# 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 in toroidal magnetic confinement systems.
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
^{[1]}

- $ 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.

## References

- ↑ K. Miyamoto,
*Plasma Physics and Controlled Nuclear Fusion*, Springer-Verlag (2005) ISBN 3540242171