No abstract

High-speed pellet fuelling experiments have been performed on the ISX-B device in a new regime characterized by large global density rise in both Ohmically and neutral-beam heated discharges. Hydrogen pellets of 1 mm in diameter were injected in the plasma midplane at velocities exceeding 1 km·s−1. In low-temperature Ohmic discharges, pellets penetrate beyond the magnetic axis, and in such cases a sharp decrease in ablation is observed as the pellet passes the plasma centre. This behaviour can be accounted for by an ablation model that includes dynamic cooling of the target plasma while the ablation proceeds. Complete penetration can be prevented by operation in low-density regimes where runaway electrons are thought to be responsible for high ablation. A similar effect is observed with moderate to large amounts of neutral-beam injection. There is a strong enhancement of the ablation rate in the outer 10-cm plasma region even for short heating intervals, which can be explained by the presence of multi-kilo-electron volt ions in the discharge. Density increases of ∼300% have been observed without degrading plasma stability or confinement. Energy confinement time increases in agreement with the empirical scaling τE ∼ ne and central ion temperature increases as a result of improved ion-electron coupling. Laser-Thomson scattering and radiometer measurements indicate that the pellet interaction with the plasma is adiabatic. The low level of power emission from the pellet-plasma interaction region is consistent with negligible charge-exchange losses; within the experimental accuracy, nearly all of the pellet mass can be accounted for in the initial plasma density rise. Penetration to r/a ∼ 0.15 is optimal, in which case large-amplitude sawtooth oscillations are observed and the density remains elevated. Gross plasma stability is dependent roughly on the amount of pellet penetration and can be correlated with the expected temporal evolution of the current density profile.

Neutral-beam injection of up to 2.5 MW into plasmas in the ISX-B tokamak (R0 = 0.93 m, a = 0.27 m, BT = 0.9–1.5 T, Ip = 70–210 kA, n̄e = 2.5–10×1013 cm−3) has created plasmas with volume-averaged beta of up to ∼ 2.5%, peak beta values of up to ∼ 9%, and root-mean-square beta values of up to ∼ 3.5%. Energy confinement time is observed to decrease by about a factor of two as beam power goes from 0 to 2.5 MW; the decrease is caused predominantly by the electron confinement time falling below the predictions of ‘Alcator scaling’ by a factor of 3–4 at high beam power. An empirical relationship of the form fits our measurements over a wide range of plasma parameters. The function f(Pb), where Pb is the beam power, is linear for Pb ≤ 1.2 MW but tends to saturate for 1.2 MW ≤ Pb ≤ 2.5 MW. Although the equilibria attained in ISX-B are predicted to be above the threshold for the ideal magnetohydrodynamic (MHD) ballooning instability, no evidence of these modes is observed.

Confinement studies on the Impurity Study Experiment (ISX-B) in beam-heated plasmas contaminated with small quantities of low-Z impurities are reported. Experimental results on the correlation of particle and energy confinement are presented. A linear relationship of energy confinement and plasma density is observed. As density is increased further, this effect saturates and energy confinement becomes independent of electron density. The experiments have been extended to higher beam power, resulting in an expansion of the ISX-B operating space. Impurities other than neon (carbon and silicon) have been tried and do not produce an enhancement in confinement. Edge cooling by the introduction of impurities has been demonstrated. The change in confinement has been shown to be correlated with changes in the normalized poloidal field fluctuation level (Sg/Bg) but not with the density fluctuation level (n e /n e ). The experimental results are compared with models of drift-wave and resistive ballooning turbulence and an explanation is offered for the difference between the results with recycling and non-recycling impurities.

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.