Disruption

A disruption is a phenomenon in a tokamak where the plasma current is rapidly lost and confinement suddenly collapses. It is known as a challenge specific to tokamaks, which rely on a large current for confinement.

Simple Definition (High School)

A tokamak uses the magnetic field created by a large current flowing through the plasma itself to achieve confinement. However, if the plasma becomes unstable for some reason, this current and the confinement can be lost all at once within a few milliseconds. This is a disruption.

To use an analogy, it is like a spinning top: while it spins fast it stands stably, but at some point it suddenly loses balance and falls over. At the moment it falls, the large amount of stored energy rushes toward the walls of the device all at once, so there is a risk of damaging the device.

Precise Definition (Undergraduate and Above)

A disruption is understood in terms of two main stages.

The first is the thermal quench. An instability grows and breaks the plasma confinement, releasing the thermal energy stored in the plasma to the walls and divertor in about one millisecond. The plasma temperature drops sharply.

The second is the current quench. As the temperature falls, the electrical resistance of the plasma rises, and the plasma current $I_\mathrm{p}$ decays over a few to a few tens of milliseconds. The rapid time variation of the current $\mathrm{d}I_\mathrm{p}/\mathrm{d}t$ induces large eddy currents in the conductors of the device, which interact with the magnetic field to produce strong electromagnetic forces.

Furthermore, the strong induced electric field that arises during the current quench can accelerate some electrons to nearly the speed of light, creating a high-energy electron beam known as runaway electrons. If runaway electrons strike the wall locally, they can cause serious damage.

Role in Fusion Research

Disruptions are a challenge specific to tokamaks, which use the plasma current for confinement. In large devices such as ITER, the greater the stored energy, the more serious the effects of heat loads, electromagnetic forces, and runaway electrons become. For this reason, research into prediction, which detects signs of instability in advance, and mitigation (disruption mitigation), which injects large amounts of gas or pellets to distribute the energy uniformly, occupies an important place in the design of practical reactors. One of the reasons stellarators attract attention as an approach that does not require a plasma current is that disruptions are, in principle, unlikely to occur.

Learn More

• Plasma Instabilities
• Tokamak

Related Terms

• Tokamak - The approach where disruptions become a problem
• Stellarator - An alternative approach that does not rely on a plasma current
• Plasma - The object that collapses
• Divertor - The component that absorbs the heat load