Bootstrap current

The bootstrap current is a Neoclassical toroidal current produced in the presence of a pressure gradient, associated with the existence of trapped (banana) particles. These trapped particles must be able to complete their (banana) orbits, so a requirement for the existence of the bootstrap current is ν_ei < ν_b (the collision frequency is less than the banana bounce frequency). The difference in particle density on banana orbits crossing a given radial position r then leads to a net toroidal current at r. The bootstrap current is estimated (roughly) as

$j_{b}\sim -\varepsilon ^{1/2}{\frac {1}{B_{p}}}{\frac {dp}{dr}}$

Here, ε is the inverse aspect ratio a/R, B_p the poloidal magnetic field, and p the pressure. More precise estimates can be made by simulating particle orbits.