Observations of sawtooth postcursor oscillations in JET and their bearing on the nature of the sawtooth collapse

Abstract

An analysis of sawtooth postcursor oscillations in JET is presented. From electron cyclotron emission measurements, the radial Te profile on the equatorial plane is obtained with high time resolution. The plasma rotation is used to reconstruct the constant temperature contours in the poloidal plane from the Te profile. The postcursor oscillation observed after a partial collapse shows a cold region with a structure very much like that of an m = 1 magnetic island, while the hot plasma core is displaced but still has concentric Te contours. After a full collapse, a cold region of similar structure is seen, while the hot core is displaced over a much larger distance and is squeezed into a crescent shape. Also a large amount of energy is lost from the hot core. It is concluded that partial and full collapses are likely to be due to the same instability and that reconnection is involved in the non-linear evolution of the instability. It is suggested that the amplitude at which the instability saturates determines the type of sawtooth collapse and the amplitude of the postcursor oscillation.