Plasma potential measurements by HIBP diagnostic in fusion plasmas: biasing experiments in TJ-II stellarator and T-10 tokamak

The influence of the edge biasing to the plasma core potential was investigated in the TJ-II stellarator and T-10 tokamak. The Heavy Ion Beam Probe (HIBP) diagnostics, which is a unique tool to study the core potential directly, was used in both machines. Experiments in TJ-II show that it is possible to modify the global particle confinement and edge plasma parameters with both positive and negative biasing. Two distinctly different scenarios of the confinement improvement were observed: significant improvement with modifications in the structure of plasma fluctuations and minor improvement without visible changes in fluctuations. This means that the biasing induced improvement of confinement is realized via multiple mechanisms. The plasma potential and electric fields relax with two different characteristic decay times. For the first time was shown that the plasma column in stellarator can be charged as whole. In spite of the fact that the experimental conditions in two machines have many important differences, the basic features of plasma potential behavior have some similarities: potential response has the same polarity and scale as biasing voltage, the fluctuations are suppressed near the electrode/limiter region. However, some differences are seen: in TJ-II both edge and core plasma potential are affected by biasing, in T-10 the potential changes mainly near the electrode. (orig.)