Anomalous pinch in the T-11M tokamak in an enhanced-collisionality regime

Abstract

The formation of a peaked bell-shaped profile of the electron density ne(r) in the T-11M tokamak (Bt=1 T, R/a = 0.7/0.2 m, Ip = 100 kA, tshot ≤ 300 ms, Li and C limiters) was observed in Li experiments carried out in the near-plateau collisionality regime (the collisionality parameter at one-half of the minor radius was v* ≥ 0.5) under the conditions of low hydrogen recycling and intense hydrogen influx from the plasma edge. It is well known that peaked ne (r) profiles are observed in collisionless regimes at v* values as low as 10−1–10−2 or in impurity-contaminated discharges, in which this effect can be attributed to the impurity accumulation on the plasma column axis. Moreover, a bell-shaped ne (r) profile in discharges with low ne can result from the ionization of hydrogen atoms at the column axis, where they arrive from the plasma edge due to cascade charge-exchange. In quasi-steady lithium discharges in T-11M, however, peaked ne(r) profiles were observed at a relatively high central electron density ne(0) and relatively high collision frequency, such that the influence of impurities on the ne(r) profile could be ignored (Zeff = 1.1±0.1). To explain this effect, one has to assume that the pinching of hydrogen ions in T-11M is anomalous. The lower estimate of the observed pinch velocity is 4 ± 1 m/s, which is three to five times higher than the velocity of the neoclassical (Ware) pinch, characteristic of these conditions. The work is devoted to the experimental study of this effect.