FEMTOSECOND ELECTRON AND X-RAY GENERATION BY LASER AND PLASMA-BASED SOURCES

Abstract

FEMTOSECOND ELECTRON AND X-RAY GENERATION BY LASER AND PLASMA-BASED SOURCES E. ESAREY AND W.P. LEEMANS Center for Beam Physics, Accelerator and Fusion Research Division Ernest Orlando Lawrence Berkeley National Laboratory One Cyclotron Road, University of California, Berkeley CA 94720 The generation of ultra-short x-rays by Thomson scattering intense laser pulses from electron beams is discussed, including recent experimental results and meth- ods for enhancing the x-ray flux. A high flux of x-rays in a femtosecond pulse requires the generation of femtosecond electron bunches and a head-on Thomson scattering geometry. The generation of ultrashort electron bunches in a plasma- based accelerator with an injection technique that uses two colliding laser pulses is discussed. Simulations indicate the bunches as short as a few fs can be produced. Conversion of the fs electron pulse to a fs x-ray pulse can be accomplished by Bremsstrahlung or Thomson scattering. Introduction The use of femtosecond x-ray pulses is becoming an increasingly important tool in the study of ultrafast structural dynamics in crystals. 1 Optical wave- lengths have been used in the past to provide indirect information on laser excited ultra-fast disordering in materials such as Si, GaAs and InSb. 2 How- ever, since these photons interact primarily with outer shell electrons, only an indirect measure of the material structure can be obtained. X-rays, on the other hand, can provide a direct measurement of the crystal structure through measurement of the diffraction pattern. To enable time-resolved measurement of structural changes requires the production of ultra-short x-ray pulses. The typical time scale for lattice changes in a crystal is related to the characteristic vibrational time, which is on the order of 100 fs. Furthermore, since ultra-short laser pulses are typically used to initiate the material changes, synchronization between laser and x-ray source is an essential requirement to allow pump-probe type experiments. In this paper, we discuss two methods for the generation of fs x-ray pulses through the interaction of a relativistic electron beam with an intense laser pulse. 3−6 The first method employs Thomson scattering an intense laser pulse off a relativistic electron beam. 7−13 The second method is a technique for the production of ultra-short electron bunches (and hence ultra-short x-ray pulses) by a laser-plasma injector, in which low emittance electron bunches are expected to be produced through the use of colliding laser pulses. 14−17 ICFA00b: submitted to World Scientific on February 11, 2000