The high-foot implosion campaign on the National Ignition Facility

Abstract

The “High-Foot” platform manipulates the laser pulse-shape coming from the National Ignition Facility laser to create an indirect drive 3-shock implosion that is significantly more robust against instability growth involving the ablator and also modestly reduces implosion convergence ratio. This strategy gives up on theoretical high-gain in an inertial confinement fusion implosion in order to obtain better control of the implosion and bring experimental performance in-line with calculated performance, yet keeps the absolute capsule performance relatively high. In this paper, we will cover the various experimental and theoretical motivations for the high-foot drive as well as cover the experimental results that have come out of the high-foot experimental campaign. At the time of this writing, the high-foot implosion has demonstrated record total deuterium-tritium yields (9.3×1015) with low levels of inferred mix, excellent agreement with implosion simulations, fuel energy gains exceeding unity, and evidence for the “bootstrapping” associated with alpha-particle self-heating.

Demonstration of High Performance in Layered Deuterium-Tritium Capsule Implosions in Uranium Hohlraums at the National Ignition Facility

“…Despite the effective hohlraum drive being nearly equal to N131119 (Au) its performance greatly improved (see Table I): The primary neutron yield increased by 50% from 5.2 to 8.0 × 10 15 , and the ion temperature T ion from 4.8 to 5.1 keV. One of the key observables that clearly improved along with implosion performance was the shape of the hot spot, see Fig.…”

“…This can be explained by a combination of impeded inner beam propagation and reduced CBET starting at ∼1.5 ns before the end of the laser drive [15]. N140120 stands out with a 4× larger P 2 swing and, coincidentally, a thinner capsule support tent (16.4 nm vs ∼45 nm).…”

“…1(a)] cooled to 18.6 K at shot time [15,24] containing plastic capsules with 2.25 mm outer diameter and 175μm ("T-1") or 195 μm ("T0") thickness [ Fig. 1(d)].…”

“…Among the benefits are a larger ablator density scale length (reducing susceptibility to ablation front RayleighTaylor growth [14]), and a shorter, simplified laser drive (three shocks rather than four). These benefits come at the price of lower fuel areal density, which requires higher implosion velocity compared to the low-entropy design [7] to achieve ignition.The first high-foot DT implosions [12,13,15] were done in gold (Au) hohlraums. These experiments incrementally increased laser peak power P laser and thus total laser energy from 351 TW=1.3 MJ on N130501 to 428 TW=1.9 MJ on N131119, which is at the current laser energy limit of the National Ignition Facility (NIF).…”

“…The first high-foot DT implosions [12,13,15] were done in gold (Au) hohlraums. These experiments incrementally increased laser peak power P laser and thus total laser energy from 351 TW=1.3 MJ on N130501 to 428 TW=1.9 MJ on N131119, which is at the current laser energy limit of the National Ignition Facility (NIF).…”