Bootstrap Current Models
Bootstrap-current models predict the self-generated plasma current that arises from the pressure gradient, a key term in current-profile and scenario codes.
Current from the pressure gradient
In a toroidal plasma, the pressure gradient combined with trapped-particle orbits drives a spontaneous current parallel to the field, the bootstrap current. It is not driven externally; it emerges from neoclassical physics. Because it can supply a large fraction of the total current, predicting it accurately is central to scenario and stability modeling.
Neoclassical origin
In toroidal geometry some particles are trapped in banana-shaped orbits on the low-field side. The asymmetry between trapped and passing populations under a pressure gradient produces a net current. Models capture this through neoclassical coefficients that depend on collisionality, aspect ratio, and the fraction of trapped particles.
How it is modeled
- Analytic formulas relate bootstrap current to the pressure and density gradients
- Coefficients depend on collisionality and trapped-particle fraction
- Modern fits cover a broad range of geometry and collisionality
- The result is a current-density profile added to the current evolution
Why it matters
A high bootstrap fraction reduces the externally driven current a device must supply, easing the demands on current-drive systems and enabling steady-state operation. But the bootstrap current is tied to the pressure profile, so it couples the current profile to the transport and pedestal, and it feeds the drive for neoclassical tearing modes.
Integration
Bootstrap models are embedded in transport and equilibrium codes, since the self-generated current reshapes the safety-factor profile and hence stability. Getting it right is part of any self-consistent scenario, and its uncertainty propagates into current-profile and stability predictions.
Bootstrap current is one of the terms that determine whether a design can approach steady-state operation with realistic external current drive.