PARTICLE EXHAUST IN RADIATIVE DIVERTOR EXPERIMENTS

Power exhaust is one of the critical questions to be solved for ITER, especially with the side condition of sufficient particle exhaust. Sin ce the H-mode threshold might require a high power flux over the separ atrix, only small power losses inside the bulk plasma might be permitt ed, and thus eventually a large fraction of the power must be removed in the scrape-off layer and divertor plasma. Neutral energy losses can only remove a small fraction of the power, and therefore most of the power flux has to be radiated, either by hydrogen isotopes, by the int rinsic impurities (mainly carbon) or by additionally seeded impurities . However, all these scenarios with reduced power flow to the divertor imply also a reduced particle flow into the divertor. This might be c ritical for particle exhaust, especially concerning the helium ash. Re duction of the power flow to the divertor by enhanced radiation losses has been investigated in the large divertor experiments like JET, DII I-D, ASDEX Upgrade, and JT-60U, and an experimental overview of the pr esent status will be given in this paper, together with a discussion o f impurity exhaust (mainly He and Ne) in these scenarios.