Effects of magnetized coaxial plasma gun operation on spheromak formation and plasma characteristics
Authors: Zhao Fan-Tao , Song Jian , Zhang Jin-Shuo , Qi Liang-Wen , Zhao Chong-Xiao , Wang De-Zhen ,   Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams of the Ministry of Education, School of Physics, Dalian Univeristy of Technology, Dalian 116024, China
Published: September 2021

Spheromak plasma formed by a magnetized coaxial plasma gun possesses high propagation velocity and electron density, which has been extensively investigated, for it has a variety of applications, such as fueling of fusion reactor, magnetized target fusion, and labratory simulations of astrophysical phenomena. Formation and optimization of the gun-type spheromak are studied by investigating the discharge characteristics of the gun and the scaling of plasma parameters with various operation conditions. Based on the spheromak formation mechanism, several significant operation parameters are identified, including peak value of gun current, bias flux, gas-puffed mass and the length of neutral gas distribution inside the gun channel: this length can be controlled by adjusting the time delay between gas injection and discharge of the capacitor bank to initiate gas breakdown and for a long time delay the current path distribution inside the gun channel can be characterized by a moving plasma ring which carries almost all of the gun current. Under a sufficient pressure of the self-generated field, the moving plasma ring with freezed toroidal field pushes the bias field into the vacuum chamber, the twisted field lines are then broken, reconnected, and thus forming a free spheromak. The injected gas is desired to exist only in the gun channel: if downstream region of the gun is filled with neutral gas, a weakly ionized and cold spheromak will be formed, which is not beneficial to practical applications. The multiple current path phenomenon is observed using two spatially separated magnetic coils inside the gun channel, excepting for the plasma ring, there are a stagnant current path and a reversed current path separately located in upstream and middle region of the gun channel. Development of the upstream current path is due to the residual charged particles deteached from the tail of accelerated plasma ring and the unswept netural particles, which reduces the energy injected into the plasma ring from capacitor bank, and thus having a negative effect on the performance of spheormak. The axial propagation velocity of spheromak, electron temperature and density are shown to increase with the capacitor bank voltage rising, which can be attributed to the elevation in energy injected into the plasma ring. Only higher electron density is obatined by increasing the gas-puffed mass, and the propagation velocity and electron temperature are observed to decrease. The energy injected into the plasma ring is independent of the gas-puffed mass, and electron density is elevated with gas-puffed mass increasing. Since the frequency of electron impact ionization increases, electrons undergo more collisions and transfer more energy to other particle species, thus the thermal energy of electrons decreases.