Progress towards a more predictive model for hohlraum radiation drive and symmetry

Abstract

For several years, we have been calculating the radiation drive in laser-heated gold hohlraums using flux-limited heat transport with a limiter of 0.15, tabulated values of local thermodynamic equilibrium gold opacity, and an approximate model for not in a local thermodynamic equilibrium (NLTE) gold emissivity (DCA_2010). This model has been successful in predicting the radiation drive in vacuum hohlraums, but for gas-filled hohlraums used to drive capsule implosions, the model consistently predicts too much drive and capsule bang times earlier than measured. In this work, we introduce a new model that brings the calculated bang time into better agreement with the measured bang time. The new model employs (1) a numerical grid that is fully converged in space, energy, and time, (2) a modified approximate NLTE model that includes more physics and is in better agreement with more detailed offline emissivity models, and (3) a reduced flux limiter value of 0.03. We applied this model to gas-filled hohlraum experiments using high density carbon and plastic ablator capsules that had hohlraum He fill gas densities ranging from 0.06 to 1.6 mg/cc and hohlraum diameters of 5.75 or 6.72 mm. The new model predicts bang times to within ±100 ps for most experiments with low to intermediate fill densities (up to 0.85 mg/cc). This model predicts higher temperatures in the plasma than the old model and also predicts that at higher gas fill densities, a significant amount of inner beam laser energy escapes the hohlraum through the opposite laser entrance hole.

Suppression of the Biermann Battery and Stabilization of the Thermomagnetic Instability in Laser Fusion Conditions

“…These conditions were motivated by radiation hydrodynamics simulations of hohlraums of the type shown in e.g. [23], in which significant volumes of the underdense plasma region satisfy the criteria for instability.…”

Investigation of the performance of mid-Z Hohlraum wall liners for producing x-ray drive

“…The value of the flux-limiter employed in a given simulation is usually adjusted to fit experimental results. Experiments on the NIF have been modelled using a range of flux-limiter values from 3% to 15% of the free-streaming limit [20]. Since the present study is entirely simulation-based, and in order to maintain consistency between the Cu-lined and pure Au hohlraum cases, the same value of 10% is used for both cases.…”

The inadequacy of a magnetohydrodynamic approach to the Biermann battery

“…Figure 1 shows the electron temperature and electric field, produced by CTC, after 1 ns of laser heating, using commonly employed values for the thermal flux limiter f = 0.15 and f = 0.05 [ 8 , 20 ]. From this figure it is clear that the thermal flux limiter makes a large difference to the electric field.…”