of limiter; the particles hit it and are rapidly cooled, remaining in the lithium. This internal pool is much easier to cool, due to its location, and although some lithium atoms are released into the plasma, its very low mass makes it a much smaller problem than even the lightest metals used previously. As machines began to explore this newly shaped plasma, they noticed that certain arrangements of the fields and plasma parameters would sometimes enter what is now known as the high-confinement mode, or H-mode, which operated stably at higher temperatures and pressures. Operating in the H-mode, which can also be seen in stellarators, is now a major design goal of the tokamak design. Finally, it was noted that when the plasma had a non-uniform density it would give rise to internal electrical currents. This is known as the bootstrap current. This allows a properly designed reactor to generate some of the internal current needed to twist the magnetic field lines without having to supply it from an external source. This has a number of advantages, and modern designs all attempt to generate as much of their total current through the bootstrap process as possible. By the early 1990s, the combination of these features and others collectively gave rise to the "advanced tokamak" concept. This forms the basis of modern research, including ITER. Plasma disruptions Tokamaks are subject to events known as "disruptions" that cause confinement to be lost in milliseconds. There are two primary mechanisms. In one, the "vertical displacement event" (VDE), the entire plasma moves vertically until it touches the upper or lower section of the vacuum chamber. In the other, the "major disruption", long wavelength, non-axisymmetric magnetohydrodynamical instabilities cause the plasma to be forced into non-symmetrical shapes, often squeezed into the top and bottom of the chamber. When the plasma touches the vessel walls it undergoes rapid cooling, or "thermal quenching". In the major disruption case, this is normally accompanied by a brief increase in plasma current as the plasma concentrates. Quenching ultimately causes the plasma confinement to break up. In the case of the major disruption the current drops again, the "current quench". The initial increase in current is not seen in the VDE, and the thermal and current quench occurs at the same time. In both cases, the thermal and electrical load of the plasma is rapidly deposited on the reactor vessel, which has