Experimental Studies Of Plasma Toroidal Rotation With Electrode Biasing At The Edge Of J-TEXT

Toroidal rotation is critical for plasma operation in tokamaks. On one hand, toroidal rotation and its shear can enhance the internal transport barrier (ITB), drive Er×B shear flows and suppress micro-instabilities (turbulence), as a result to improve plasma confinement; on the other hand, toroidal rotation with sufficient large amplitude can also stabilize some dangerous magnetohydrodynamic (MHD) instabilities, including neoclassical tearing modes (NTMs) and resistive wall modes (RWMs), conducive to the steady-state operation of plasmas. Therefore, how to understand the whole picture of toroidal rotation, and find a way to optimize and control the rotation profile, becomes an important issue in the tokamak physics research. At present, some difficulties still remain in the toroidal rotation related studies, two of which are the driving mechanism(s) of intrinsic (toroidal) rotation and the underlying physics of (toroidal) momentum transport.In this dissertation, we perform experiments on J-TEXT tokamak to investigate the driving mechanism of intrinsic rotation and momentum transport at plasma edge. As the basis of experimental studies, we design and construct an electrode biasing (EB) system, which is used as an adjusting method in the experiments to change local Er×B shear and drive local toroidal rotation at plasma edge.It is observed that, plasma (mainly the particle) confinemet can be improved with both positive and negative bias, accompanied with the enhancement of radial electric field (Er) shear at the edge as well as the suppression of local turbulent particle flux.The intrinsic toroidal rotation of J-TEXT edge plasma is in the co-Ip direction (the same direction of plasma current,IP). By applying electrode biasing, edge toroidal rotation can be modified greatly:it changes towards co-Ip direction with positive bias, but changes towards counter-Ip direction (the opposite direction of plasma current) with negative bias; further statistical analysis shows that edge toroidal rotation scales approximately linearly with bias current. The (toroidal) Reynolds stress term always dominates the turbulent momentum flux at the edge. With the enhancement of local Er shear under bias, both the Reynolds stress and total turbulent momentum flux are all suppressed, suggesting the universal role of Er×B shear flows in suppressing turbulent transport. The toroidal rotation is successfully cancelled at the edge under negative bias (with about-60A bias current) in the experiment, and both local residual stress and intrinsic torque density distributions are obtained; comparison shows that the torque density contributed by the residual stress is in reasonable agreement with the intrinsic torque density, providing direct evidence to the theoretical hypothesis that residual stress is the driving mechanism of intrinsic rotation.The radial profiles of toroidal momentum transport coefficients (diffusivity X(?) and convective velocity Vconv) of local plasma (normalized minor radius p=0.68-0.9) are also preliminarily evaluated based on pertabative analysis technique by using electrode biasing modulation. The results show that, X(?) is always positive, and increases with the minor radius, with its value (in the order of Im2/s) obviously exceeds the prediction of neoclassical theory; Vconv is always negative, suggesting the pinch effect of toroidal momentum.