Magnetic fields and thermal flux inhibition in inertial confinement fusion

Of critical concern in directly driven laser fusion systems is the understanding of electron thermal transport between the region where laser light is absorbed and the ablation layer where the high pressures which accelerate the imploding shell are generated. Evidence has accumulated over the past several years which indicates that the heat flow may be strongly inhibited. Computer hydrodynamics models used to interpret experimental results have typically employed, in an ad hoc fashion, strong flux-limited diffusion which has led to fairly widespread acceptance of a flux inhibition value near f=0.03. It is shown in this paper that the observed flux inhibition can stem directly from the strong magnetic fields generated at the ablation layer as a result of modest laser asymmetries. These fields are shed from the ablation layer and fill the overdense region which strongly influences the thermal transport. The self-consistent numerical simulation of thermal transport in this environment shows strong thermal flux inhibition as interpreted from the following computational measurements: (1) reduced ablation pressures, (2) reduced implosion velocities, (3) reduced mass ablation rates, (4) density profile flattening, and (5) reduced classical absorption; all of which have been experimentally observed. The mass ablation rates obtained from the self-consistent two-dimensional model agree well with a one-dimensional model using an imposed flux inhibition factor of 0.03. (author)