A. Zingale , N. Czapla, D. M. Nasir, S. K. Barber, J. H. Bin, A. J. Gonsalves, F. Isono, J. van Tilborg, S. Steinke, K. Nakamura , G. E. Cochran, J. Purcell , W. P. Leemans,2∥ C. G. R. Geddes, C. B. Schroeder, E. Esarey, and D.W. Schumacher The Ohio State University, Columbus, Ohio 43210, USA Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, California 94550, USA

High-resolution (sub-%), single-shot energy distribution, and emittance measurements of GeV-class electron beams generated by laser plasma accelerators (LPAs) have been enabled through the use of an active plasma lens combined with relatively short dipole magnets. This setup, with only 60 cm between the LPA source and the final diagnostic screen, was facilitated through the use of a replenishable ∼20–40 nm thick liquid crystal plasma mirror to separate remnant laser radiation from the electron beam without emittance degradation. As LPAs capable of generating GeV-class electron beams at cm-scale distances become increasingly ubiquitous, there is a need to supplement these compact accelerators with equally compact diagnostics.

In this report, we demonstrate precise control of a laser plasma accelerator (LPA) through characterization of a shock-induced density downramp injector. Experiments systematically varied the shock injector profile, including shock angle, shock position, up-ramp width, and acceleration length. As a result, an electron beam that was highly tunable from 30 to 300 MeV with 8.5% energy spread, 1.5 mrad divergence and 0.35 mrad pointing fluctuation was produced. We also report the progress of a project focusing on demonstrating the crucial enablers of a compact LPA based MeV Thomson photon source in an integrated experiment and a dedicated 100 TW laser facility built at the Berkeley Lab Laser Accelerator (BELLA) Center, Lawrence Berkeley National Laboratory (LBNL).

Electrically discharged active plasma lenses (APLs) are actively pursued in compact high-brightness plasma-based accelerators due to their high-gradient, tunable, and radially symmetric focusing properties. In this manuscript, the APL is experimentally compared with a conventional quadrupole triplet, highlighting the favorable reduction in the energy dependence (chromaticity) in the transport line. Through transport simulations, it is explored how the non-uniform radial discharge current distribution leads to beam-integrated emittance degradation and a charge density reduction at focus. However, positioning an aperture at the APL entrance will significantly reduce emittance degradation without additional loss of charge in the high-quality core of the beam. An analytical model is presented that estimates the emittance degradation from a short beam driving a longitudinally varying wakefield in the APL. Optimizing laser plasma accelerator operation is discussed where emittance degradation from the non-uniform discharge current (favoring small beams inside the APL) and wakefield effects (favoring larger beam sizes) is minimized.

Laser-plasma accelerators (LPAs) have the potential to drive compact free-electron lasers (FELs). Even with LPA energy spreads typically at the percent level, the e-beam brightness can be excellent, due to the low normalized emittance (< 0.5 mu m) and high peak current (multi-kA) resulting from the ultra-short e-beam duration (few fs). It is critical, however, that in order to mitigate the effect of percent-level energy spread, one has to actively manipulate the phase-space distribution of the e-beam. We provide an overview of the methods proposed by the various LPA FEL research groups. At the BELLA Center at LBNL, we are pursuing the use of a chicane for longitudinal e-beam decompression (therefore greatly reducing the slice energy spread), in combination with short-scale-length e-beam transportation with an active plasma lens and a strong-focusing 4-m-long undulator. We present ELEGANT & GENESIS simulations on the transport and FEL gain, showing strong enhancement in output power over the incoherent background, and present estimates of the 3D gain length for deviations from the expected e-beam properties (varying e-beam lengths and emittances). To highlight the role of collective effects, we also present ELEGANT & GENESIS simulation results.