The Linac Coherent Light Source (LCLS)

- M. Cornacchia

SSRL is engaged in a design study of a Free Electron Laser operating in the x-ray region. The study is a collaboration that involves several divisions of SLAC and other scientific institutions, and is planned for completion in August, 1997.

The LCLS is designed to be a research facility for the production and utilization of high power coherent radiation in the x-ray region, covering a wavelength range of 1.5 Å to 15 Å. Operating on the principle of the single pass free electron laser (SASE: Self-Amplified Spontaneous Emission), it would create a photon beam of unprecedented brightness, coherence, and peak beam power, far surpassing anything available in third generation sources today. Design goals are a brightness of 10³⁴ photons/(s - mm² - mrad² - 0.1% bandwidth) and a peak power of 15 to 30 GW.

The LCLS would utilize the last one-third of the SLAC linac to accelerate electrons to a range of energies between 5 and 15 Gev, for photon wavelengths of 1.5 Å to 15 Å respectively. Because of the high beam quality requirements, an rf photocathode gun would be the source of the electrons. With one nanocoulomb per bunch, and a two-stage bunch compressor, a peak current of 5,000 Å in a bunch 100 femtosecond long (rms length) appears to be achievable. Assuming a normalized emittance from the gun and linac of 1.5 pi mm mrad and a rms momentum spread of 2x10^-4, theory and simulations show that the radiation would be amplified from noise and reach saturation level in an undulator length of about 100 m.

The study is aimed at proving that the physical and technological bases for a successful experiment exist. Several technical challenges are crucial to achieving the performance goals. The understanding of the Self-Amplified-Spontaneous-Emission mechanism, radiation start-up from noise, and gain guiding is based on theory and simulations. Experiments are being planned (at Brookhaven National Laboratory, University of California at Los Angeles and Los Alamos National Laboratory) that will enhance confidence in the physics model. These experiments will provide SASE radiation in the micron wavelength region; the extrapolation to 1.5 Å is one of the main research tasks of the LCLS. Other important technical factors include the generation and preservation of emittance and momentum spread during acceleration. SLAC, with its experience of several years of SLC running and with the large R&D effort devoted to problems of similar nature in the context of the Next Linear Collider studies, is in an ideal situation to spearhead the research on ways to create and achieve high energy electron beams of high brightness.

The design of the undulator poses considerable technical challenges. The present design envisages a hybrid permanent magnet design that incorporates strong focusing elements. Its length, 100 m, the tight constructional tolerances, and the severe demands on the electron orbit trajectory will require advanced alignment techniques, as well as a refined beam monitoring system. Alignment and beam position monitoring must guarantee that the orbit does not deviate more then a few microns from the ideal trajectory. Here again, the LCLS will benefit from the advanced research carried out on alignment and beam monitoring at the SLC and the Final Focus Test Beam.

The photon optics also face important challenges, as well as the opportunity to improve the technology. For the preservation of transverse coherence, high quality mirrors will be a necessity. Crystal optics will be used for wavelengths below 6 Å. With a peak power six orders of magnitude above proven techniques, the study is exploring ways to reduce the possibility of damage to optical elements. Grazing incidence optics and photon beam spreading (by waveform manipulation or with a long drift space) are solutions that we are exploring. Ultimately, of course, experiments will provide the understanding to advance the technology, and this is one of the research goals of the LCLS.

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December 2, 1996