Theory of ECE fluctuations in a tokamak plasmas

摘要

Fluctuations of equilibrium plasma parameters such as density, temperature, electric and magnetic fields are closely associated with many important physical phenomena and can serve as a source of valuable diagnostic information. A well-established way to observe fluctuations in a tokamak plasma interior is collective scattering of electromagnetic waves. Alternatively the fluctuations can be studied using the plasma radiation itself, as, for example, at the electron cyclotron frequency (ECE). In this case the measurements are spatially resolved and radial scans are easily obtained. Although turbulent oscillations produce no waves, which propagate outside the plasma, these oscillations are capable of modulating the EC emission, which makes them observable. To discriminate this plasma turbulence induced modulations from the thermal carrier fluctuations a correlation technique is used. This method consists in correlating signals received by two radiometric channels at slightly displaced sample volumes. The feasibility of this technique has been successfully demonstrated recently in a number of experiments. In this work a microscopic theory of coupling between weak low-frequency plasma turbulence and the microwave emission is presented for the case of high optical thickness of ECR layer. A general expression for the fluctuating part of the ECE signal in terms of the turbulence spectrum is obtained. Effect of turbulence induced fluctuations of the wave damping is taken into account.