Investigation of ion kinetic effects in direct-drive exploding-pusher implosions at the NIF

Abstract

Measurements of yield, ion temperature, areal density (ρR), shell convergence, and bang time have been obtained in shock-driven, D 2 and D 3 He gas-filled "exploding-pusher" inertial confinement fusion (ICF) implosions at the National Ignition Facility (NIF) to assess the impact of ion kinetic effects. These measurements probed the shock convergence phase of ICF implosions, a critical stage in hot-spot ignition experiments. The data complement previous studies of kinetic effects in shock-driven implosions. Ion temperature and fuel ρR inferred from fusion-product spectroscopy are used to estimate the ion-ion mean free path in the gas. A trend of decreasing yields relative to the predictions of 2D draco hydrodynamics simulations with increasing Knudsen number (the ratio of ion-ion mean free path to minimum shell radius) suggests that ion kinetic effects are increasingly impacting the hot fuel region, in general agreement with previous results. The long mean free path conditions giving rise to ion kinetic effects in the gas are often prevalent during the shock phase of both exploding pushers and ablatively-driven implosions, including ignition-relevant implosions.

Assessment of ion kinetic effects in shock-driven inertial confinement fusion implosions using fusion burn imaging

“…Ratio of measured DD fusion yields to hydrodynamics-simulated yields (yield over clean, YOC) as a function of the Knudsen number (NK ) for an indirectdrive exploding pusher on NIF (red diamond), 23 three polardirect-drive (PDD) 24 exploding pushers on NIF, 22 and directdrive exploding pushers on OMEGA (green circles). 6 Fusion burn profile measurements of the OMEGA experiments are described herein.…”

“…The trend of measured yields relative to hydrodynamicallysimulated yields with respect to Knudsen number in these experiments on the OMEGA laser facility, 20 as well as in other exploding pusher experiments at the National Ignition Facility (NIF), 21 previously shown in Ref. [22], is presented for background in Figure 1. Overall, these different experiments illustrate how ion kinetic effects start to become significant in implosions with N K > ∼ 0.1.…”

Refractive Index Seen by a Probe Beam Interacting with a Laser-Plasma System

“…However, assessing this quantitatively requires multi-ion-fluid simulations and is considered outside the scope of this study. Species separation is an increasingly active field of research in the ICF community [28][29][30][31][32].…”

Note: A monoenergetic proton backlighter for the National Ignition Facility

“…This relative timing is consistent with earlier observations. 22 The spatial profile of D 3 He proton emission was measured with a variant of the proton penumbral imaging setup fielded previously at the OMEGA laser. [37][38][39] In this case, two orthogonal images were obtained simultaneously: one along the polar axis (θ = 0 • ), and one along an equatorial axis (θ = 90 • , ϕ = 78.75 • ).…”

“…Mono-energetic proton sources have been wellcharacterized at the OMEGA laser facility, 21 and generation of protons at the NIF has been demonstrated using all 192 NIF beams. 22 This note reports the spectral, spatial, and temporal measurements of 15 MeV proton emission from a capsule implosion using only 24 beams at the NIF, which leaves 168 beams in reserve to drive a subject target.…”