Control of low-mode drive asymmetry in an efficient long pulse low-density filled hohlraum

One anticipated advantage of a 4-shock CH design (~2x longer laser pulses than typical HDC designs) is the ability to achieve high compression of the DT fuel, essential for the design of a high gain target. It has been shown experimentally that hohlraums filled with gas at low densities (< 0.6 mg/cc) have higher efficiency because of reduced backscatter losses due to laser plasma instabilities. Thus, combining longer drives in low gas-density hohlraum has been considered challenging since high-Z plasma ablated off the hohlraum wall is less tamped and will potentially interfere with the transport of the inner cone beam power during the last stages of the drive pulse.

Through a series of NIF experiments using subscale targets (5.75 mm diameter hohlraums, filled at 0.45 mg/cc He, and 980 um outer radius capsules), we demonstrated that a symmetric implosion using a long duration low-adiabat drive is possible. We confirmed the original prediction that the wall motion dynamics of the adiabat controlled 4 shock drive pulse is modified by its long low power (< 1 TW) trough acting to temporarily interrupt the ballistic wall expansion, allowing for adequate inner beam propagation even 14 ns after laser start. To simultaneously mitigate P2 and P4 drive asymmetry swings caused by the drift of the laser spots, a temporal beam staggering^[1] (TBS) technique was employed.

Finally, we demonstrated successful mitigation of the time-integrated P2 asymmetry using cross beam energy transfer from outer to inner cone beams. In summary excellent symmetry control of the main intrinsic asymmetry modes P2 and P4 throughout the pulse is possible for long pulse low-density filled hohlraums.

[1]R. Turner et. al. Phys Plasmas 7 (2000) 333.