Abstract
Recent work in indirectly-driven inertial confinement fusion implosions on the National Ignition Facility has indicated that late-time propagation of the inner cones of laser beams (23° and 30°) is impeded by the growth of a “bubble” of hohlraum wall material (Au or depleted uranium), which is initiated by and is located at the location where the higher-intensity outer beams (44° and 50°) hit the hohlraum wall. The absorption of the inner cone beams by this “bubble” reduces the laser energy reaching the hohlraum equator at late time driving an oblate or pancaked implosion, which limits implosion performance. In this article, we present the design of a new shaped hohlraum designed specifically to reduce the impact of this bubble by adding a recessed pocket at the location where the outer cones hit the hohlraum wall. This recessed pocket displaces the bubble radially outward, reducing the inward penetration of the bubble at all times throughout the implosion and increasing the time for inner beam propagation by approximately 1 ns. This increased laser propagation time allows one to drive a larger capsule, which absorbs more energy and is predicted to improve implosion performance. The new design is based on a recent National Ignition Facility shot, N170601, which produced a record neutron yield. The expansion rate and absorption of laser energy by the bubble is quantified for both cylindrical and shaped hohlraums, and the predicted performance is compared.
Highlights
Recent work in indirectly-driven inertial confinement fusion implosions on the National Ignition Facility has indicated that late-time propagation of the inner cones of laser beams (23 and 30) is impeded by the growth of a “bubble” of hohlraum wall material (Au or depleted uranium), which is initiated by and is located at the location where the higher-intensity outer beams (44 and 50) hit the hohlraum wall
We present the design of a new shaped hohlraum designed to reduce the impact of this bubble by adding a recessed pocket at the location where the outer cones hit the hohlraum wall
Inertial confinement fusion (ICF) implosion experiments are being conducted at the National Ignition Facility (NIF)1 with a goal of compressing a spherically layered cryogenic shell of deuterium tritium (DT) fuel2 to a sufficient areal density to inertially confine the hot fuel for a sufficient duration to sustain a self-propagating burn wave
Summary
Inertial confinement fusion (ICF) implosion experiments are being conducted at the National Ignition Facility (NIF) with a goal of compressing a spherically layered cryogenic shell of deuterium tritium (DT) fuel to a sufficient areal density (qR) to inertially confine the hot fuel for a sufficient duration to sustain a self-propagating burn wave. Depending on the hohlraum and implosion details, the inner cone energy can become completely blocked, and the resulting lack of drive at the equator generates a non-recoverably oblate (or pancaked) implosion core This impaired inner-beam propagation appears to be a reproducible phenomenon across a wide range of low to intermediate gas-filled hohlraums as documented in Ref. 9 with fill densities ranging from 0.3 to 0.6 mg/cc, capsule ablator materials varying from CH to Be to high-density carbon (HDC), and hohlraum sizes ranging from 5.75 to 6.72 mm diameter. Other options for mitigating the Au bubble in low gasfilled hohlraums include adding a low-density foam material, again to tamp the bubble expansion, but in a more localized manner Such a technique is currently being investigated experimentally on NIF in hohlraums with reduced gas fill density ($0.3 mg/cc), and may be able to minimize the detrimental impacts of LPI (Laser Plasma Interaction) caused by higher gas fills. These are still in the testing phase, but do raise additional difficulties in target fabrication and increase concerns about debris from the fragile foam structures falling on and perturbing the implosion of the capsule
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