Neon soft x-ray yield optimization in spherical plasma focus device

Abstract

A spherical plasma focus (SPF) (KPU200 (Maslov 2014 Instrum. Exp. Tech. 57 131–4)) operated with neon is used for soft x-ray optimization in this work. This device with 135 kJ stored energy has 432 µF capacitor bank, 8 cm inner and 14.5 cm outer electrode radii. Inductance and charging voltage of this device are 36 nH and 25 kV. Optimization study is conducted with a spherical magnetohydrodynamics model. Soft x-ray optimization is done in four steps. In the first step, gas pressure is varied from 0.1 Torr until 10 Torr with small increments, and optimum gas pressure is found as 1.15 Torr with fixed anode and cathode radii (8 and 14.5 cm). Soft x-ray yield with optimum gas pressure alone is found to be 3046 J (2.26% of 135 kJ bank energy). Then anode and cathode radii are optimized separately and they are found as 8.458 and 11.1 cm, respectively. Soft x-ray yield with separately found optimum pressure, anode radius, and cathode radius (1.15 Torr, 8.458 cm, and 11.1 cm) is increased to 3543 J (2.63% of 135 kJ bank energy). In the second step, anode and cathode radii are optimized with respect to two fixed cathode-to-anode ratios (14.5/8 = 1.8125 and 11.1/8.458 = 1.3124) for various pressures ranging from 0.5 Torr until 4 Torr with 0.25 Torr increments. Soft x-ray yield of 4500 J (3.33% of 135 kJ bank energy) is calculated in this step at 0.5 Torr with cathode-to-anode ratio of 1.3124. In the third step, optimization of external inductance (ranging from 10 to 100 nH) is done for optimum soft x-ray yield. Soft x-ray yield in SPF reaches 4684 J (3.47% of 135 kJ bank energy) after optimizing external inductance (54 nH). In the last step, the effect of charging voltage on soft x-ray yield is investigated. Charging voltage is varied from 20 kV until 35 kV with 1 kV increments. After optimizing charging voltage (31 kV) in addition to the other optimum parameters, soft x-ray yield reaches 6348 J (3.07% of 207 kJ bank energy). Even though soft x-ray yield is increased with increasing charging voltage, soft x-ray yield as % of bank energy is decreased. The device also achieves maximum current of 1852 kA (4 Torr, 30 kV). Overall, soft x-ray yield is increased from 3046 J (with optimum pressure alone) to 6348 J after optimizing gas pressure, anode radius, cathode radius, external inductance, and charging voltage in SPF.