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

The formation of a peaked bell-shaped profile of the electron density n{sub e}(r) in the T-11M tokamak (B{sub t}=1 T, R/a = 0.7/0.2 m, I{sub p} = 100 kA, t{sub shot} {=} 0.5) under the conditions of low hydrogen recycling and intense hydrogen influx from the plasma edge. It is well known that peaked n{sub e} (r) profiles are observed in collisionless regimes at v* values as low as 10{sup -1}-10{sup -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 n{sub e} (r) profile in discharges with low n{sub e} 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 n{sub e}(r) profiles were observed at a relatively high central electron density n{sub e}(0) and relatively high collision frequency, such that the influence of impurities on the n{sub e}(r) profile could be ignored (Z{sub eff} = 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.