Heavy Neutral Beam Probe Space Potential Measurements of the Helimak Experiment (Te $\sim$ 10 eV)

The Heavy Neutral Beam Probe (HNBP) for the Helimak low temperature plasma experiment has been developed at the University of Texas at Austin (UT-Austin). The HNBP is based on the highly successful Heavy Ion Beam Probe (HIBP), but is engineered to work in the low temperature plasma regime ($T_e < 40$ eV). The greatest difficutly to operation at low electron temperatures is that the measurement signal is enabled by electron-impact ionization events, which become increasingly rare when the temperature dips to $\sim 10$ eV. This problem is overcome by probing the plasma with a neutral alkali metal (Na) and by modulating the probing beaming with a square wave (chopping) and recovering the signal with phase sensitive detection. The Helimak experiment at UT-Austin approximates the infinite cylindrical slab with open field lines1. The geometry is like a torus, but with a rectangular cross-section and with vertical field coils, that combined with the toroidal field coils, give rise to a helical magnetic field inside the device. Because of the curved, sheared magnetic field, and its gradient, the Helimak simulates the scrape off layer (SOL) of a tokamak.