Near-vacuum hohlraums for driving fusion implosions with high density carbon ablatorsa)

Abstract

Recent experiments at the National Ignition Facility [M. J. Edwards et al., Phys. Plasmas 20, 070501 (2013)] have explored driving high-density carbon ablators with near-vacuum hohlraums, which use a minimal amount of helium gas fill. These hohlraums show improved efficiency relative to conventional gas-filled hohlraums in terms of minimal backscatter, minimal generation of suprathermal electrons, and increased hohlraum-capsule coupling. Given these advantages, near-vacuum hohlraums are a promising choice for pursuing high neutron yield implosions. Long pulse symmetry control, though, remains a challenge, as the hohlraum volume fills with material. Two mitigation methodologies have been explored, dynamic beam phasing and increased case-to-capsule ratio (larger hohlraum size relative to capsule). Unexpectedly, experiments have demonstrated that the inner laser beam propagation is better than predicted by nominal simulations, and an enhanced beam propagation model is required to match measured hot spot symmetry. Ongoing work is focused on developing a physical model which captures this enhanced propagation and on utilizing the enhanced propagation to drive longer laser pulses than originally predicted in order to reach alpha-heating dominated neutron yields. V C 2015 AIP Publishing LLC. INTRODUCTIONAchieving ignition-where the energy output from inertial confinement fusion (ICF) reactions is higher than the energy input to drive the reaction-requires reaching sufficiently high implosion densities and temperatures. Proximity of the implosion to ignition conditions can be characterized through the Ignition Threshold Factor (ITF) 1-3 ITF / v 8 a À4 1 À 1:2 DR R hotspot 4 M clean M DT 0:5 ;(1)where v denotes the implosion velocity; a is the adiabat at peak velocity; the third term captures degradation due to deviation (DR) from spherical symmetry (where R is the 1D hotspot radius); 4,5 M clean is the mass of fuel without ablator mix; and M DT is the initial mass of deuterium-tritium fuel. As denoted through Eq. (1), reaching ignition has a strong dependence on implosion velocity, which highlights the need for a highly efficient hohlraum to drive the implosion in indirectdrive ICF. 6 In this paper, we describe recent efforts at the National Ignition Facility (NIF) 7,8 to develop a more efficient hohlraum with progressively reduced adiabat implosions as well as associated methodologies for symmetry control. The NIF is composed of 192 laser beams that, after frequency tripling to 351 nm, are incident on the two endcaps (laser entrance holes or LEHs) of a gold, cylindrical hohlraum. Half of the beams enter each LEH and propagate to the hohlraum wall where conversion to x-ray radiation occurs. This radiation then ablates and implodes a deuterium-tritium (DT) fuel-filled capsule. On the NIF, the laser beams are grouped into "inner" and "outer" beams, which are pointed to provide x-ray drive to the waist and pole, respectively, of the imploding capsule.Low-Z gas-filled hohlraums are typically utilized due to the pulse length...

Symmetry control in subscale near-vacuum hohlraums

“…One approach to mitigating this is known as dynamic beam phasing, wherein the late "pole-hot" drive is compensated for by an earlier "waist-hot" radiation flux in order to obtain a round hot spot at peak compression. 17 However, recent work has shown that radiation flux symmetry swings can degrade performance (even if the hot spot is round at the time of peak emission) due to deviation between the hot spot core shape and the fuel areal density at peak compression, which results in poorly stagnating implosions (residual kinetic energy). 10 An alternative mitigation strategy is to provide more room for inner beam propagation by increasing the case-tocapsule ratio (CCR).…”

Cryogenic tritium-hydrogen-deuterium and deuterium-tritium layer implosions with high density carbon ablators in near-vacuum hohlraums

“…This method, called dynamic beam phasing, is described in more detail in a paper by Berzak Hopkins. 20 The inner-cone fraction for the first 2.5 ns of N130920 is indeed 20%, an inner:outer balance of 1:4.…”

“…Recent experiments on the NIF in near-vacuum hohlraums have typically resulted in more prolate ("sausage") implosions than predicted by HYDRA. 20 The basic physics of symmetry control for vacuum hohlraums is essentially the same as for more conventional gas-filled hohlraums. The NIF laser beams are grouped into an inner cone with polar angles of 23.5 and 30 relative to the hohlraum axis and an outer cone with polar angles of 44.5 and 50 relative to the hohlraum axis.…”

“…It has been known for some time that the interaction of ablated gold and capsule material in a vacuum hohlraum is initially collisionless, 24 as the ion-ion mean-free-paths of such flows are hundreds of microns. 20 Dealing with this transition in a physically meaningful way is an active area of research in the indirect drive ICF program.…”

“…The inner and outer laser pulses for the experiments described here utilized "dynamic beam phasing" to generate a round hot spot at stagnation. 20 In this case, the inner cone laser beam power is increased faster than the outer cone power during the rise to peak power, when the laser propagates freely to the hohlraum wall. This is illustrated in Fig.…”