Numerical studies on dynamics of Z-pinch dynamic hohlraum driven target implosion

The dynamic hohlraum is a possible approach to driving inertial confinement fusion. Recently, dynamic hohlraum experiments on the primary test stand (PTS) facility were conducted, and preliminary results show that a dynamic hohlraum is formed, which can be used for driving target implosion. In this paper, the implosion dynamics of Z-pinch dynamic hohlraum driven target implosion with the drive current of PTS facility is numerically investigated. A physical model is established, in which a dynamic hohlraum is composed of a cylindrical tungsten wire-array and a CHO foam converter, and the target is composed of a high density CH ablator and low density DT fuel. The drive current is calculated by an equivalent circuit model, and the integrated simulations in (r, Z) plane by using a two-dimensional radiation magneto-hydrodynamics code are performed to describe the overall implosion dynamics. It is shown that the wire-array plasma is accelerated in the run-in stage, and in this stage the target keeps almost immobile. As the accelerated wire-array plasma impacts onto the low-density foam converter, a local region with high temperature and high pressure is generated near the W/CHO boundary due to energy thermalization, and this thermalization process will last several nanoseconds. This high temperature region will launch a strongly radiating shock. At the same time, high temperature radiation also appears and transfer to the target faster than the shock. When the high temperature radiation transfers to the surface of the target, the ablator is heated and the ablated plasma will expand outward, and a high-density flying layer will also be generated and propagate inward. After the high-density layer propagates to the ablator/fuel boundary, the DT fuel will be compressed to a high-density and high-temperature state finally. At the same time, the cylindrical shock, which is generated from the impact of the wire-array plasma on the foam converter, will gradually propagate to the ablator plasma. After it propagates over the converter/ablator boundary, it will be decelerated by the ablation pressure, which is beneficial to isolating the fuel compression from the direct cylindrical shock. It is shown that though the trajectories of the outer boundaries of the ablator at the equator and at the poles are completely different due to shock interaction at the equator, the fuel compression is nearly uniform due to radiation compression. It is shown that the asymmetry of fuel compression is mainly caused by the non-uniformity of the hohlraum radiation at the equator and at the poles. Generally, there are two differences between the radiation temperatures at the equator and at the poles, namely the time difference due to the finite velocity of radiation transfer, and the peak temperature difference due to energy coupling. If the target is small, the peak radiation temperature at the equator is almost the same as at the pole. The fuel at the equator is first compressed just because the radiation first transfers to the target equator. As the size of the target is increased, the difference in peak radiation temperature will be more serious, thus causing weaker fuel compression at the equator than at the poles. Certainly, if the target size is too large, the cylindrical shock will directly interact on the target at the equator, resulting in complete asymmetry at the equator with respect to the shock at the poles, which should be avoided. Furthermore, it is shown that as the target size is increased, the final neutron yield will first increase and then decrease, which means that there is a relatively optimal size selection for target implosion.