<title>Laser system power balance effects from stimulated rotational Raman scattering in air</title>

In the OMEGA Upgrade laser system 60 beams are used to symmetrically irradiate the target. To achieve optimum fuel compression, the beams must irradiate the target uniformly, requiring excellent beam quality in each beam and nearly identical temporal waveforms so that target irradiation remains balanced throughout the course of the implosion process. Propagation of high intensity, pulse-shaped IR and UV laser beams through long air paths leads to losses and decreased beam quality due to stimulated rotational Raman scattering (SRRS). With short pulses, it is necessary to use transient Raman theory to include build up and relaxation of the medium. Imperfect beam quality leads to intensity not spots that can lower the effective threshold for Raman scattering. A detailed analysis of the system to determine energy efficiency and beam quality necessitates a four-dimensional (4-D) treatment of both the optical beam and the air path. In this paper we outline the 4-D modeling of the OMEGA Upgrade laser and describe full-scale experimental investigations of SRRS in air.