Simultaneous Measurements of Gas-Puff Z-Pinch Parameters Using Visible Spectroscopy

Characterizing gas-puff Z-pinch is of significant importance for the alternate fusion schemes and intense X-ray sources. Key parameters historically difficult to measure in the understanding of Z-pinch dynamics are magnetic field and current distributions as a function as time and space. One of the reasons is that gas-puff Z-pinches display many different plasma conditions and instabilities over tens to hundreds of nanoseconds, hence characterizing the dynamics of the implosion requires a complex set of measurements. One way to mitigate instabilities is to apply an axial magnetic field. This may also decrease the temperature and density that the plasma might reach by limiting the compression. Consequently, characterizing the plasma conditions in accordance with an externally applied B-field is of paramount importance. The Weizmann Institute of Science (WIS) in Israel developed different spectroscopic techniques to simultaneously measure $\mathrm{T}_{\mathrm{e}}, \mathrm{n}_{\mathrm{e}}, \mathrm{B}_{\mathrm{z}}$ and $\mathrm{B}_{\theta}$ on Z-pinches. We present here experimental results from Oxygen gas-puff Z-pinch carried out at and in collaboration with WIS using a ~300 kA current driver with a $1.6\ \mu \mathrm{s}$ rise time. $\mathrm{B}_{\mathrm{z}}$ and $\mathrm{B}_{\theta}$ measurements (“-’ 0.3 T to 4 T) were based on Zeeman splitting and polarization of visible emission lines while $\mathrm{T}_{\mathrm{e}}(\sim 5\ \text{eV}$ to 15 eV) and $\mathrm{n}_{\mathrm{e}}(\sim 10^{18}-10^{19}\mathrm{g}/\text{cm}^{3})$ were estimated with line ratio and line shape analysis, respectively, using the atomic physics code CRETIN. For each shot, all measurements were time gated and spatially resolved allowing a full characterization of the O gas-puff implosion. Such data allowed interesting comparisons with MHD code TRAC-II that showed good agreement with the experiment. This work also aims to compare Z-pinch dynamics for different drivers. The data collected at WIS will be eventually compared to similar measurements performed at UCSD on a linear transformer driver (~800 kA and 160 ns rise). As a first look, we also present here some gas-puff Z-pinch temperature measurements made at UCSD showing how the plasma conditions differ from the ones at WIS.