Formation Mechanism of Dense High-Speed Plasma Flow in Tapered Capillary Discharge

High-speed plasma flows have attracted much interest for conducting laboratory astrophysics experiments [1]. The formation of dense, high-speed plasma is essential for those applications and the investigation of related plasma phenomena. Devices such as a laser [2], z-pinch [3], or arc channel [4] are conventionally used to drive the highspeed flow. However, dense, high-speed flow is difficult to form by hydrodynamic acceleration because of rarefaction waves accompanying the acceleration process. Also, a strong correlation among velocity, temperature, and density in hydrodynamically accelerated plasma is undesirable for almost all applications, especially parametric studies in astrophysics. Pinching plasmas in a plasma focus system can form high-energy density, high-speed plasmas [1, 5]. Electromagnetic acceleration can avoid the need to heat the plasma in order to increase the axial velocity. Therefore, a dense plasma can be formed by a compression process in a z-pinch system. However, the plasma parameters are difficult to control owing to nonuniform gas discharge. We propose a new type of plasma device for the formation of dense high-speed plasma flow, in which the plasma parameters can be controlled by manipulating the operating conditions. Figure 1 shows a schematic diagram of the proposed scheme. The current sheet in a tapered capillary radially compresses and axially accelerates the plasma. Using this scheme, an argon plasma was accelerated to 700 km/s by a tapered pinch discharge with a fast pulse power generator, which drove a load current of 80 kA with a pulse width of 70 ns [6]. The electromagnetic energy is converted to the thermal energy and axial kinetic energy of the moving plasma. In this device, the geometry of the tapered capillary is expected to play an important role in distributing of the energy. We intend to clarify the relationships between the experimental conditions (i.e.,