RADIATION PHENOMENA AT EDGE AND DIVERTOR

The concept of a cold radiative plasma boundary is presented as a possible solution of the energy exhaust problem in a fusion reactor. The most relevant processes which determine level and location of the radiation from low-Z impurities are discussed in detail. Experimental results are used to demonstrate the general feasibility of generating a stable and quasi-stationary plasma with a cold radiating layer on a high power level. Furthermore, the limitations of the concept are briefly analyzed addressing feed-back control of the impurity level, thermal stability, particle transport and impurity contamination. I. THE CONCEPT OF THE "COLD RADIATIVE PLASMA BOUNDARY" One of the outstanding challenges on the way to a fusion reactor is the problem of energy exhaust1. The Į-particle heating will generate a radial power flow of about 0.3 MW/m2 which ultimately has to be exhausted via the wall structures (ITER: PĮ § 100MW). If most of this power would enter the scrape-off-layer (SOL) convectively the power flowing parallel to the fieldlines would be in the order of 500 MW/m2. Even with optimistic assumptions about possible means for reducing the power load density on the wall surface, e.g. by an inclined target plate, the peak load on the first wall will come close to 20 MW/m2- a value too high for stationary operation. The necessary reduction of the power load can be achieved by distributing a substantial fraction of the power on other parts of the wall. A promising means to achieve this is given by line radiation from impurities, which allows to distribute the power load on large areas (the whole vessel/divertor wall). In case this line radiation would be confined to the plasma boundary, cooling the plasma edge and leaving the plasma core essentially unaffected, we have a situation as considered in the concept of the "cold radiative plasma boundary". I.A. The relevant processes and the radiation characteristics Impurities enter the plasma in general as neutral particles- as atoms or molecules. Due to electron impact the particles are dissociated and ionized. The characteristic penetration depth Ȝi of the particles is given by their ionization tim