Approaching a burning plasma on the NIF

Abstract

The locus of National Ignition Facility (NIF) inertial confinement fusion (ICF) implosion data, in hot-spot burn-average areal density (ρR) and Brysk temperature (T) space, is shown and illustrates that several implosions are nearing a burning plasma state, where α-heating is the dominant source of plasma heating. A formula for diagnosing a burning plasma using measured/inferred data from ICF implosion experiments is given with the underlying derivation. Plotting ICF implosion performance against inferred hot-spot energy illustrates the key need to maximize the delivery of energy to an implosion hot-spot. A very compact analytical equation for α-heating is given, which shows that fundamentally, α-heating provides a discrete “boost” to pVγ of an implosion hot-spot. It is then shown numerically that the analytical expression for the amplification of pVγ is simply related to yield amplification and to other more famous metrics used for inferring yield amplification in experiments. Interestingly, the argument of the pVγ boost equation appears to provide a fundamental ignition condition that implicitly includes the effects of asymmetry, and it also exposes the origin of why there is uncertainty in defining single ignition criteria. The resulting analysis of NIF implosion data indicates that an increase in burn-average hot-spot temperature will be needed in order to ignite, and the strategy being pursued to achieve this goal is outlined.

Preparations for a European R&D roadmap for an inertial fusion demo reactor

“…A recent study suggests that the resulting mix of ablator material and the dense fuel into the hot spot significantly raises the hot spot’s fuel adiabat (i.e. the ratio α of the fuel pressure to the ideal Fermi degeneracy pressure) from the designed low-level of α = 1.5 to an estimated [ 2 , 16 ]. This is due to the combined effects of fuel preheat, vorticity and mix of fuel and pusher (due to the Rayleigh–Taylor hydrodynamic instability and residual drive asymmetries) [ 17 – 19 ].…”

“…The experimental data are consistent with the observed trend that higher adiabat target designs perform closer to predictions than those designed for a lower adiabat. Following the first entry into this regime in 2014, there has been substantial progress in improving the fusion yield by the use of improved target and laser pulse-shape designs [ 14 , 16 , 18 ]. These advances indicate that entry into the burning plasma regime, where alpha-particle energy deposition fully dominates the hot-spot hydrodynamics, is tantalizingly close.…”

Energy transfer between lasers in low-gas-fill-density hohlraums

“…An excess of drive on the waist of the hohlraum can be used advantageously to drive larger capsules which reduces the energy requirement for ignition due to higher convergence. Using δn sat e /n e = 2 − 2.5x10 −3 we estimate that transfer to the inner beams, resulting in 1.7-1.9x increase in average laser power, could enable driving >25% larger capsules (>1400µm in outer radius), with the same thickness, which could result in more than 2.5 times yield improvement from analytical scaling of yield with capsule size [49] and more if alpha heating is considered. For this design study ∆Λ = 5Å between the inner and outer beams was used, consistent with N161103, which gave reduced laser backscatter compared to N170706 [42].…”

Scaling of Laser Fusion Experiments for DD-Neutron Yield

“…Many efforts have been devoted to optimizing the heating of fusion targets by laser pulses to initiate different fusion reactions producing e.g., neutrons and alpha particles through fusion DD, DT and p 11 B reactions that carry released energy [4][5][6]. One of the important objectives for developing a usable fusion energy source is just to find an effective way to enhance the laser energy coupling to the target in the context of inertial confinement fusion research [7].…”