Rotation scalings and momentum confinement in neutral-beam-injected ISX-B plasmas

Abstract

Scalings of the central rotation in non-gettered, co-injected ISX-B discharges have been measured as a function of beam power, electron density and plasma current. Extensive studies are made possible by exploiting charge-exchange excitation (CXE) of 0 VIII lines to measure Doppler shifts. The rotation velocity, vϕ(0), tends to saturate at (1.0 − 1.2) × l07 cm·s−1 when Pb≅0.5 MW, showing little further increase up to the maximum input of 2 MW; vϕ(0) is independent of ne and Ip. Momentum confinement times in quasi-steady plasmas are 10–16 ms for n̄e = 4.5 × 1013 cm−3. Counter-injection discharges always disrupt, but before this event vϕ(0) is the same as for co-injection plasmas. The addition of a third beam line, permitting injection of up to 2 MW of balanced neutral-beam power, has allowed comparisons of the energy and particle confinement in rotating and non-rotating plasmas with the same total neutral-beam input. In those cases where impurity buildup can be avoided, it is found that the ISX-B empirical scaling of energy confinement time is reproduced with balanced injection. Thus, the unfavourable dependence of is not the result of rotation. Studies of impurity behaviour under differing injection conditions have been extended to include fully stripped low-Z ions. The results are consistent with previous investigations of metallic elements which revealed strong dependences on the sense (co versus counter) of injection. The potentials calculated from momentum balance, using measured rotation profiles and typical plasma density and temperature profiles, are in qualitative agreement with the potentials measured directly for various combinations of co- and counter-injection.