Kinetic simulations exploring the focusability of backward Raman amplified pulses in randomly inhomogeneous plasma *

Nearly relativistic non-focused intensities might be reached by backward Raman amplification in plasma.  Further efficient vacuum focusing to the unprecedented ultrarelativistic intensities is enabled by using a prefocused seed pulse, as it can remain well-focused during the amplification process [1]. However, it is predicted that in an inhomogeneous plasma, the focusability might be spoiled when the amplification length is longer than a threshold [2]. Here, by using the two dimensional kinetic particle-in-cell simulations, we examine the above defocusing effect of plasma inhomogeneities on the amplified pulse. Furthermore, to correct the phase distortions induced by the plasma density inhomogeneity, we propose to use a phase conjugated seed pulse for the amplification, with a phase front, when exiting the plasma, that enables full convergence under the vacuum focus .

[1] G. M. Fraiman, N. A. Yampolsky, V. M. Malkin, and N. J. Fisch, Phys. Plasmas 9, 3617 (2002).
[2] A. A. Solodov, V. M. Malkin, and N. J. Fisch, Phys. Plasmas 10, 2540 (2003).