Spontaneous appearance of q=2 Snakes in JET Optimised Shear Discharges

摘要

Snakes, which get their name from their characteristic signature when viewed by soft X-ray (SXY) cameras [1], have been observed in many tokamaks. They apear as localised structures on a q=1 (or q=3/2) field line. They ususally form as molid D2 pellets cross the ratioal q surface or following the onset of sawteeth. For the frist time snales with an (m,n)=(2,1) topology are regularly seen at JET in Optimised Shear (OS) discharges following formation of a strong Internal Transport Barrier (ITB). These snakes form spontaneously with no obvious trigger and are observed in cases where a strong ITB is formed inside, but close to, the q=2 magnetic surface. Their formation is believed to be dependent on the low magnetic shear and low values of transport coefficients that prevail locally under these conditions. Like their q=1 counterparts, the q=2 snakes display remarkable stability over their lifetime.