Laser-plasma interactions from thin tapes for high-energy electron accelerators and seeding compact FELs

For over 10 years, laser plasma acceleration (LPA) has been a rapidly growing technology/nused to create electron beams on length-scales much smaller than that of a conventional/nRF-accelerator [1]. As electron beam properties improve, research for LPAs is expanding to/ntake advantage of the creation and accessibility of high-quality electron beams from plasma/ntargets. Two applications which are currently being explored are a multi-stage plasma/naccelerator to reach energies greater than those a single-stage accelerator can achieve and/nexploring the possibility of an LPA based free-electron laser (FEL) light source. Research/nsupporting both of these efforts has been performed on the 50 TW TREX laser system at/nthe BELLA Center at the Lawrence Berkeley National Lab, and the results of these efforts/nare described in this dissertation./nUsing chirped-pulsed amplification to produce high-quality laser pulses up to petawatt/nlevels, experimental results have yielded laser driven electron beam energies up to 4.25/nGeV [2]. By tuning the density of the target, the accelerating gradients sustained by the/nplasma can grow beyond 100 GeV/m [3] (10^3 times larger than that of a conventional RF/naccelerator). However, limiting factors such as dephasing of the electron beam from the/nplasma wake, defocusing of a laser pulse, and energy depletion of the laser into the plasma/nlimit the maximum sensible length of a plasma accelerator. Staging the LPA with two or/nmore accelerating modules could be the next step towards producing beams with energies/ngreater than those possible with a single stage./nOne requirement for staged acceleration is that the laser pulse used to drive the first/naccelerating stage must be coupled out of the beamline, and a fresh laser pulse must be/ncoupled in for the second stage to post accelerate the electrons. To do this while maintaining/na short scale length between the two stages requires an optic to be placed near the final focus of the second laser pulse. Because damage will occur when the laser pulse interacts with a steering optic near focus, the coupling optic must be capable of replacing the surface/nfollowing damage on each successive shot. This thesis comprises a detailed investigation of the physics of using a plasma mirror (PM) from a tape by reflecting ultrashort pulses from/na laser-triggered surface plasma. The tapes used in the characterization of the PM are VHS/nand computer data storage tape. The tapes are 6.6 m (computer storage tape) and 15 m/n(VHS) thick. Each tape is 0.5 inches wide, and 10s of meters of tape are spooled using a tape drive; providing thousands of shots on a single reel of tape. The amount of reflected energy of the PM was studied for different input intensities. The fluence was varied by translating the focus of the laser upstream and downstream of the tape, which changed the spot size on the tape surface and hence changed the fluence. This study measured reflectances from both sides of the two tapes, and for input light of both s and p-polarizations. Lastly, an analytic model was developed to understand the reflectance as a function of fluence for each tape material and polarization./nAnother application that benefits from the advancements of LPA technology is an LPAbased/nFEL. By sending a high quality electron bunch through an undulator (a periodic/nstructure of positive and negative magnetic poles), the electrons oscillate transversely to the/npropagation axis and produce radiation. The 1.5 m THUNDER undulator [4] at the BELLA/nCenter has been commissioned using electron beams of 400MeV beams with broad energy/nspread (35%) [5]. To produce a coherent LPA-based FEL, the beam quality would need to/nimprove to sub-percent level energy spread. A seed source could be used to help induce/nbunching of the electron beam within the undulator./nThis thesis described the experimental investigation of the physics of using solid-based/nsurface high-harmonic generation (SHHG) from a thin tape as a possible seed source for/nan FEL. A thin tape placed within centimeters of the undulator's entrance could act as/na harmonic generating source, while simultaneously transmitting an electron beam. This/nremoves the need for transport optics for the XUV photons and the need for additional/noptics to overlap the seed beam with the electron beam at the undulator entrance./nBy operating at sub-relativistic laser strengths, harmonics up to the 17th order of 800/nnm light are produced using an SHHG technique known as coherent wake emission (CWE)./nCWE pulse properties such as divergence, energy, conversion efficiency, and spectrum are/nmeasured for a wide range of tape materials and drive laser conditions. A clear correlation/nbetween surface roughness and harmonic beam divergence is found. The measured pulse/nproperties for the 15th harmonic from VHS tape (conversion efficiency 6.5x10^-?7 and/nan rms divergence of 12 mrad), the 100 mJ-level, 40-50 fs-class drive laser, produces peak/npowers of several MW's of XUV pulses. The results of a 1D model indicate that these CWE/npulses with MW level powers are sufficient for seed-induced FEL gain.

Advisor: Leemans, Wim P
Attwood, David