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

TL;DR

This paper demonstrates that in backward Raman amplifiers, the maximum spike intensity limit can be surpassed by sub-dominant spikes, and delaying filamentation instability can enable higher fluences.

Contribution

It reveals that sub-dominant spikes can exceed the relativistic intensity limit of the leading spike and that delaying filamentation allows for greater amplification in plasma.

Findings

Sub-dominant spikes can surpass the leading spike's intensity limit.

Delaying filamentation instability enables higher pulse fluences.

Amplification can be extended beyond traditional relativistic limits.

Abstract

The leading amplified spike in backward Raman amplifiers can reach nearly relativistic intensities before the saturation by the relativistic electron nonlinearity, which 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 sub-dominant spikes, which surprisingly outgrow the leading spike after its nonlinear saturation. Furthermore, an initially negligible group velocity of the amplified pulse in strongly under-critical plasma appears to be capable of delaying the filamentation instability in the nonlinear saturation regime. This enables further amplification of the pulse to even larger output fluences.