A future fusion reactor will need to maintain a high current over a long time scale in order to produce a decent amount of energy.
By ramping an external current, a voltage swing is produced which gives some inductive current. However, this is inherently limited. More current is produced by neutral beam heating and radio frequency heating.
The bootstrap current is a godsend for plasma physics in a tokamak as it is the current produced by radial diffusion in the plasma itself and not from any external source (altougth the conditions for the current to start booting itself must first be created).
The bootstrap current is a non-inductive current which results from a complex interplay between trapped and untrapped particles in a tokamak.
Theorists predicted the current before it was experimentally observed, first in the Japanese tokamak JT-60. The Onsager symmetry relations imply that if there is a density gradient in a plasma there must be a corresponding current somewhere. The exchange of momentum between the trapped and passing particles results in a net current; the bootstrap current. It value depends on the inverse aspect ratio of the tokamak (a geometric factor), the pressure gradient and density gradient plus the value of βpoloidal (i.e. the ratio of the kinetic and magnetic pressures).
When the plasma ions and electron are heated considerably (to achive fusion reactions) a high βpoloidal and a peaked pressure profile results. Therefore, a high bootstrap current is a natural and very welcome by-product.
Bootstap currents that account for up to 80% of the total plasma current have been observed. In a future tokamak, it may account for close to 100%.