In the paper quoted in the title (Anderson et al., 1991) it is claimed that it has been found that non-inductive current drive scenarios, as considered for Tokamak reactors with ITER-like scalings for the energy confinement time, are unstable (or at best marginally stable). The phenomenon is qualitatively transparent: for example, a positive plasma temperature excursion increases the non-inductively driven current (at fixed heating/current drive power and in the absence of a controlled external toroidal electric field) which in turn improves the global energy confinement properties so that the plasma temperature increases further. Since the time scale of the variation of the plasma current is the global skin time tauJwhich is longer than any other characteristic time intervening, the growth time of the instability is of the order of tauJ, i.e. 1000 s or more in practical cases.
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