New methods for diagnosing and controlling hohlraum drive asymmetry on Nova

Abstract

A novel method to control lowest-order (P2) flux asymmetry in Nova cylindrical hohlraums [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] with fixed laser beams is to use a pair of axial gold disks of varying radii to partially block the capsule view of the laser-entrance holes. Some advantages in using axial disks include the prospect for added drive on target, the potential for P4 control when used in tandem with laser pointing, and possibly reduced time-dependent P2(t) flux asymmetry swings at early time. Neutron-based diagnostics have provided some suggestion of increased drive, but a more direct measure of drive enhancement is with the use of backlit, low-density (0.3 g/cc) foam surrogate targets. In this scheme, an ablatively driven, inwardly propagating shock is imaged in time using backlighting from an irradiated Ti disk placed outside of the hohlraum. The benefit in using low-density surrogate targets is an amplified shock speed that enables easier detection of both average shock motion (drive) and distortion (flux asymmetry). Experiments and calculations are in excellent agreement over a nearly 10% enhancement in peak drive temperature in the presence of axial gold disks. Measurements of lower-order distortion, P2(t) and P4(t), versus time for several laser pointings (without axial disks) using this technique have also been carried out and show good agreement between experiment and simulations. Efforts to further control time-dependent flux asymmetry using multiple ring, beam-phasing techniques on Nova, as will be required for the National Ignition Facility [J. Lindl, Phys. Plasmas 2, 3933 (1995)], are under development. Current designs indicate an appreciable reduction in P2(t) is possible. Significant control of time-integrated P4 flux asymmetry with appreciable inner and outer ring separation also appears possible.

Asymmetrically driven implosions

“…[10][11][12][13] Drive symmetry in such experiments is measured by following the position of the ablation and shock fronts in the sphere, where the speed of the shock varies as the square root of the incident drive flux. 14 To maximize the responsiveness of a diagnostic, the density is made as low as possible, consistent with subsonic radiation transport in the foam.…”

Using laser entrance hole shields to increase coupling efficiency in indirect drive ignition targets for the National Ignition Facility

“…The final improvement is the addition of LEH shields. 6,7 The LEH shields are high Z disks that are placed in the hohlraum between the capsule and LEH. The LEH shields serve to block the capsule's view of the LEH, which is cold compared to the hot hohlraum walls.…”

Lawrence Livermore National Laboratory's activities to achieve ignition by X-ray drive on the National Ignition Facility

“…LASNEX and simple analytic scaling shows that a 10% modulation in the drive (P2/P0) produces a 2-3 pm modulation in the observable a2 (second Legendre coefficient in radius). On Nova, measurements of a2 have been made with error bars as small as 1 pm (Amendt et al 1997). In addition to the second Legendre coefficients the fourth, sixth, and possible eighth coefficients need to be measured and controlled to achieve ignition on the NIF.…”

“…Earlier work developed the technique of imaging the shape of the x-ray emission of an implosion, and imaging in hard x-rays the motion of the x-ray emitting area to measure drive asymmetry (Suter et al 1994). Another technique to measure the time dependence of the drive asymmetry is from x-ray backlit images of the ablation front in a low density witness ball placed at the center of a hohlraum (Amendt et al 1997). LASNEX and simple analytic scaling shows that a 10% modulation in the drive (P2/P0) produces a 2-3 pm modulation in the observable a2 (second Legendre coefficient in radius).…”