Plasma potential measurements by the heavy ion beam probe diagnostic in fusion plasmas: Biasing experiments in the TJ-II stellarator and T-10 tokamak

The effect of edge biasing on plasma potential was investigated in the TJ-II stellarator and the T-10 tokamak. The Heavy Ion Beam Probe (HIBP) diagnostic, a unique tool for studying the core potential directly, was used in both machines. Experiments in TJ-II show that it is possible to modify the global confinement and edge plasma parameters with limiter biasing, illustrating the direct impact of radial electric fields on TJ-II confinement properties. For the first time it was shown that the plasma column in a stellarator can be charged as a whole for a hot, near-reactor-relevant plasma. The plasma potential and electric fields evolve on two different characteristic time scales. Although the experimental conditions in the two machines have many important differences, the basic features of plasma potential behavior have some similarities: The potential response has the same polarity and scale as the biasing voltage, and the fluctuations are suppressed near the electrode/limiter region. However, whereas both edge and core plasma potential are affected by biasing in TJ-II, the potential changes mainly near the biased electrode in T-10.