Sprecher
Beschreibung
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 [1]. Inserting a plasma mirror (PM) near the LPA exit to retro-reflect spent LPA drive pulses onto trailing electron bunches [2] 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 [3], 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.
Reference:
[1] D. P. Umstadter, Contemp. Phys. 56, 417 (2015).
[2] 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).
[3] J. P. Couperus et al., Nat. Comm. 8, 487 (2017).
| Working group | Secondary radiation generation & applications |
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