Potential Measurement with the 6-MeV Heavy Ion Beam Probe of LHD

Abstract

The potential in the plasma core of the Large Helical Device (LHD) was measured with the 6 MeV Heavy Ion Beam Probe (HIBP). The radial profile of the potential in the region where the normalized minor radius, ρ, is less than 0.5 was measured and the electric field obtained from the fitting function of the experimental data was compared with that calculated from neoclassical ambipolarity. The experimental results coincide well with the theoretical calculations. Negative pulses were observed in the potential signal in the inward shifted magnetic configuration. The time constants of these pulses were less than the energy confinement time. Potential fluctuations of the coherent modes were also observed, and one of their frequencies coincided with the geodesic acoustic mode (GAM). In this report, the present status of potential measurements with the HIBP system in LHD is described.