UC Berkeley

For over 10 years, laser plasma acceleration (LPA) has been a rapidly growing technology

used to create electron beams on length-scales much smaller than that of a conventional

RF-accelerator [1]. As electron beam properties improve, research for LPAs is expanding to

take advantage of the creation and accessibility of high-quality electron beams from plasma

targets. Two applications which are currently being explored are a multi-stage plasma

accelerator to reach energies greater than those a single-stage accelerator can achieve and

exploring the possibility of an LPA based free-electron laser (FEL) light source. Research

supporting both of these efforts has been performed on the 50 TW TREX laser system at

the BELLA Center at the Lawrence Berkeley National Lab, and the results of these efforts

are described in this dissertation.

Using chirped-pulsed amplification to produce high-quality laser pulses up to petawatt

levels, experimental results have yielded laser driven electron beam energies up to 4.25

GeV [2]. By tuning the density of the target, the accelerating gradients sustained by the

plasma can grow beyond 100 GeV/m [3] (10^3 times larger than that of a conventional RF

accelerator). However, limiting factors such as dephasing of the electron beam from the

plasma wake, defocusing of a laser pulse, and energy depletion of the laser into the plasma

limit the maximum sensible length of a plasma accelerator. Staging the LPA with two or

more accelerating modules could be the next step towards producing beams with energies

greater than those possible with a single stage.

One requirement for staged acceleration is that the laser pulse used to drive the first

accelerating stage must be coupled out of the beamline, and a fresh laser pulse must be

coupled in for the second stage to post accelerate the electrons. To do this while maintaining

a 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

following 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

a laser-triggered surface plasma. The tapes used in the characterization of the PM are VHS

and computer data storage tape. The tapes are 6.6 m (computer storage tape) and 15 m

(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.

Another application that benefits from the advancements of LPA technology is an LPAbased

FEL. By sending a high quality electron bunch through an undulator (a periodic

structure of positive and negative magnetic poles), the electrons oscillate transversely to the

propagation axis and produce radiation. The 1.5 m THUNDER undulator [4] at the BELLA

Center has been commissioned using electron beams of 400MeV beams with broad energy

spread (35%) [5]. To produce a coherent LPA-based FEL, the beam quality would need to

improve to sub-percent level energy spread. A seed source could be used to help induce

bunching of the electron beam within the undulator.

This thesis described the experimental investigation of the physics of using solid-based

surface high-harmonic generation (SHHG) from a thin tape as a possible seed source for

an FEL. A thin tape placed within centimeters of the undulator's entrance could act as

a harmonic generating source, while simultaneously transmitting an electron beam. This

removes the need for transport optics for the XUV photons and the need for additional

optics to overlap the seed beam with the electron beam at the undulator entrance.

By operating at sub-relativistic laser strengths, harmonics up to the 17th order of 800

nm light are produced using an SHHG technique known as coherent wake emission (CWE).

CWE pulse properties such as divergence, energy, conversion efficiency, and spectrum are

measured for a wide range of tape materials and drive laser conditions. A clear correlation

between surface roughness and harmonic beam divergence is found. The measured pulse

properties for the 15th harmonic from VHS tape (conversion efficiency 6.5x10^-􀀀7 and

an rms divergence of 12 mrad), the 100 mJ-level, 40-50 fs-class drive laser, produces peak

powers of several MW's of XUV pulses. The results of a 1D model indicate that these CWE

pulses with MW level powers are sufficient for seed-induced FEL gain.