Study of Plasma Fluctuations in the TUMAN-3M Tokamak during Various Types of L–H Transitions

The reflectometer method is used to comparatively study plasma fluctuations in the edge plasma of the TUMAN-3M tokamak during L–H transitions initiated by different methods. It is shown that the width of the spectrum of backscattered microwave radiation is the most representative parameter when comparing the results obtained in different confinement regimes. The following methods for affecting the edge plasma were applied: gas puffing, a fast current ramp-up, a rapid increase in the toroidal magnetic field, and ion cyclotron heating. The studies were performed at different positions of the cutoff of O- and X-mode probing waves. A similar behavior of the spectral width was observed during transitions triggered by the fast current ramp-up and the rapid increase in the toroidal field. This provides evidence that the mechanism for transition to the H-mode is the same in both cases in spite of the different character of the evolution of the current density profiles. The fastest and strongest narrowing of the spectra was observed during the transition triggered by ion cyclotron heating. Possible reasons for similarities and differences in the behavior of the spectra during the transitions to the improved confinement regime are discussed.