Developing the next generation of coherent light sources based on laser-plasma accelerators
In addition to the advantage of large accelerating gradients, LPAs intrinsically produce ultra-short (femtosecond) electron bunches, providing unique opportunities for radiation sources. The ultra-high accelerating gradients motivate active research on LPAs as compact sources of energetic beams for many applications. We have for example experimentally demonstrated GeV beams in cm-scale plasmas with tens of picocoulombs of charge, few-percent relative energy spread, and ultra-low transverse emittance. Because of the short beam duration, the peak current is high (1–10 kA). All these factors make it natural to consider LPA electron beams as drivers for a free-electron laser (FEL), producing coherent, high-peak- brightness beams of electromagnetic radiation. Researchers at the BELLA Center are actively working toward demonstration of a compact LPA-driven FEL.
The six-dimensional beam brightness of the LPA electron beam is comparable to that of state-of-the-art photocathode-based sources; however the phase-space distribution of the beam is not optimized for the FEL application. LPA research at the BELLA Center is focused on methods to provide detailed control of the injection of background plasma electrons into the plasma wave, thereby controlling the beam phase space characteristics and to improve the shot-to-shot stability and tunability of the beam parameters. Although LPA beam phase space properties continue to improve, application of FEL beams may be accomplished using present experimentally-demonstrated LPA beam properties with proper e-beam phase space manipulation following the LPA. Beam phase-space manipulation to enable FEL lasing of LPA beams has been a focus of research at the BELLA Center.
Novel methods of seeding the FEL are also being investigated, and a coherent seed source of XUV light based on laser high-harmonic generation (HHG) at a solid surface has been developed at the BELLA Center. Laser harmonics at XUV wavelengths may be generated efficiently by the interaction of a moderately-intense laser with an overcritical plasma (solid) by the coherent wake emission (CWE) mechanism. A spooling tape-based solid target configuration enables high-repetition rate (multi-Hz) operation over thousands of shots. The intrinsic synchronization of the electron beam and the MW HHG pulses, as well as the elimination of a HHG transport line, makes this source attractive as a seed for a few-Hz LPA-driven FEL operating in the extreme ultraviolet (XUV) range.