Diagnosis and Analysis of Load Current Divergence in Z-Pinch Experiments

During the later stage of stagnation in Z-pinch experiments, a current divergence phenomenon often occurs that downstream current (load current measured in a vacuum) declines slowly to zero, while upstream current (current measured outside vacuum) normally decays with quasi-sinusoidal oscillation. To investigate the formation mechanism of this current divergence, the possible abnormal discharge channels in vacuum are captured by optical imaging in this article. Both time integral image and time-resolved image show that current divergence is accompanied by the surface flashover on the vacuum insulator. The time-resolved images also verified that the time when downstream current starts slow decline corresponds to the time when surface flashover occurs. Surface flashover changes the circuit topology and forms the freewheeling branch of the load inductance L. An equivalent circuit is also established to analyze the current divergence. The discharge channel with low resistance R results in large time constant (L/R) and makes the downstream current drop slowly, while the external impedance can make the upstream current keep the waveform of oscillation attenuation even under the condition of internal flashover. Moreover, the influence caused by current divergence is discussed in the aspect of load voltage and the energy dissipation on generator. According to the simulation results, with an appropriate resistance, an additional branch can help protect the upstream components.