In a toroidal fusion power device, a

plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...

is confined within a donut-shaped cylinder. If the gas pressure of the plasma varies across the radius of the cylinder, a self-generated current will spontaneously arise within the plasma, due to

collisions In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great f ...

between

trapped particles Trapped may refer to: Films * ''Trapped'' (1931 film), a crime drama short starring Lina Basquette * ''Trapped'' (1937 film), an American western starring Charles Starrett * ''Trapped'' (1949 film), a semidocumentary film noir directed by Richar ...

and passing particles. This current is called the bootstrap current, and is commonly found in tokamak fusion devices. The tokamak uses a combination of external magnets and a current driven in the plasma to create a stable confinement system. One goal of advanced tokamak designs is to maximize the bootstrap current, and thereby reduce or eliminate the need for an external current driver. This could dramatically reduce the cost and complexity of the device. From a kinetic point of view, the bootstrap current is the effect of trapped particles (which practically lie on poloidal plane) and density gradient: the poloidal current due to trapped particles motion is unbalanced since the density is not constant, therefore the bootstrap current is needed to "close the circuit".

References

"Self Generated “Bootstrap” Current Contains Magnetic Fusion Plasma"
https://web.archive.org/web/20160303224504/http://fusion.gat.com/images/3/3a/PolitzerAPS04.pdf "Archive"], General Atomics
"Plasma Dictionary"
https://web.archive.org/web/20181003220902/http://www.ees.nagoya-u.ac.jp/~web_dai5/english/B.html "Archive"], Nagoya University {{nuclear-power-stub Fusion power