A process is said to be unstable when an initially slight perturbation induces a force that intensifies this perturbation.
Such instabilities can hamper plasma confinement. They lead to undesirable deformation of the confinement geometry and – at worst – disruption of the plasma discharge.
The number of instabilities possible is very large. Identifying their causes and finding remedies was one of the main fields of endeavour particularly in the early days of fusion research. In most cases, it required lengthy series of experiments and intensive collaboration of experimental and theoretical physicists.