Measurement of the impedance for a hybrid X-pinch load

For a hybrid X-pinch load assembled in parallel with the return-current rods, the impedance of the hybrid X-pinch load is important for current division between the load and the current rods. In this paper, the experimental results of the investigation of the impedance of a hybrid X-pinch load on the Qiangguang facility (1.5 MA, 100 ns) were reported. The current of the X-pinch load was measured using a serial nickel foil resistor, and the voltage was measured using a two-stage resistive divider. The inductance of the X-pinch load was obtained by comparing the differential signal of the current with the voltage waveform, and it is about 4.7 nH, which is nearly equal to the calculated total inductance of the metal wire and the two X-pinch electrodes. The resistance of the X-pinch was determined by subtracting a part of the inductance from the measured voltage, and the following results were obtained. At the start of the current, the resistance of the X-pinch increases quickly from the metallic resistance of 0.16 Ω at room temperature for a 2-mm-length 30-μm-diameter Mo wire to 0.8 Ω in about 10 ns, and then, it falls fast to nearly zero, which suggests that the electrically exploding phase of the metal wire ends and the high conductive plasma is formed. As the current continues to flow through the Mo wire, the resistance increases once again from nearly zero to about 0.4 Ω at the moment when the X-pinch begins to burst x-ray radiation, which reflected that the micro-pinch dynamical process occurs and a hot and tight plasma zone is formed. Assuming that the tight plasma zone is 300 μm long with a 10 µm diameter and the temperature of the plasma is ∼1 keV, we can obtain that the resistance of the tight X-pinch spot is about 0.6 Ω, which is close to the value of 0.4 Ω obtained in the experiments.