Soft x-ray diagnostics of toroidal plasmas

Abstract

This work is a presentation of the application of soft x-ray measurements to the diagnostics of toroidal plasmas currently of interest in the controlled thermonuclear reaction research program. Soft x-ray techniques are here used in a study of auxiliary plasma heating at the second harmonic of the electron cyclotron resonance (ECRH) in the small (R = 0.3 m, a < 0.1m) TOSCA tokamak. The importance of soft x-ray diagnostics in relation to the parameters of current fusion plasma experiments is shown by detailing the dependence of the soft x-ray radiation power on the plasma conditions, through a consideration of the contributing atomic processes. The contribution from soft x-ray measurements to the existing body of knowledge of tokamak physics is outlined by way of a review of the more significant results obtained to date. It is shown how radially resolved measurements of the soft x-ray flux may be used in conjunction with soft x-ray spectral measurements to deduce the form of the distribution in minor cross-section of the local electron temperature in a tokamak. The profiles obtained in this way from TOSCA are comparable with laser scattering measurements performed on the Alcator A and TM-3 tokamaks. This work is extended to the study of the effect of ECRH in TOSCA, which is found to be strongly localised in major radius at electron densities well above the extraordinary mode cut-off condition, and also strongly asymmetric with respect to the minor axis. Some progress is made in evaluating the electron temperature profile during ECRH. The local electron temperature at the ECR could at least be doubled. The significant role of trapped electrons under ECRH is discussed.