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.

Disruptions and related Vertical Displacement Events pose a major problem to the design and operation of future tokamak reactors. The cause and dynamics of disruptions will be described for the many different scenarios that these violent events can follow. Possibilities will be discussed to avoid or at least to ameliorate the damaging effects of disruptions. IntroductionDisruptions have been with us as long as tokamaks exist. In the early years they were mainly regarded as a nuisance and the l i i t e d time and spatial resolution of the diagnostics of those days prevented much progress in undentanding. In present-day machines of the size of ET, TFTR, and JT-6OU disruptions are more than a nuisance, the are able to cause considerable structures, melting or erosion of plasma facing components and short circuits in extemal supplies. The situation will be even worse in future burning fusion devices, like ITER, for which forces and heat loads are foreseen which are two orders of magnitude larger than in present-day machines. Therefore disruptions feature hkh on the physics R&D pmgramme damage. Forces up to 2 MN and heat loads up to 2 MJ/m T have caused deformation of of ITER [f]. The sophistication of present-day diagnostks enabl& us to make such-physics studies fruitfuL Classification of observed disruption scenariosThe chain of events altogether called 'disruption' is sketched in figure 1: initiating event, precursor, thermal quench and current quench. Many different initiating events, different precursor scenarios and current quench scenarios are possible. These could not always be distinguished in the past, which caused confusion. Until recently it was thought that there was only one physics mechanism underlying the thermal quench nonlinear interaction between low-mode number tearing modes, seeHowever, with the better diagnostics it has become clear that also for this very short duration event there are at least two different types of thermal quenches with different physics mechanisms: *) The aulhor acts mekly as rappimew on Le state-of-the-art in tokamak disruptions as established during the ITER Workshop on Disruptions and Vertical Displacement Events organized as a " i c a l Meeting of the ITER Disruptions, Plasma Conaol and MHD Expen Group m Garching, Germany, Febnrary 13-17.1995. Much of Le material presented is the unpublished work of participants which has been kindly made available to the author.

Oscillating MHD modes in JET are often observed to slow down as they grow and generally stop rotating (lock) when the amplitude exceeds a critical value, then continue to grow to large amplitudes (b̃r/Bθ ∼ 1%). The mode can grow early in the current rise or after perturbations, such as a pellet injection or a large sawtooth collapse, and maintain a large amplitude throughout the remainder of the discharge. Such large amplitude quasistationary MHD modes can apparently have profound effects on the plasma, including stopping central ion plasma rotation, reducing the amplitude and changing the shape of sawteeth, flattening the temperature profile around resonant q surfaces and reducing the stored energy. Perhaps most important, large amplitude locked modes are precursors to most disruptions. Some large amplitude modes can be avoided by proper programming of the q evolution. The apparent reasons for the mode locking in a particular location are discussed and a comparison with theory is made.

An experimental study of the generation of runaway electrons in TEXTOR has been performed. From the infrared synchrotron radiation emitted by relativistic electrons, the number of runaway electrons can be obtained as a function of time. In low density discharges (ne < 1 × 1019 cm-3) runaways are created throughout the discharge and not predominantly in-the startup phase, From the exponential increase in the runaway population and the ongoing runaway production after the density is increased, it is concluded that the secondary generation, i.e. the creation of runaways through close collisions of already existing runaways with thermal electrons, provides an essential contribution to the runaway production. The effective avalanche time of this secondary process is determined to be teff = 0.9 ± 0.2 s

Experiments with strong localized electron cyclotron heating (ECH) in the RTP tokamak show that electron heat transport is governed by alternating layers of good and bad thermal conduction. For central deposition hot T e filaments are observed inside the q = 1 radius. Moving the ECH resonance from the centre to the edge of the plasma results in discrete steps of the central electron temperature. The transitions occur when the minimum q value crosses q = 1, 2, 5/2 or 3, and correspond to the loss of a transport barrier situated close to the rational q value. Close to the transitions a new type of sawtooth activity is observed, characterized by the formation of sharp off-axis maxima on the T e profile, which collapse abruptly. The formation of the off-axis maxima is attributed to heat deposition precisely 'on top of' a transport barrier.