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.

Analysis of MHD activity in pellet enhanced performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase (m,n)=(1,1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events usually is n=3, whereas the mode following them is dominantly n=2. Toroidal linear MHD stability calculations assuming a non-monotonic q-profile with an off-axis minimum decreasing from above 1 to below 1 describe this sequence of modes (n=3,1,2), but always give a larger growth rate for the n=1 mode than for the n=2 mode. This large growth rate is due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m,n)=(1,1) mode is observed as a hot spot with a ballooning character on the low field side. The hot spot has some of the properties of a 'hot' island consistent with the presence of a region of negative shear

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.

The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 × 10l6 s−1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 × 1019 m−3. The corresponding fusion product nD(0)τETi(0) is between 7.0 and 8.6 × 1020 m−3·s·keV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.

The magnetohydrodynamic (MHD) activity during density limit disruptions in tokamaks is modelled numerically by three-dimensional resistive reduced MHD simulations with a simple transport model including radiation losses. The simulations reproduce experimentally observed phenomena such as the destabilization of MHD modes near the plasma edge during the early profile contraction phase, followed by growth of the m = 2/n = 1 mode to large amplitude, a sequence of minor disruptions and the major disruption. A new theoretical model is given for the major disruption, which takes place in two phases: (1) an internal relaxation flattens the temperature in the central part of the discharge and (2) the current profile broadens. The internal instability of the first phase has a mainly m = 1/n = 1 convection pattern, but, because of non-linear coupling to the large m = 2/n = 1 mode, the magnetic perturbation has a strong m = 3/n = 2 component. During the internal relaxation, the large amplitude 2/1, 1/1 and 3/2 perturbations break up the magnetic surfaces isolating the q = 1 region from the stochastic region around q = 2, and the magnetic field becomes stochastic in the entire q <, 2 region. In the second phase of the major disruption, MHD turbulence first develops on the stochasticized fields, resulting in current filamentation, initially in the central region where q ^ 2. This leads to a broadening of the central current profile and a strong instability of the 2/1 mode. The disruption ends with rapid growth of the m > 2/n = 1 modes. The result is stochastic magnetic fields across the entire plasma and a large scale broadening of the current profile.