Analysis of a direct-drive ignition capsule designed for the National Ignition Facility

This paper reviews the current direct-drive ignition capsule designed for the National Ignition Facility (NIF) [M. D. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41, B39 (1999)]. The ignition design consists of a cryogenic deuterium-tritium (DT) shell contained within a very thin CH shell. To maintain shell integrity during the implosion, the target is placed on an isentrope approximately three times that of Fermi-degenerate DT (alpha =3). One-dimensional studies show that the ignition design is robust. Two-dimensional simulations examine the effects on target performance due to laser imprint, power imbalance, and inner- and outer-target-surface roughness. Results from these studies indicate that the capsule gain can be scaled to the ice/vapor surface deformation at the end of the acceleration stage of the implosion. The physical reason for gain reduction as a function of increasing nonuniformities is examined. Simulations show that direct-drive target gains in excess of 30 can be achieved for an inner-ice-surface roughness of 1 mum rms, an on-target power imbalance of 2% rms, and by using the beam-smoothing technique SSD with 1 THz and two color cycles. (C) 2001 American Institute of Physics.