NIF double-shell ignition designs: stability of short-wavelength perturbations
Abstract
Previously reported [1] detailed multi-mode 2-D simulations of a proposed double-shell design [2] for the NIF showed large growth of high Legendre mode numbers (l>200) on the outer surface of the inner shell leading to shell breakup. The physical mechanism invokes the large outward expansion of the outer surface [due to ablation by the L-shell (> 8 keV) radiation from the high-Z hohlraum walls] and subsequent recompression by the converging outer shell. Here, we predict successful control of this instability by introducing a tamping material on the outer surface of the inner shell. This modification has a two-fold effect: a) reduced outward excursion of the inner shell and b) reduced Atwood number. Results with two tamping materials (CH, Ti) are presented. By direct numerical simulations of the CH-tamped capsule, we are able to demonstrate that the inner shell is able to survive disruption for perturbations containing mode number beyond l>1000. Furthermore, the calculated mix-width at the pusher/tamper interface approaches a constant value (approximately 40 % of the shell width at peak compression), irrespective of the maximum l used beyond l=600. In addition, recognizing the stabilizing effects of a density-gradient scalelength, we present results on a new target design with a graded density inner shell that avoids the instability altogether. [1] J.L. Milovich et al., BAPS 47, 27 (2002). [2] P. Amendt et al., PoP 9, 2221 (2002).
- Publication:
-
APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- October 2003
- Bibcode:
- 2003APS..DPPLO2007M