SHORT-WAVELENGTH FELS USING THE SLAC LINAC

Recent technological developments have opened the possibility to const ruct a device which we call a linac coherent light source (LCLS) (C. P ellegrini et al., Nucl. Instr. and Meth. A 331 (1993) 223; H. Winick e t al., Proc. IEEE 1993 Particle Accelerator Conf., Washington, DC, May 1993; C. Pellegrini, Nucl. Instr. and Meth. A 341 (1994) 326; J. Seem an, SPIE Meet. on Electron Beam Sources of High Brightness Radiation, San Diego, CA, July 1993 [1-4]); it would be a fourth-generation light source, with brightness, coherence, and peak power far exceeding othe r sources. Operating on the principle of the free electron laser (FEL) , the LCLS would extend the range of FEL operation to much shorter wav elength than the 240 nm that has so far been reached. We report the re sults of studies of the use of the SLAC linac to drive an LCLS at wave lengths from about 3 to 100 nm initially and possibly even shorter wav elengths in the future. Lasing would be achieved in a single pass of a low emittance, high peak current, high-energy electron beam through a long undulator. Most present FELs use an optical cavity to build up t he intensity of the light to achieve lasing action in a low-gain oscil lator configuration. By eliminating the optical cavity, which is diffi cult to make at short wavelengths, laser action can be extended to sho rter wavelengths by self-amplified-spontaneous-emission (SASE), or by harmonic generation from a longer wavelength seed laser. Short wavelen gth, single pass lasers have been extensively studied at several labor atories and at recent workshops (M. Cornacchia and H. Winick (eds.), S LAC Report 92/02; 1. Ben-Zvi and H. Winick (eds.), BNL report 49651 [5 ,6]). The required low-emittance electron beam can be achieved with re cently-developed rf photocathode electron guns (B.E. Carlsten, Nucl. I nstr. and Meth. A 285 (1989) 313; J. Rosenzweig and L. Serafini, Proc. IEEE 1993 Particle Accelerator Conf., Washington, DC, 1993 [7,8]). Th e peak current is increased by about an order of magnitude by compress ing the bunch to a length of about 0.2 ps (rms). Techniques for beam t ransport, acceleration, and compression without emittance dilution hav e been developed at SLAC as part of the linear-collider project (J. Se eman, Advances of Accelerator Physics and Technologies, ed. H. Schoppe r (World Scientific, Singapore, 1993 [9]). The undulator length requir ed to saturate the laser varies from about 15 m for a 100 nm FEL to ab out 60 m at 3 nm. Initial experiments, at wavelengths down to about 50 nm are planned using the 25-m-long Paladin undulator now located at L LNL. In a proposed future LCLS R&D facility the short wavelength light pulses are distributed to multiple end stations using grazing-inciden ce mirrors. About 10(14) photons per pulse can be produced at a 120 Hz rate, corresponding to average brightness levels of about 10(21) phot ons/s/mm2/mrad2 Within 0.1% BW and peak brightness levels of about 10( 31) photons/S/MM2/Mrad2 within 0.1% BW. Peak power levels are several hundred megawatts to several gigawatts. Electron energies required ran ge from about 500 MeV for the 100 nm FEL to about 7 GeV for 3 nm.