Spontaneous appearance of snakes in JET

Tokamaks exhibit a wide variety of mhd activity on the q=1 surface such as sawteeth and the recently discovered snake which is a surprisingly long lived compact region of high density plasma formed on the q=1 surface following pellet injection. More recently, snakes have been observed to form spontaneously in JET at the onset of the sawtooth and also during neutral beam injection. A typical sawtooth onset case shows that when q becomes 1 in or near the plasma centre mhd oscillations are seen with m=n=1 and a variety of frequencies and amplitudes. Immediately after the first sawtooth these are replaced by a snake, which gradually decays in amplitude, but survives several sawtooth crashes. It is probable that these spontaneous snakes have been previously observed in many other machines but classified as m=1 instabilities. For example, during the 'O' mode of sawtooth onset of DIII, or the mhd activity observed early in PBX discharges. Further, JET discharges from as long ago as 1984 had similar mhd activity at early times which, in retrospect, can also probably be identified as due to snakes. It appears that snakes are a much more general phenomena than originally realized. (author) 5 refs., 5 figs