Electron cyclotron heating at down-shifted frequencies in existing tokamak devices

Abstract

Plasma heating in existing tokamak devices by electron cyclotron waves with frequency (f) significantly smaller than the electron gyrofrequency (f_c) is investigated for the case of Maxwellian plasmas. It is shown that for central electron temperatures larger than 3 keV, strong absorption of extraordinary waves can occur at values of the toroidal field for which the condition f = f_c is not satisfied in the plasma region. The cases of f = 60 GHz and f = 100 GHz are discussed for the PLT and TFTR devices as being representative of medium (≈30 kG) and high (≈50 kG) magnetic field tokamaks, respectively. Numerical calculations with a ray tracing code indicate that most of the RF energy is absorbed in a central plasma region. These results are of practical interest because they significantly simplify the main technical problem of ECH in a tokamak reactor, i.e. the development of high-frequency and high-power microwave sources.