Interpretation of perturbed temperature based on X-ray emissivity in fusion plasma experiments

摘要

The relationship between the dynamical response to perturbations of the soft X-ray emissivity (δE), the electron temperature (δT_e), the electron density (δn_e) and the impurity concentration (δn_i) for a Maxwellian plasma is analysed in detail. In particular, the so-called 'impurity function' F(Z_(eff)) is also strongly dependent on Te via the direct radiative recombination (DRR) contribution to the X-ray emission, which significantly affects the relation between the perturbed quantities as derived from the popular expression E ∝ F(Z_(eff))n~2_eT~α_e even if the impurity content (or Z_(eff)) remains constant. In order to overcome this difficulty, a simple analytical approximation is derived for F(Z_(eff),T_e) that can be used as a formula to relate the perturbed quantities δE, δT_e, δn_e and δF with ease and accuracy. This simple approximation is illustrated by studying saw-toothing discharges on the Tokamak de Varennes (TdeV) with Te, ne and E measured by the Thomson scattering, the FIR interferometer and the X-ray camera diagnostics, and its accuracy is tested against the predictions of a full X-ray modelling code.