Indirect-drive noncryogenic double-shell ignition targets for the National Ignition Facility: Design and analysis

Abstract

The central goal of the National Ignition Facility (NIF) is demonstration of controlled thermonuclear ignition. The mainline ignition target is a low-Z, single-shell cryogenic capsule designed to have weakly nonlinear Rayleigh-Taylor growth of surface perturbations. Doubleshell targets are an alternative design concept that avoids the complexity of cryogenic preparation but has greater physics uncertainties associated with performance-degrading mix. A typical double-shell design involves a high-Z inner capsule filled with DT gas and supported within a low-Z ablator shell. The largest source of uncertainty for this target is the degree of highly evolved nonlinear mix on the inner surface of the high-Z shell. High Atwood numbers and feed-through of strong outer surface perturbation growth to the inner surface promote high levels of instability. The main challenge of the double-shell target designs is controlling the resulting nonlinear mix to levels that allow ignition to occur.Design and analysis of a suite of indirect-drive NIF double-shell targets with hohlraum temperatures of 200 eV and 250 eV are presented. Analysis of these targets includes assessment of two-dimensional radiation asymmetry as well as nonlinear mix. Twodimensional integrated hohlraum simulations indicate that the x-ray illumination can be adjusted to provide adequate symmetry control in hohlraums specially designed to have high laser-coupling efficiency [Suter et al., Phys. Plasmas 5, 2092(2000l. These simulations also reveal the need to diagnose and control localized 10-15 keV x-ray emission from the high-Z hohlraum wall because of strong absorption by the high-Z inner shell. Preliminary estimates of the degree of laser backscatter from an assortment of laser-plasma interactions suggest comparatively benign hohlraum conditions. Application of a variety of nonlinear mix models and phenomenological tools, including buoyancy-drag models, multimode simulations and fallline optimization, indicates a possibility of achieving ignition, i.e., fusion yields greater than 1 MJ. Planned experiments on the Omega laser to test current understanding of high-energy radiation flux asymmetry and mix-induced yield degradation in double-shell targets are described.

Nonlinear Structure of the Diffusing Gas-Metal Interface in a Thermonuclear Plasma

“…It is the purpose of this Letter to work out the basic structure of this layer as it develops from an initially sharp interface-a problem that has not previously been studied. We focus on heavy metal pushers as used in the double shell capsule designs [4][5][6].…”

Hohlraum-Driven Ignitionlike Double-Shell Implosions on the Omega Laser Facility

“…The second (and related) goal is the control of mix between the fuel and pusher (induced by Rayleigh-Taylor instability growth) to ensure appreciable compression neutron production. A useful figure-of-merit for controlling mix is the "fall-line" delay to the origin [5]. Physically, the fall-line is the trajectory of free-falling interfacial material after deceleration onset.…”

“…A key feature of these experiments is that preheat M-band x-ray radiation (2-5 keV) from the laser-irradiated gold hohlraum walls causes the inner shell to expand appreciably before arrival of the ablatively-driven first shock, resulting in only premature "first-shock" [11] neutron burn. However, a key ingredient of ignition-like behavior is the participation of a second shock originating from the rarefaction fan in the outer shell [5].…”

“…Although the gain (of 2 to 4) is comparatively low with current double-shell target designs [4,5] for the National Ignition Facility (NIF) [6], the lower threshold ignition temperature of ≈4 keV makes ignition easier by relaxing the requirements on implosion symmetry. However, the main challenge with double-shell ignition is the required control of mix of high-Z pusher material and DT fuel to low levels.…”

“…First, the goal was to have the compression stage of (clean) neutron production largely dominate the earlier shock-flash neutron burst in order to mimic the behavior of a proposed igniting double-shell on the NIF [5]. The second (and related) goal is the control of mix between the fuel and pusher (induced by Rayleigh-Taylor instability growth) to ensure appreciable compression neutron production.…”