First HIBP measurement of plasma potential during the H-mode transition on the TUMAN-3M tokamak

The difficulty of Heavy Ion Beam Probe (HIBP) application on the TUMAN-3M (R=0.53m, a=0.22m, BdT=0.8T, Idp=140kA, Tde=0.5keV, n < 4 10 super 19m super -3) - significant toroidal shift of beam trajectory - is caused by high ratio of poloidal field to toroidal one. Strong UV radiation from the plasma loads the energy analyzer's detector and complicates the problem even more. This paper presents the results of first measurement of plasma potential evolution in the discharges performed in ohmic H-mode using 80 keV Ku+ beam and a Proca-Green secondary ion energy analyzer. Spatial region covered by the diagnostic in the experiments discussed was 0 < r < 0.6a. Spatial scan was performed utilizing the toroidal field decrease due to capacity power supply battery discharge. The change in plasma potential of the order of 100V has been measured during the H-mode formation. The potential in core plasma (r < 0.6a) starts to change simultaneously with L-H transition, and than changes during 6-8ms after the transition. Thus, the potential changes rather slowly in a comparison with L-H transition timescale ( 2ms for TUMAN-3M ohmic H-mode). Possible explanation to the slow change in central plasma potential may be a formation of potential well structure at the plasma edge, in which radial electric field changes direction. This kind of structure is beneficial for the edge turbulent transport suppression because of high | d Edr/ d r|, but not necessary requires a strong change in central plasma potential to occur immediately. The results from microwave reflectometry support this hypothesis.