Impurity accumulation in ISX-B during counter-injection. Are alternative hypotheses valid

Neutral-beam-heated experiments in the ISX-B tokamak have been interpreted as showing impurity accumulation with counter-injection but not with co-injection. It has recently been suggested that there is no difference of transport in ISX-B during these operations and that the experimental results are artifacts of a recombining plasma caused by greater influxes of impurities during counter-injection. It is shown here that this interpretation does not realistically take account of atomic processes and impurity ionization balances. In addition, several new pieces of data are presented that support the previous conclusions of the ISX-B group. These include observations of accumulation of fully stripped light ions by charge-exchange spectroscopy, for which detailed analysis of the atomic physics is not required, and the measurement of almost identical production rates and influxes of limiter material for both senses of injection. (author)