Application of cross-beam energy transfer to control drive symmetry in ICF implosions in low gas fill Hohlraums at the National Ignition Facility

Cross beam energy transfer (CBET), invoked by setting a wavelength difference, Delta lambda, between inner and outer beam cones, can be used to increase the drive on the waist in indirectly driven inertial confinement fusion experiments at the National Ignition Facility (NIF). Historically, hot spot symmetry control in capsule implosions in high (>= 0.9mg/cm(3 4)He) gas fill Hohlraums was enabled by substantial CBET. However, these implosion designs suffered from inflight symmetry swings, high SRS backscatter on the inner cones, and significant hot electron generation posing a threat to DT fuel preheat. Subsequent experiments in larger, low (<= 0.6mg/cm(3 4)He) gas fill Hohlraums demonstrated round implosions by varying the inner cone fraction throughout the laser drive at Delta lambda=0 angstrom while keeping backscatter and hot electron generation very low. To enable driving larger capsules at a given Hohlraum size, additional tools for implosion symmetry control are required. With this goal in mind, this paper presents a detailed experimental study of using CBET in low gas fill Hohlraums near NIF's current peak power capability. We find a similar to 2.5x higher sensitivity of the P-2 Legendre mode with respect to Delta lambda changes compared to that of high gas fill designs. We attribute this observation to the fact that backscatter remains very low and that CBET remains in a linear regime, as suggested by simulations. As a result, a much smaller Delta lambda of order 1 angstrom is sufficient for sustaining implosion symmetry while keeping laser-to-Hohlraum coupling high and hot electron generation very low. While this study used plastic ablator capsules, our findings can be generalized to other ablator materials and, hence, show great promise for using wavelength detuning as a strong lever for implosion symmetry control in future low gas fill designs that require smaller case to capsule ratios in order to increase the energy coupled to the capsule.