SURVEY OF TARGET PLATE HEAT-FLUX IN DIVERTED DIII-D TOKAMAK DISCHARGES

A series of observations is presented concerning divertor heat flux, q (div), in the DIII-D tokamak, and it is shown that many features can b e accounted for by assuming that the heat flux flows preferentially al ong field lines because tau(parallel to) < tau(perpendicular to) in th e scrape-off layer (SOL). Exceptions to this agreement are pointed out and the discrepancies explained by means of two dimensional (2-D) eff ects. About 80% of the discharge input power can be accounted for. The power deposited on the target plate due to enhanced losses during edg e localized modes (ELMs) is less than 10% of the total target power in most cases. X point height scans for lower single null (LSN) diverted discharges show that the peak heat flux variation is primarily due to flux expansion and secondarily due to transport of energy across the magnetic field in the divertor. At the outer strike point q(div,peak) proportional to P-in(I-p - )(1/B-t)(4/9)(B-div/B-mp)f(L-div,chi(perpen dicular to)), where G is a linear function of the inner gap, g(in), ov er a specified range and f describes cross-field energy transport in t he divertor. Evidence of radial in-out asymmetries (comparing the oute r strike point with the inner strike point or centre-post) and toroida l asymmetries in q(div) is presented and the heat flux peaking due to tile gaps and misalignment of tiles is examined. For magnetically bala nced double null (DN) discharges with downward del B ion drift, it is found that q(div) is inherently higher in the lower divertor than in t he upper divertor, having a 3:1 downward bias. Examples of heat flux r eduction by gas puffing deuterium or neon in LSN and DN discharges are given. At least a threefold reduction of the peak heat flux in both t he upper and lower diverters of a DN discharge, using D-2 puffing, is reported.