PFC / JA - 96 - 17 Screening of Recycling and Non - Recycling Impurities in the Alcator C - Mod Tokamak

The behavior of argon and nitrogen, injected into single null diverted ohmic discharges, has been compared. The screening of the divertor and scrape-off layer plasma is studied by comparing the number of impurity atoms in the core with the number injected by a calibrated gas puff. The effect of the poloidal position of injection on screening has been investigated. The core nitrogen density is dependent of the poloidal position of injection while the argon density is not. The spatial distribution of low charge states in the divertor has also been studied using a multichord visible spectrometer. Initial results of nitrogen modeling with the Monte Carlo code DIVIMP are presented. Introduction. One of the principal functions of a divertor is the removal of the plasma material interaction to a remote location where the impurities produced have a small probability of reaching the confined plasma and polluting it. The action of the divertor can reduce impurities in two ways, firstly, by having a plasma temperature gradient between the confined plasma and the divertor target which reduces the impurity sputter yield and, secondly, by reducing the probability of impurities produced at the target reaching the confined plasma[1,2]. A high ionization rate in the SOL, far from the separatrix, is generally likely to improve the screening since it will decrease the cross-field diffusion-rate into the core. However there are parallel temperature gradient forces which can push the impurity ions towards the confinement region. Good screening depends on having the parallel force due to the friction of the plasma flowing towards the target greater than the ion temperature gradient force acting away from it [3,4]. 12th PSI Conf., San Raphael, May 1996 1 10:14 May 16, 1996 The screening process in the SOL of the high density tokamak, Alcator C-Mod [5] has been studied using gaseous impurities. Gas puffing has the advantage of giving independent control of the impurity injection rate under a range of operating conditions so that the screening process can be studied independently of the impurity production process. In the present paper the effect of different poloidal injection positions on the screening of impurities has been extensively studied Impurity behavior can be classified as either recycling or non-recycling. The rare gases He, Ne and Ar are typical recycling species. Since they are chemically inert, ions arriving at a surface are backscattered with some fraction of their thermal energy plus the energy acquired in the plasma sheath potential at the target. After recycling they may therefore penetrate deeply into the plasma before ionization. Non-recycling impurities are typified by carbon and metals which are expected to have a low back-scattering probability. Somewhat surprisingly it has been found that nitrogen gas also acts as a non-recycling impurity [6], presumably due to its chemical interaction with solid surfaces.