A key step on the way to ignition on NIF is for the energy generated through fusion reactions in an inertially confined fusion plasma to exceed the amount of energy deposited into the deuterium-tritium (DT) fusion fuel and hot spot during the implosion of the target capsule—a condition known as “fuel gain.” An article in the Feb. 12 online issue of the journal Nature reports that through the use of the “high-foot” implosion process developed by the NIF/Weapons and Complex Integration team, fusion fuel gains exceeding unity have been achieved for the first time on any facility.
The experiments show an order-of-magnitude improvement in yield performance over previous NIF shots as well as a significant contribution to the yield from alpha-particle self-heating, in which the alpha particles (helium nuclei) produced in the DT fusion process deposit their energy in the DT fuel. The alpha particles further heat the fuel, increasing the rate of fusion reactions, thus producing more alpha particles. This “bootstrapping” process is the mechanism required to accelerate the DT fusion burn rate to eventual self-sustaining fusion burn and ignition.
The high-foot experiments feature a higher initial laser pulse than previous “low-foot” shots, shorter pulse duration, and three laser shocks on the target rather than four. This configuration is designed to reduce ablation-front instability growth and thereby inhibit plastic ablator material from mixing into and contaminating the DT hot spot. The essential stability benefits of the high-foot scheme can be seen in a reduction in the linear growth rate of ablation-driven Rayleigh-Taylor hydrodynamic instability.
“The increase in density gradient scale-length of the ablation front is primarily due to the stronger first shock, which increases the adiabat (internal capsule energy) of the implosion and prevents the ablator from becoming so highly compressed (risking break-up) during the implosion,” the authors said. For more information, see the LLNL News Release.
Results of Initial High-Foot Experiments Also Reported in Physical Review Letters
The performance of the first DT layered implosions on NIF using a high-foot, higher adiabat (internal capsule energy) design also was reported in a Physical Review Letters paper published online on Feb. 5.
The intensity of the laser pulse delivered to the hohlraum changes with time. Previous NIF experiments used a “low-foot” drive (dotted yellow line), in which the first stage of the pulse (the “foot”) delivered a relatively low power. Using a high-foot drive (solid yellow line) instead delivered a higher neutron yield that was more consistent with simulations. The radiation temperature, which characterizes the radiation field in the hohlraum that drives the implosion, is plotted against time. (Credit: APS/Joan Tycko) The paper also was the subject of a “Viewpoint” article, “Encouraging Signs on the Path to Fusion,” by Steven Rose, professor of plasma physics at Imperial College London, in the American Physical Society’s online journal Physics.
The authors said the high-foot implosions generated YOC (yield over simulation predictions of yield) greater than 50 percent, bringing experiment and simulations into closer agreement. DT neutron yields exceeding 1015 (one quadrillion) in cryogenic layered implosions were achieved for the first time; and a 50-percent yield boost due to self-heating of the fuel by alpha particles (helium nuclei) emitted in the initial reactions was observed, another first.
“The results have been very encouraging,” the authors said. “The comparison with the low-foot, low-adiabat series of implosions is interesting and instructive.” The high-foot series used a shorter, three-shock drive with a higher foot, and produced a higher fuel adiabat, higher yields, less mixing of ablator material into the DT hot spot, but modest fuel areal densities. The low-foot series of implosions used a longer, four-shock drive, and produced a lower fuel adiabat, lower yields, higher hot-spot mix, but higher fuel areal densities.
“Creating a higher adiabat and higher radiation temperature early in the drive appears to be an effective means for reducing hot-spot mix by reducing ablation front (hydrodynamic) instability growth,” the authors said. “Future work will focus on avenues for increasing the fuel velocity and areal density while holding ablation-front (instability) growth and hot-spot mix under control, to approach the conditions required for ignition.”
A companion paper describing the design and theory behind the high-foot experiments was published in the same issue of Physical Review Letters.
