Thomson backscatter (TBS) of near-IR (hνL ~ 1eV) laser pulses from laser-plasma-accelerated (LPA) electron bunches (200 < γe < 4000) provides a compact source of bright, tunable, ultrashort x-rays (0.1 < 4γe2hvL < 100 MeV) for radiography and nuclear science . Inserting a plasma mirror (PM) near the LPA exit to retro-reflect spent LPA drive pulses onto trailing electron bunches  is an exceptionally compact, inexpensive, simple way to convert even the smallest LPAs into robust TBS sources, but raises as-yet-unanswered questions about x-ray beam quality: Is the spent LPA pulse spectrum (and thus TBS) severely broadened by its interaction with LPA and PM? Do pre-pulses pre-expand the PM surface, degrading its reflectivity, and TBS efficiency? Do LPA electrons generate broadband bremsstrahlung in the PM that pollutes narrowband TBS? Can the spectrum of MeV x-rays from a PM-based LPA-TBS source be characterized with compactness, simplicity and frugality matching those of the source, without resorting to bulky pair-production spectrometers? Here, through systematic experiments using 6 J, 800nm, 25fs pulses from HZDR’s DRACO laser facility to drive a 250 MeV (10% spread), 500pC LPA in a nitrogen-doped He gas jet , we answer these questions. Our main findings are: spent drive pulses broaden, but contribute less to x-ray spectral broadening than electron energy spread; the PM remains highly reflective even near the LPA exit, where drive pulse amplitude reaches a0~3; background bremstrahlung becomes negligible compared to TBS with thin (~25µm) low-Z PMs; we reconstruct accurate single-shot x-ray spectra near 1 MeV by recording and analyzing particle cascades that x-rays generate in an inexpensive calorimetric stack consisting of ~20 image plates or scintillators separated by converters. Reconstructed x-ray spectra reveal both TBS and bremsstrahlung, and track centroid shifts accompanying changes in γe and spectral broadening accompanying nonlinear TBS.
 D. P. Umstadter, Contemp. Phys. 56, 417 (2015).
 H.E. Tsai et al., Phys. Plasmas 22, 023106 (2015); A. Döpp et al., Plasma Phys. Control. Fusion 58, 034005 (2016); C. Yu et al., Sci. Rpts. 6, 29518 (2016).
 J. P. Couperus et al., Nat. Comm. 8, 487 (2017).
|Working group||Secondary radiation generation & applications|