In JET, both high density and low-q operation are limited by disruptions. The density limit disruptions are caused initially by impurity radiation. This causes a contraction of the plasma temperature profile and leads to an MHD unstable configuration. There is evidence of magnetic island formation resulting in minor disruptions. After several minor disruptions, a major disruption with a rapid energy quench occurs. This event takes place in two stages. In the first stage there is a loss of energy from the central region. In the second stage there is a more rapid drop to a very low temperature, apparently due to a dramatic increase in impurity radiation. The final current decay takes place in the resulting cold plasma. During the growth of the MHD instability the initially rotating mode is brought to rest. This mode locking is believed to be due to an electromagnetic interaction with the vacuum vessel and external magnetic field asymmetries. The low-q disruptions are remarkable for the precision with which they occur at qψ = 2. These disruptions do not have extended precursors or minor disruptions. The instability grows and locks rapidly. The energy quench and current decay are generally similar to those of the density limit.

No 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.

No abstract

No abstract