gain, self-amplified free-electron laser are measured, including single-shot analysis and statistics over an ensemble of many shots. Excellent agreement with the theory of free-electron laser (FEL) and photon statistics is found. This is an important step towards understanding and controlling the radiation in such FEL pulses.
Next generation x-ray sources require very high-brightness electron beams that are typically at or beyond the present state-of-the-art, and thus place stringent and demanding requirements upon the electron injector parameters. No one electron source concept is suitable for all the diverse applications envisaged, which have operating characteristics ranging from high-average-current, quasi-CW, to high-peak-current, single-pulse electron beams. Advanced Energy Systems, in collaboration with various partners, is developing several electron injector concepts for these x-ray source applications. The performance and design characteristics of five specific RF injectors, spanning "L" to "X"-band, normal-conducting to superconducting, and low repetition rate to CW, which are presently in various stages of design, construction or testing, is described. We also discuss the status and schedule of each with respect to testing.
Most proposed linac-based light sources, such as single-pass free-electron lasers and energy-recovery-linacs, require very high-brightness electron beams in order to achieve their design performance. These beam requirements must be achieved not on an occasional basis, but rather must be met by every bunch produced by the source over extended periods of time. It is widely assumed that the beam source will be a photocathode electron gun; the selection of accelerator technique (e.g., dc or rf) for the gun is more dependent on the application.
The current state of the art of electron beam production is adequate but not ideal for the first generation of linac-based light sources, such as the Linac Coherent Light Source [ ] (LCLS) x-ray free-electron laser (X-FEL). For the next generation of linac-based light sources, an order of magnitude reduction in the transverse electron beam emittance is required to significantly reduce the cost of the facility. This is beyond the present state of the art, given the other beam properties that must be maintained. The requirements for current and future linac-based light source beam sources are presented here, along with a review of the present state of the art. A discussion of potential paths towards meeting future needs is presented at the conclusion.
Construction of a single-pass free-electron laser (FEL) based on the self-amplified spontaneous emission (SASE) mode of operation is nearing completion at the Advanced Photon Source (APS) with initial experiments imminent. The APS SASE FEL is a proof-of-principle fourth-generation light source. As of January 1999 the undulator hall, end-station building, necessary transfer lines, electron and optical diagnostics, injectors, and initial undulators have been constructed and, with the exception of the undulators, installed. All preliminary code development and simulations have also been completed. The undulator hall is now ready to accept first beam for characterization of the output radiation. It is the project goal to push towards full FEL saturation, initially in the visible, but ultimately to UV and VUV, wavelengths.
Preliminary calculations using the computer code PARMELA indicate that it is possible to achieve peak currents on the order of 1 kA using a thermionic-cathode rf gun and ballistic bunch compression. In contrast to traditional magnetic bunching schemes, ballistic bunch compression uses a series of rf cavities to modify the energy profile of the beam and properly chosen drifts to allow the bunching to occur naturally. The method, suitably modified, should also be directly applicable to photo injector rf guns. Present work is focusing on simultaneously compressing the bunch while reducing the emittance of the electron beam. At present, the calculated normalized rms emittance is in the neighborhood of 6.8 (pi) mrad with apeak current of 0.88 kA, and a peak bunch charge of 0.28 nC from a thermionic-cathode gun.
The far-infrared portion of the electromagnetic spectrum has remained largely unexplored, partially as a consequence of a lack of suitable sources. The Stanford Far-Infrared Free- Electron Laser has recently demonstrated lasing at 86 micrometers . With component costs, space, shielding requirements, and complexity reduced by up to an order of magnitude from conventional designs, this places the far-infrared free-electron laser within the reach of an individual researcher or a research department. Results from the most recent experimental run are presented. The accelerator section is currently undergoing redesign to make it more suitable for free-electron laser use.
A possible approach to high efficiency FEL operation is to combine a microwave linear accelerator and magnetic wiggler into a single structure. As the electrons lose energy to the radiation at the FEL oscillation wavelength (e.g. 10 micrometers ), energy is replaced by the microwave linac. The electron beam acts as a catalyst for the conversion of microwave power to infrared power. Several advantages to the accelerator/wiggler are: it is possible to obtain high conversion efficiency in a short length; small- signal gain reduction can be avoided; power extraction may be increased by increasing length; there is little detrapping; and electron beam energy out of the wiggler is relatively monochromatic, permitting efficient energy recovery. The expected performance of the high efficiency FEL will be calculated by computer simulation based on parameters measured on a full scale, six period model. The gain and efficiency will be estimated with and without the presence of the accelerating microwave field. Simulation results predict an efficiency greater than 15% in a 135 m long accelerator/wiggler. A fast (< 1 microsecond(s) time constant) variable attenuator will be used to modulate the microwave field in time so that small signal gain reduction is avoided. The optimum value of microwave field as a function of time will be calculated and the sensitivity of the gain to the microwave field will be discussed.