Measurements of plasma space potential and radial electric field in TEXT with a heavy ion beam probe

A heavy ion beam probe (HIBP) was used to measure plasma potential and radial electric field during Ohmic Heating (OH), high magnetohydrodynamic (MHD), electron cyclotron heating (ECH) and ergodic magnetic limiter (EML) discharges in the Texas Experimental Tokamak (TEXT). A negative central potential phi(0) approx. = -1 kV and a radial electric field E(sub r) approx. = -40 V/cm were observed in ohmically heated plasma. The measured space potential profile is consistent with the ion radial momentum balance equation using experimental values of density and ion temperature, and assuming neoclassical plasma poloidal rotation velocity and no toroidal velocity. With these assumptions, it is expected that phi(sub 0) approx. = (2 to about 2.5) T(sub i)(0). For B(sub T) = 20 kG, I(sub p) = 195 kA OH discharges, the measured phi(sub 0) varies as (n sub e0)(sup 1/3), consistent with Artismovich ion temperature scaling.