Transient magnetic field diffusion considerations relevant to magnetically assisted indirect drive inertial confinement fusion

Abstract

Application of a magnetic field to an indirect drive inertial confinement fusion target requires diffusion of the field through the high-Z and electrically conducting Hohlraum. The onset of the external field generates eddy currents in the Hohlraum wall that result in (1) a reduction of the peak field at the capsule, (2) heating of the Hohlraum wall through Ohmic dissipation, and (3) wall movement due to the inward force from the eddy current interacting with the field. Heating of the wall causes an increase in blackbody radiation which can preheat the capsule and frozen deuterium–tritium fuel, while wall motion leads to potential misalignment of the lasers at the Hohlraum wall. Limiting these detrimental effects sets requirements on the tolerable magnitude of each effect. We present a nonlinear model for B-field diffusion through an infinitely long thin-walled cylinder with a temperature dependent resistivity, to show that a 15 μm thick wall of pure gold fails to meet these requirements. A new Hohlraum material made from an alloy of Au and Ta has a measured resistivity of ≥60 times that of Au and is shown with the nonlinear model to meet the requirements for magnetization. We compare the nonlinear model to simulations of the actual Hohlraum target using a finite element code which includes temperature-dependent Hohlraum resistivity.

Increased Ion Temperature and Neutron Yield Observed in Magnetized Indirectly Driven D2 -Filled Capsule Implosions on the National Ignition Facility

“…The hohlraum is ∼20 μm thick of a novel high electrical resistivity, ∼200 μΩ cm, alloy of 20% atomic Au and 80% atomic Ta [33][34][35][36] (pure Au has resistivity ∼2 μΩ cm) with a 120 μm thick epoxy overcoat for mechanical stability. The B-field diffusion time through the hohlraum wall is measured with a B-dot probe to be about 20 ns and matches the theoretical estimate [37]. The hohlraum is magnetized by running current through 26-gauge Kapton-coated Cu wire wound on the hohlraum with 3 turns above and 3 below the midplane [see Fig.…”

Boosting Inertial-Confinement-Fusion Yield with Magnetized Fuel

“…The theoretical part of this work showed that implosions close to triggering a fusion burn can be pushed over the threshold with a 50-T seed field in the fuel. A second three-year project at LLNL has recently started with the goal of demonstrating key scientific elements of magnetized NIF implosions experimentally [9]. If magnetized fuel in a NIF implosion shows close to the anticipated performance improvement, then this technology will become available to augment any implosion design on NIF.…”

Tantalum Suboxide Films with Tunable Composition and Electrical Resistivity Deposited by Reactive Magnetron Sputtering

“…Tantalum oxide (Ta 2 O 5 ) films are used for a wide range of (opto)electronic applications due to useful properties, including a high refractive index, optical transmission, and dielectric constant [1][2][3]. Tantalum-based alloys are also attractive hohlraum materials for magnetized indirect-drive inertial confinement fusion (ICF) [4][5][6][7]. A hohlraum is a heavy-metal-based sphero-cylindrical can, typically ∼5 mm in diameter and ∼10 mm long, which hosts a hydrogen fuel capsule [8].…”

“…In magnetized ICF, a seed magnetic field of ∼50 Tesla is applied shortly before the implosion. During the implosion the magnetic flux density in the capsule is significantly amplified, reaching values in excess of 10 kT, positively influencing the implosion [4,5,9]. Such pulsed magnetic fields are generated by a solenoid wound around the hohlraum, which in turn necessitates that the hohlraum have a high electrical resistivity (ρ) of 100 µΩ cm at cryogenic temperatures [4,5].…”

“…During the implosion the magnetic flux density in the capsule is significantly amplified, reaching values in excess of 10 kT, positively influencing the implosion [4,5,9]. Such pulsed magnetic fields are generated by a solenoid wound around the hohlraum, which in turn necessitates that the hohlraum have a high electrical resistivity (ρ) of 100 µΩ cm at cryogenic temperatures [4,5]. The typical hohlraum materials, pure Au and U, have rather low ρ in these conditions, which impedes the soak in of the magnetic field flux at the microsecond time scales of magnetic field transients [4,5].…”