High-Adiabat High-Foot Inertial Confinement Fusion Implosion Experiments on the National Ignition Facility

Abstract

This Letter reports on a series of high-adiabat implosions of cryogenic layered deuterium-tritium (DT) capsules indirectly driven by a "high-foot" laser drive pulse at the National Ignition Facility. High-foot implosions have high ablation velocities and large density gradient scale lengths and are more resistant to ablation-front Rayleigh-Taylor instability induced mixing of ablator material into the DT hot spot. Indeed, the observed hot spot mix in these implosions was low and the measured neutron yields were typically 50% (or higher) of the yields predicted by simulation. On one high performing shot (N130812), 1.7 MJ of laser energy at a peak power of 350 TW was used to obtain a peak hohlraum radiation temperature of ∼300 eV. The resulting experimental neutron yield was (2.4±0.05)×10(15) DT, the fuel ρR was (0.86±0.063) g/cm2, and the measured Tion was (4.2±0.16) keV, corresponding to 8 kJ of fusion yield, with ∼1/3 of the yield caused by self-heating of the fuel by α particles emitted in the initial reactions. The generalized Lawson criteria, an ignition metric, was 0.43 and the neutron yield was ∼70% of the value predicted by simulations that include α-particle self-heating.

On the importance of minimizing “coast-time” in x-ray driven inertially confined fusion implosions

“…680 ns. Also, as noted previously 18 fuel compression was observed to dramatically improve when coast-time was reduced from $2 to < 1 ns.…”

“…The 2D simulations, with burn-on, are a much better match to the inferred data down to coast-times of 750 ps, but are optimistic for coast-times < 750 ps. Observations [13][14][15][16]18,25 show that the high-foot campaign implosions were asymmetric, so it is expected that 2D simulations would be a better match than 1D as been shown previously. 11,22 The relation shown in Eq.…”

“…19,20 So, it appears that the only way a change in drive conditions (i.e., ablationpressure) associated with coast-times $500 ps could impact the implosions would be if the ablator thickness is actually thinner than modeled in the 1D simulation above or if the implosions were actually asymmetric [either thin-regions in the shell (DT fuel þ remaining ablator) of the implosion or overall hot-spot asymmetry] which would allow some characteristics (pressure disturbances) to arrive in the hot-spot sooner than in an equivalent 1D implosion. In fact it is known that the high-foot experiments had, in some cases, thinner ablators 14 (by design) and were asymmetric [13][14][15][16]18,25 (not by design) in many cases.…”

“…Implosion data from the high-foot implosion campaign [13][14][15][16][17][18] showed that many ignition relevant parameters (neutron yield, stagnation pressure, etc.) varied strongly with coast-time 19,20 especially as t coast was reduced to a mere hundreds of pico-seconds.…”

Modeling of Intense Laser Driven High Energy Density Plasmas

“…1.2. Relating to these issues, significant progress has been made in the two year period since the the end of NIC in 2012 [78,158,172,99,41,42,45,44], and the promise of the indirect drive ICF approach to achieving fusion energy gain remains substantial.…”