Magnetic Fields in Indirect-drive Inertial Confinement Fusion

Plasma magnetisation effects are routinely ignored in the design and interpretation of laser-driven inertial confinement fusion (ICF) experiments. The presented work uses the 3-D extended-magneto-hydrodynamics code Chimera to simulate both the impact of self-generated magnetic fields on the hotspot cooling process and assess the anticipated increase in fusion performance through the application of external magnetic fields.

Magnetic fields are spontaneously generated during ICF implosions by the Biermann battery process when the capsule is not spherically symmetric. During stagnation, the hotspot edge contains large magnetic field intensities, estimated to be up to 10,000T in strength [1]. Subsequent magnetisation of the electron population can reduce thermal conductivities by 90%. Further extended-MHD processes, such as the Nernst term advecting fields out of the hotspot, result in the estimated change in yield due to self-generated fields being small. Righi-Leduc heat-flow also significantly modifies the hotspot cooling process and acts to increase the hot-spot deformation.

Externally-applied magnetic fields can be used to enhance the fusion yield by magnetising both the electron and α-particle populations in the hotspot, increasing the energy containment. Modifications to hotspot shape are explored by using perturbations relevant to the high-foot (radiation asymmetry and tent scar) and HDC (fill-tube) campaigns. High-mode perturbations are also simulated, suggesting reductions of mix into the hotspot due to magnetic tension suppressing vortices. Extended-MHD effects are also important in this regime, with the cross-gradient-Nernst term twisting the magnetic field and enhancing energy containment. Finally, the overall increase in yield by applying magnetic fields to current NIF implosions will be estimated.

[1] C. A. Walsh et al. Physical Review Letters 118, 155001 (2017)