Numerical studies on the formation process of Z-pinch dynamic hohlruams and key issues of optimizing dynamic hohlraum radiation

Dynamic hohlraum is a possible selection to drive inertial confinement fusion. Currently, the ~8 MA PTS facility in China has been completed, which provides a powerful experimental platform of relatively large drive current for researches of dynamic hohlraums and dynamic hohlraum driven inertial fusion. To understand the formation processes and the main characteristics of the dynamic hohlraum, and explore the most important issues affecting the optimization of hohlraum radiation, is not only fundamental in the research of dynamic hohlraums, especially for the experimental design, but also can provide a physical insight for the experimental diagnosis. In this paper the implosion dynamics of a tungsten wire-array Z-pinch embedded with a CH foam converter, especially the impaction interaction of the wire-array plasma with the converter plasma, is numerically investigated using a one-dimensional non-equilibrium radiation magnetohydrodynamic code. In simulations the tungsten plasma is assumed as a plasma shell with a width of 1 mm, and the CH converter plasma is assumed to be uniform with an initial temperature of 0.1 eV. The overall implosion is driven by an assumed current with a peak value of 8 MA and a rise time of 66.4 ns. It is shown that a local high pressure region, which is generated by the impaction of the tungsten plasma with the converter plasma, is crucial to launch the strongly radiating shock wave and to form the dynamic hohlraum. Due to the supersonic radiation transfer in the low opacity CH converter plasma, which is also produced in the high pressure region, there exists a hohlraum region inside the front of the shock wave, in which the radiation is high. At the same time, the plasma pressure is uniform in this hohlraum region, so the plasma will not be disturbed before the shock arrives. As the shock propagates to the axis, the hohlraum becomes small and the radiation temperature is also increased. Basically, the hohlraum radiation is determined by the detailed profiles of plasma conditions when the wire-array plasma impacts onto the CH converter plasma. And these profiles are determined by many factors, such as the drive current, initial masses and radii of the wire-array and the converter, as well as the material of the converter. When the drive current is fixed, the optimal wire-array can be determined. Firstly, the mass ratio of the wire-array to the CH converter is varied. Numerical calculations show that as this ratio is decreased, the shock velocity is increased and the radiation temperature is increased as well. Additionally, the time duration of the radiation pulse before the shock arrives at the axis is remarkably increased. It is also found that when this mass ratio is slightly lower than unity, for example 0.75, a relative optimal dynamic hohlraum can be produced. Secondly, if the mass ratio is fixed and the initial radius of the converter is decreased, it is found that the shock velocity is just slightly changed. However, the peak hohlraum radiation temperature is increased and the radiation pulse becomes remarkably narrow. A suitable radius ratio of the wire-array to the converter, neither too large to induce strong Magneto-Rayleigh-Taylor (MRT) instability nor too small to gain a small kinetic energy of the wire-array before impacting onto the converter surface, should be selected. In the future we will develop two-dimensional code to investigate the effect of MRT instability on the formation of dynamic hohlraums.