First results of a neutral beam probe for space potential measurements of low temperature plasmas

A Neutral Beam Probe (NBP) has been developed for measuring the space potential of the Helimak low temperature (T_e ~ 10 eV) plasma experiment; the NBP is a variation on the well known Heavy Ion Beam Probe (HIBP). The NBP consists of a Na ion beam that was chopped (modulated) before entering a heat pipe neutralizer consisting of Cs vapor; the beam next enters a toroidal magnetic field region where the probing atoms are ionized; the Na ions then follow a path to a calibrated energy analyzer. The calibration and first results have been obtained and are presented. The Texas Helimak experiment approximates a sheared cylindrical slab intended to study turbulent-driven transport. The dominant magnetic field is toroidal with a weak vertical component. Vertical propagation of turbulence in Helimak is on the order of 1000 m/s and radial electric fields are thought to play a strong role. The NBP is capable of radial potential measurements, shedding light on sheared flow turbulence. Heavy ion beam probing of low temperature plasmas is difficult because the signal depends on electron impact ionization of the probing beam. With low electron temperature and density (n_e ~ 10¹¹ cm^-3), the signal expected was in the range of 1-10 nA, and the pure electronics noise was ~ 2 nA (rms). Because of the very low signal levels that were expected, a phase-sensitive signal processing scheme was implemented to recover the signal from the noise. The modulation was initially implemented as a triangular sweep in order to locate the signal. Once found, an on-off modulation scheme was implemented to increase total signal strength. The detected signal and interpretation are presented along with noise characterization.