Stable and clean AC tokamak discharges with two, four and eight cycles of alternating plasma current have been achieved in the CT-6B tokamak. Although there are differences in the plasma-wall interactions in the two halves of each AC cycle, the purity of the plasma is not significantly affected. The finite electron densities during the current reversal phases of each cycle are attributed to a non-zero particle confinement time even at zero current. The plasma parameters in the boundary region during the AC operations were measured by a triple probe system.

Coherent structures of density fluctuations have been observed in the shear region of poloidal rotation of the CT-6B tokamak plasma by analyzing the measured H ␣ line emission with a wavelet correlation technique. The structures exhibit strong temporal and spatial intermittency with a lifetime of 20-100 s and a radial size greater than or equal to 1 cm. The fluctuation shows a skewed non-Gaussian probability density function in the region where the coherent structures occur. ͓S1063-651X͑98͒09905-X͔ PACS number͑s͒: 52.35. Ra, 52.25.Gj, 52.55.Fa

Electron cyclotron wave current startup has been investigated in a wide range of toroidal magnetic fields on the CT-6B tokamak. Two kinds of drive mechanisms of current startup have been identified experimentally, according to the dependence of the wave started current on the vertical field. Their contributions to the current have been estimated quantitatively for different resonance scenarios. The wave started current may occur at a downshifted resonance frequency even if the resonance layer is located outside the vacuum vessel.

The electron density fluctuations of CT-6B Tokamak plasma have been studied in detail through the observation of Hα emission intensity by multichannel optical diagnostics. Their various behavior, such as power spectrum, fluctuation level and correlation,have been measured. The experimental results show that some characteristics of density fluctuations are founded in CT-6B Tokamak plasma: the density fluctuation level is about 10%-90% in the radial range r=4-10cm; the point in the range draws, nearer to the edge of plasma the level lecomes higher. A broadband power spectrum is observed at observed points along the plasma radius. There exists a local correlation structure with short correlation time, about 0.2-0.5ms, and short correlation distance , about 0.5-1cm,in the edge of plasma.

A currentless plasma has been obtained in a tokamak device using a powerful microwave at electron cyclotron frequency generated by gyrotron systems. Its electron density is up to 8×1012cm-3; the electron temperature is up to 50 eV; the particle confinement time is about. 0.5ms; the heating efficiency is above 25%. The effect of the plasma on the toroidal breakdown and the heating mechanism have been studied.