Turbulence in Gas-Puff Z-Pinches: Applying Thomson Scattering to Diagnosing Turbulent Density and Velocity Fluctuations

We present measurements of velocity distributions and electron density fluctuations in a neon gas-puff z-pinch. These experiments were performed on the 0.9 MA, 240 ns rise-time COBRA generator at Cornell University. Time-resolved electron plasma wave (EPW) and ion acoustic wave (IAW) Thomson scattering data were obtained using a 526.5 nm laser (E=10 J, $\Delta$t= 2.3 ns) and two streak cameras. IAW spectra from two differently sized scattering volumes at the same radius were also collected. We observe that including a velocity distribution in the IAW fit improves the fit pre-pinch. The width of the distribution does not scale with the size of the scattering volume, indicating that the distribution is not due to a continuous velocity gradient, but more likely to turbulent eddies.

The EPW provides measurements of electron temperature ($T_e$) and electron density ($n_e$). The width of the EPW depends on $T_e$, but also on non-uniform $n_e$. We look for local density variations by comparing the values of $T_e$ derived from the IAW and EPW. This comparison indicates non-uniform density in the scattering volume. With the velocity distributions observed via the IAW, this points to turbulent motion. Based on the smaller but extant difference between $T_e$(IAW) and $T_e$(EPW) in the sheath, turbulence has begun to develop already in the imploding plasma. At the axis, it is fully developed, indicated by the difference between $T_e$(IAW) and $T_e$(EPW) (non-uniform density), and large velocity distributions.