Radial Density Profile in the SSX Plasma Wind Tunnel using a Double Langmuir Probe

We present preliminary results from a moveable double Langmuir probe in the present plasma wind tunnel configuration of SSX. The probe is designed to measure radial profiles of electron density ($n_e$) and electron temperature ($T_e$) across the midplane with a $1~cm$ resolution. Line-averaged densities from He-Ne interferometry show densities of $1-5 \times 10^{15}~cm^{-3}$. In addition to mean values, we will also present electrostatic fluctuations and correlations with magnetic field measurements. The double Langmuir probe also measures local $T_e$. Line-averaged measurements from VUV spectroscopy indicate $T_e \sim 10~eV$. The Langmuir probe stalk diameter measures $6.5~mm$ and tip spacing is $1.1~mm$. The SSX plasma wind tunnel has dimensions $L\cong 1~m$ and $R=0.08~m$. Plasma flow speeds are $v\ge 50~km/s$. The cylindrical copper boundary and probe surfaces are baked and cleaned in a $He$ glow discharge to maintain excellent vacuum and surface conditions. Electrostatic measurements during merging will be presented if available.