As shown in Chap. 8, disruptions are disasters. Magnetic containment is suddenly lost, and the plasma drifts vertically into the walls, depositing all its thermal energy. The tokamak current tries to keep itself going as the plasma goes away, so very high voltages are generated. Runaway electrons of MeV energies are created by the high voltages, and these electrons crash into the walls, generating high-energy X-rays. The plasma current is used to generate the poloidal magnetic field, and as this field decays with the current, large forces are applied to the magnetic coils and conducting parts of the tokamak structure. The entire energy in the plasma, magnetic field, and tokamak current is something like 500 MJ, and in a disruption this is all dumped into the structure of ITER in 1/30th of a second [18]. This is like an explosion of 120 kg (260 lbs.) of TNT. Disruptions are expected in ITER, and its parts are designed to withstand them. Disruptions have to be eliminated in reactors, which would be so heavily damaged as to require lengthy shutdowns for repair.

There is a possible scenario of how a change in the magnetic structure of the tokamak discharge, such as a coalescing of magnetic islands, can cause a disrup­tion. It has been confirmed in experiment that staying well below the known stability limits, such as the density limit, can avoid disruptions. A reactor, however, needs to operate at the highest level to lower the cost of electricity (COE). Since a disruption is now known to be a vertical displacement of the plasma, there are ideas on stopping these displacements with a coil or coils inside the chamber. Such a coil is included in Fig. 9.27. Though it is not possible to stop a disruption once it starts, there are ways to mitigate the damage. Disruptions have magnetic precursors which can be detected, and fast action can be taken. Injection of liquid jets or solid pellets of a frozen gas have been tried, but these have led to creation of too many runaway electrons. A large puff of a gas like argon can be driven well into the plasma, be ionized into high-Z ions, and increase the resistivity so that the current dies more gently. Fast gas valves have been developed for this purpose. There is then a smaller tendency to induce currents elsewhere, lower forces on the structure, and fewer runaway electrons. After a disruption, there is only gas left in the vacuum chamber. This has to be pumped out and the discharge started all over again.