Demonstration scheme for a laser-plasma driven free-electron laser

By A. Maier. 2012

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Demonstration scheme for a laser-plasma driven free-electron laser A. R. Maier, 1, 2, 3, ⇤ A. Meseck, 4 S. Reiche, 5 C. B. Schroeder, 6 T. Seggebrock, 3 and F. Gruner 2, 1, 3 Center for Free-Electron Laser Science, Notkestrasse 85, 22607 Hamburg, Germany Institut fur Experimentalphysik, Universitat Hamburg, 22607 Hamburg, Germany Department fur Physik, Ludwig-Maximilians Universitat, 85748 Garching, Germany Helmholtz-Zentrum Berlin fur Materialien und Energie, 14109 Berlin, Germany Paul Scherrer Institut, 5232 Villigen PSI, Switzerland Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA (Dated: August 9, 2012) Laser-plasma accelerators are prominent candidates for driving next-generation compact light sources, promising high-brightness, few-fs x-ray pulses intrinsically synchronized to an optical laser and thus are ideally suited for pump-probe experiments with fs resolution. So far, the large spectral width of laser-plasma driven beams has been preventing successful FEL demonstration using such sources. In this work we study the application of an optimized undulator design and bunch decom- pression to large energy spread beams in order to permit FEL amplification. Numerically, we show a proof-of-principle scenario to demonstrate FEL gain in the VUV range with electron beams from laser-plasma accelerators as currently available in experiments. PACS numbers: 41.60.Cr; 52.38.Kd I. INTRODUCTION Envisioning a compact and affordable electron accel- erator on a laboratory scale, laser-wakefield accelera- tors (LWFA) [1, 2] are attracting increasing interest, es- pecially as candidates for driving next-generation free- electron lasers (FELs) [3–5]. Without doubt, increased availability of such a compact, high-brightness x-ray source, featuring a few-femtosecond (fs) pulse length combined with intrinsic few-fs temporal synchronization to an optical laser beam, would have enormous impact on many scientific disciplines [6]. Insufficient electron beam quality, foremost the elec- tron beam energy spread, which is typically on the per- cent level for current LWFA generated beams [7–9], is usually considered the greatest obstacle in realizing a laser-plasma driven FEL, and so far no laser-plasma based FEL has been demonstrated. However, a consequent adjustment of the FEL setup for increased energy spread acceptance, using large un- dulator parameters and longitudinal bunch decompres- sion, can permit FEL amplification even for beams of relatively large spectral width. Based on recent novel diagnostic experiments, which characterized the typical LWFA emittance [9–12], bunch length [13, 14] and slice energy spread [15], we exemplarily demonstrate the appli- cation of an optimized undulator and longitudinal bunch decompression to currently available laser-plasma gener- ated beams. We conclude with a start-to-end simulation of a proof-of-principle experiment, which solely focuses on demonstrating detectable FEL gain well above the spontaneous emission background using a lab-size, 2 me- ter long undulator. We find that laser-plasma beams very andreas.maier@desy.de similar to already experimentally demonstrated beams may be sufficient to generate experimentally detectable FEL gain. Knowledge of the slice energy spread is a crucial pa- rameter and still difficult to access experimentally and thus imposes an uncertainty on the experiment design. Therefore, after optimizing the undulator design (Section II) and defining parameters of a typical electron bunch, where the electron beam energy spread ˆ ⌘ / and bunch charge Q are free parameters, we study different scenarios to determine the minimum requirements on the electron beam for demonstrating FEL gain: First, we as- sume an uncorrelated energy spread (Section III) in our studies, followed by the assumption of a correlated energy spread (Section IV) as motivated by recent experiments. Adding a simple magnetic chicane for bunch decompres- sion can further increase the performance of the FEL (Section V) and eventually we find that experimentally already demonstrated laser-plasma electron beams could show FEL gain. II. UNDULATOR DESIGN A highly relativistic electron beam of normalized en- ergy , propagating through an undulator characterized by the dimensionless parameter K ⇡ 0.93 u [cm] B[T], with u the undulator period, and B the on-axis mag- netic field, emits on-axis radiation of wavelength u (1 + K /2)/(2 ). For high electron density beams, the emitted radiation field couples to the electron bunch, causing an energy modulation and consequently a den- sity modulation (microbunching) of period along the bunch, leading to coherent emission of radiation with ra- diated pulse energies many orders of magnitude above the purely spontaneous emission. The dimensionless Pierce

Subfields: Electrical and Electronic Engineering, Nuclear and High Energy Physics, Radiation

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