Study of particle behaviour at the edge in HT-7 tokamak

The cross-field diffusion coefficient (D_⊥) at the edge in the HT-7 tokamak is close to the Bohm value when the line average electron density ranges from 1.5\times10^19 to 3.0\times10^19m^-3. The energy profile of the particles is derived directly from the H_\alpha(D_\alpha) line shape; the dissociative excitation of molecules is dominating when the local electron temperature is above 10eV. By means of the Monte Carlo method the D_\alpha line shape is also simulated. We find that the molecular dissociation contributes to 57% of neutral atoms and 53% of emission intensity in front of the limiter, and 85% of neutral atoms and 82% of emission intensity in front of the wall. The influence of atomic and molecular processes on the energy balance is discussed for the scrape-off layer (SOL), and the power loss from molecular dissociation is found to be 6\times10^4kW at the SOL. The ion Bernstein wave (IBW) can effectively suppress the magnetohydrodynamic behaviour, the fluctuation levels and the turbulence; the D_⊥ in front of the limiter declines from 0.84 to 0.2m^2\cdots^-1 and the particle confinement time rises from 9 to 12ms.