Experimental study of the relationship between micro-turbulence and magnetohydrodynamics on the HT-7 tokamak

Electron density fluctuations were measured by a CO2 laser collective scattering diagnostic system on the HT-7 tokamak. The relationship between electron density fluctuations and magnetohydrodynamics has been studied under three ohmic plasma discharge conditions (kθ = 12 cm-1): (i) only Mirnov activities, (ii) only big sawtooth activities and (iii) Mirnov activities, sawtooth activities and slight disruptions coexisting. It has been shown that electron density fluctuations obviously increase during sawtooth activities or Mirnov activities. A periodic burst in the frequency spectrum of electron density fluctuations has been observed during case (iii); detailed investigations suggest magnetic energy, the equilibrium current gradient and the heat pulse may supply sources of free energy to micro-turbulence, leading to the micro-turbulence growing rapidly and significantly. (paper)