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

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.

Figure 1

The amplified pulse amplitude $|b_1|$ vs. the delay time $\zeta$ at several values of the amplification time $\tau$ in extremely undercritical plasma.

Figure 2

The maximal amplitude of the amplified pulse ${max}_{\zeta }\left|b_1\right|$ as a function of the amplification time $\tau$ in extremely undercritical plasma.

Figure 3

The amplified pulse amplitude $|b_1|$ vs. the delay time $\zeta$ at several values of the amplification time $\tau$ and the dispersion parameter $Q$.

Figure 4

The maximal amplitude of the amplified pulse ${max}_{\zeta }\left|b_1\right|$ as a function of the amplification time $\tau$ for several values of the dispersion parameter $Q$.