Strong shock wave generation by fast electron energy deposition in shock ignition relevant plasmas

The potential role of fast electrons is one of the major unknowns in shock ignition inertial con- finement fusion. Of particular interest is the possibility that they may play a beneficial role in the generation of the ignitor shock by contributing to the ablation pressure. Here, some of the fundamental relations governing fast electron driven shock wave generation in dense plasmas are determined. To that end, a 1D planar hybrid model of fast electron transport through dense plasmas is presented. It is found that, using quasi-realistic electron populations, it is possible to generate shock waves with peak pressures that agree with a simple scaling law and have sustained shock pressures of several hundred Mbars. However, the spatial and temporal scales required for shock waves to fully develop increase with fast electron temperature and can become significant. Careful consideration of this effect is needed when assessing their usefulness as shock wave drivers. A characteristic time of shock wave formation is reinterpreted as the definitive time taken for a localised source of internal energy in an otherwise uniform fluid to drive a blast wave containing its maximum kinetic energy. This relation is of utility in inertial confinement fusion where ignition relies on the conversion of kinetic energy to internal energy at implosion stagnation. However, it is not straightforwardly reproducible by fast electron heating, which highlights the difficulties that may be encountered if fine control over shock wave formation is required. The shape of the density profile seems to be of secondary importance when compared with the consequences of heating using hotter electron populations. When heating times are on the time scale of the ignitor pulse, the density profile affects the efficiency of shock wave formation by determining the transition from an explosive regime to a driven regime of shock wave forma- tion. However, the time taken for the shock wave to contain its maximum kinetic energy is not significantly affected. It is shown that an externally applied magnetic field can constrain the range of fast electrons in solid density planar plastic targets, and enhance localised energy deposition. This mitigates the need for significant spatial and temporal scales when using fast electron populations with extended energy distributions to drive shock waves. However, this comes at the expense of the strength of the shock wave.