Exceeding the leading spike intensity and fluence limits in backward Raman amplifiers

Abstract

The leading amplified spike in backward Raman amplifiers can reach nearly relativistic intensities before the saturation by the relativistic electron nonlinearity. The saturation sets an upper limit to the largest achievable leading spike intensity. It is shown here that this limit can be substantially exceeded by the initially subdominant spikes, which surprisingly outgrow the leading spike after its nonlinear saturation. Furthermore, an initially negligible group velocity dispersion of the amplified pulse in strongly undercritical plasma appears to be capable of delaying the longitudinal filamentation instability in the nonlinear saturation regime. This enables further amplification of the pulse to even larger output fluences.

Beyond nonlinear saturation of backward Raman amplifiers

“…First, let us recall in Fig. 1 the nonlinearly saturated (multi-spike) solution for extremely under-critical plasmas as found by Malkin et al [14,15]. We solve Eqs.…”

“…1, we have ǫ = 0.1 so ξ M = 5.5. In the upper panel we plot the analytical solution (15) for the maximal amplitude of the leading spike, B max (τ ), (black line with squares). In the lower panel of the figure, we compare our theoretical prediction (20) for the phase mismatch, Φ, of the leading spike (black line with squares) with simulation results (blue dashed-dotted line).…”

“…We adopt the dimensionless quasi-static 3-wave interaction model including dispersion, detuning, and relativistic nonlinearity [15,17,33] …”

“…Most of these deleterious processes can be mitigated by proper preparation of laser pulses and plasmas, choosing parameter ranges and selective detuning of the Raman resonance. Ultimately, the output intensity limit appears to be imposed primarily by the relativistic electron nonlinearity, causing a phase mismatch of the Raman resonance that results in a saturation of the dominant leading spike growth [14] as well as of the secondary higher spikes in the multi-spike wave train solution [15].…”

“…The major physical processes that may affect BRA include the amplified pulse filamentation [2,[11][12][13], detuning due to the relativistic electron nonlinearity [14,15], parasitic Raman scattering of the pump and amplified pulses by plasma noise [2,13,[16][17][18], generation of superluminous precursors of the amplified pulse [19], pulse scattering by plasma density inhomogeneities [20], pulse depletion and plasma heating through inverse bremsstrahlung [21,22], the resonant Langmuir wave Landau damping [21,[23][24][25] or breaking [2,13,16,26,27], and other processes (see, for examples, Refs. [28][29][30][31]).…”