Dependence of Plasma-Current Coupling on Current Rise Time in Gas-Puff Z-Pinches

Experiments measuring the azimuthal magnetic field of gas-puff Z-pinch implosions on two drivers of significantly different timescales are presented. Using a Zeeman-based spectroscopic technique, measurements of $B_{\theta }$ are compared with calculated values using Ampere’s law and the load current measured by inductive probes to define the “plasma-current coupling.” Oxygen gas-puff implosions carried out on the compact experimental system for Z-pinch and ablation research (CESZAR) linear transformer driver (LTD) with a peak current of ~500-kA and ~180-ns rise time exhibited substantially lower plasma-current coupling when compared to implosions on the Weizmann Institute of Science (WIS) current driver (300-kA peak and 1600-ns rise), 48% ± 18% and 95% ± 6%, respectively. Potential causes for the significant differences in plasma-current coupling between the two drivers are examined. Shunted current across instability structures present in the CESZAR implosions is ruled out as a source of current loss. The radial charge state distribution is discussed, and a trailing plasma composed of higher charge states is hypothesized to carry a significant portion of the driver current.