The interplay between the lower hybrid heating and plasma turbulence in the scrape-off layer of the Tore Supra tokamak -

Abstract

Thermonuclear fusion is one of the most promising options for generating energy in the future. Lower hybrid (LH) current drive waves has proved to be one of the most efficient techniques regarding current drive in plasma fusion devices called tokamaks. LH propagation into closed field lines and mainly the interaction with the edge and the scrape-off layer (SOL) decreases its efficiency in driving current. For this reason, it is important to understand and quantify the interaction between the LH waves and the plasma edge. This interaction is characterized in the SOL by electrostatic probes. Using DFLUC data acquisition resolving frequencies below 0.5 MHz, we characterize the SOL properties in the presence of LH waves. The normalized level of fluctuations is found independent of the power and average density. The intermittent events reflecting convective transport decrease as LH power is increased while remaining unchanged with respect to the plasma density. The convective velocity, detected by the cross-correlation is found to increase with PLH but the distribution of turbulence among the scales was not found to be modified. As the core density is increased, the convective velocity and the typical scales did not change. Using the DCEDRE data acquisition, resolving frequencies 0.1f100 MHz, the existence of an ion sound quasi-mode was detected. The propagation velocity as well as the wavelength of this mode were calculated and found in agreement with the theory. Moreover, the amplitude of this mode increased to the square of the LH power clearly showing that it is caused by a Parametric Decay Instability. The level of turbulence was found about 10 times that of the coherent mode and it is decreasing with increasing LH power and is increasing with increasing plasma density.