Stabilization of Radiation‐Condensation Instability by Light Impurity Injection

Abstract

AbstractAs it has been shown in [1,2], Radiation‐Condensation Instability (RCI) may initiate Microfaceted Asymmetric Radiation from the Edge (MARFE) in tokamaks (see also review papers [3‐5]). Nevertheless, experiments demonstrate the stable regimes with strongly radiated edge plasmas after Ne injection [6‐8] or in siliconized discharges. Two effects destabilize radiative plasmas, the decrease of radiation losses Q with the electron temperature Te increase, and the increase of Q with electron and impurity densities rise. The finite relaxation time of impurity distribution over ionization states [6] as well as the thermal force acting on the growth rate doesn't shift the instability margin. Hence, one can examine the stability margin using the approximation of the coronal equilibrium. Radiation losses of intrinsic impurities like beryllium, carbon and nitrogen usually decrease with the temperature increase at the temperature range typical for the edge (see Fig. 1, curve 1). The situation may be significantly different for impurity mixtures. Radiation losses L ≡ Q /(nenI)normalized by electron and impurity densities ne and nI for the mixture of carbon and neon are shown in Fig. 1, curves 2‐5. One can see that ∂Q/∂T > 0 for practically any temperature at the edge if the concentration ratio nNe/nC ≥ 5. Hence, one can expect the stabilization of RCI by injection of additional impurity and achievement of stable regime with the strongly radiated edge plasmas. The stability of plasmas with few impurity mixtures is examined in the present paper numerically (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)