Power threshold for neutral beam current drive

For fully non-inductive current drive in tokamaks using neutral beams, there is a power and density threshold condition, setting a minimum value for P3/2/n2. If this condition is not met, a stationary state cannot occur, and a tokamak discharge will collapse. This is a consequence of the coupling between current and electron temperature, or between current drive efficiency and energy confinement time. (author). Letter-to-the-editor. 4 refs, 4 figs