Possible origins of a time-resolved frequency shift in Raman plasma amplifiers

Abstract

Raman amplification is a resonant process in which the energy of a long pump pulse is transferred to a short seed pulse by a plasma wave. There has been a significant effort to identify a window in parameter space within which the interaction is expected to be highly efficient and not degraded by competing instabilities or excessive damping. However, experimental results have thus far failed to approach the theoretical limits. Recent amplified signal spectra display a characteristic blue shift, which evolves within the seed pulse duration and suggests that the mechanism responsible for this shift is also limiting amplification in these experiments. We present the evidence and explore different hypotheses for the origins of the shift—namely localization in density minima along the axis of laser propagation induced by an ion acoustic wave that could arise from the Langmuir decay instability, filamentation which could also modulate the plasma density but in the plane transverse to laser propagation, particle trapping, and additional ionization induced by the amplified seed field.

Spectral characteristics of ultra-short laser pulses in plasma amplifiers

“…40,41 The obtained spectrum resembles strongly the one presented here for the 80 fspulse, i.e., the SRS-and SBS-signatures do not overlap. The presence of SRS and SBS is due to the simultaneous excitation of the modes and their competition might indicate a saturation mechanism.…”

Broadband stimulated Raman backscattering

“…However, for this parameter range the best numbers for the conversion efficiency obtained with simulation do not agree with the experimental results. Additionally the experiments suffer from simultaneously occurring plasma effects like particle trapping [17] and ion acoustic waves originating from the Langmuir decay instability [18]. Besides particle trapping and wave-breaking, most of the additional unwanted effects appear mainly in low plasma density configurations, where the coupling between density fluctuations at w pe and w 2 pe is expected to lead to a saturation of SRBS [19] amplification.…”

Raman Amplification with a Flying Focus

“…While early experimental efforts [3,9,10] on Raman-based plasma amplifiers appeared promising, progress has slowed and numerous attempts have been made recently to elucidate the shortcomings of experiments. A consensus is emerging that thermal effects [11][12][13][14][15][16][17][18] and the amplification of precursors growing from noise ahead of the seed pulse [2,11,[19][20][21][22] issues that are related to the pump's traversal of ionized plasma prior to meeting the seed -may be among the most pervasive issues degrading performance.…”