Implementing a microphysics model in hydrodynamic simulations to study the initial plasma formation in dielectric ablator materials for direct-drive implosions

A microphysics model to describe the photoionization and impact ionization processes in dielectric ablator materials like plastic has been implemented into the one-dimensional hydrodynamic code LILAC for planar and spherical targets. At present, the initial plasma formation during the early stages of a laser drive are modeled in an ad hoc manner, until the formation of a critical surface. Implementation of the physics-based models predict higher values of electron temperature and pressure than the ad hoc model. Moreover, the numerical predictions are consistent with previous experimental observations of the shinethrough mechanism in plastic ablators. For planar targets, a decompression of the rear end of the target was observed that is similar to recent experiments. An application of this model is to understand the laser-imprint mechanism that is caused by nonuniform laser irradiation due to single beam speckle.

Figure 1

An outline of the regions where the microphysics model and the normal LILAC method is implemented after the critical surface is formed. In the figure, laser light is incident on the target from the right.

Figure 2

A schematic of the photoionization, impact ionization, and recombination processes that determine the free electron density in the conduction and valence band of the material.

Figure 3

A flowchart detailing the different physical processes and the implementation sequence inside LILAC.

Figure 4

(a) A solid plastic sphere of $430\text{−}\mu \mathrm{m}$ radius is irradiated with (b) a laser pulse of 250-J energy.

Figure 5

The plasma profiles from the microphysics model and the ad hoc model are plotted in red and blue respectively. The laser light is incident on the target from the right. (a)–(c) The top row shows the laser intensity deposition profiles (in dashed red for microphysics model and solid blue circles for ad hoc model) and the corresponding free electron density (in solid colors). The critical surface formation occurs when the free electron density rises to $9\times {10}^{21}{\mathrm{cm}}^{-3}$ for UV light. (d)–(f) The middle row shows the rise in the electron temperature (in solid) predicted by the microphysics model and the ion temperatures (in dashed). (g)–(i) The mass density profile (in solid) and the difference in the pressure profiles (in dashed) is evident in the lowermost row.

Figure 6

The fraction of the incident laser energy absorbed over time. The absorption fraction between the microphysics model and the ad hoc model is different initially since the energy deposition is initially restricted to the surface of the target for the ad hoc model. Beyond the critical surface formation, the absorption profiles are the same since the plasma profile in the ablation region is controlled by the ad hoc model.

Figure 7

Pulse shape used to irradiate a planar plastic foil of $37\text{−}\mu \mathrm{m}$ thickness.

Figure 8

The plasma profiles from the microphysics model and the ad hoc model are plotted in red and blue respectively. The laser light is incident on the target from the right. (a)–(c) The top row shows the laser intensity deposition profiles (in dashed red for microphysics model and filled blue circles for ad hoc model) and the corresponding free electron density (in solid colors). (d)–(f) The middle row shows the rise in the electron temperature (in solid) predicted by the microphysics model and the ion temperatures (in dashed). (g)–(i) The mass density profile (in solid) and the difference in the pressure profiles (in dashed) is evident in the lowermost row. Note that the plastic is released in the rear end of the foil before the shock wave arrives due to the increase in the electron densities from the laser energy deposition. This leads to the material release over a larger length scale at 2 ns in the rear end of the foil as predicted by the microphysics model.

Figure 9

The temporal distribution of the fraction of laser energy absorbed by the microphysics model shows lower absorption than the ad hoc model initially until the critical surface formation. Over time, that the absorption profile has a similar trend as the distance between the critical surface from the two methods decreases and they overlap.