An effective technique for wall conditioning by injecting an ion cyclotron resonant frequency wave, RF coating, was developed in the HT-7 tokamak. Under different wall conditions, namely before and after lithium/silicon coating, the boundary plasma behaviour was investigated using a reciprocating Langmuir probe system. On reduction of impurity radiation, strong shears of the radial electric field and the poloidal velocity accompanied by turbulence suppression and de-correlation were observed in the edge region of the coated wall plasma. This led to a reduction in edge transport and improvement in plasma confinement. An enhanced shear of the radial electric field was observed on increasing the electron density, which may account for the improvement in particle confinement. The results suggest a close link between wall conditions and boundary plasma. The features of ionization and radiation driven turbulence, e/Te∼ñe/ne and θ_eñe∼π, had been found in the plasma edge region. The interplay between atomic processes in the boundary plasma and the formation of the radial electric field shear may be responsible for this behaviour.
Sheared flows and turbulence in fusion plasmas