Visualizing x-ray drive symmetry using detailed measurements in NIF hohlraums*

Achieving an efficient capsule implosion in NIF indirect drive target experiments requires symmetric hohlraum x-ray drive for the duration of the laser pulse. Experimental evidence indicates that symmetric drive becomes limited in time by the hohlraum wall plasma as it expands into the hohlraum cavity and impairs the propagation of the laser beams. This effect is enhanced in low gas-fill targets [1] where, as seen in simulations, the ablator and gas plasma cannot provide sufficient back pressure to limit wall blow-in. In order to understand this, we performed a thorough experimental characterization of laser beam transport in hohlraums using x-ray visualization measurements. A sequence of hohlraum experiments collected data quantifying: (1) laser power delivered to the hohlraum wall, (2) blow-in of the ablated wall plasma into the laser path, and (3) the symmetry of the hot-spot [2]. Each experiment is identical except for one single parameter change. We develop multi-parameter experimental scalings and compare with radiation-hydrodynamic simulations to develop a picture of how hohlraum parameters affect laser power transport.

References

[1] D. A. Callahan, et al., Phys Plasmas 25, 056305 (2018)

[2] N. Izumi, N. B. Meezan, S. Johnson, B. N. Woodworth, J. D. Moody, et al., submitted to Rev. Sci. Instrum.