The Bootstrap Current

The bootstrap current is plasma current the pressure gradient generates by itself; a high bootstrap fraction is central to steady, efficient operation.

Current the plasma makes for free

A confined plasma with a pressure gradient spontaneously drives a toroidal current: the bootstrap current. It arises from the collisional coupling of trapped and passing particles and scales with the pressure gradient and, favorably, with the shape and beta of the plasma. It is 'free' in that it needs no external drive power.

Spherical tokamaks running at high beta can produce a large bootstrap fraction, and Hyperion's design leans on this. The more of the 9.66 MA that bootstrap supplies, the less external current drive is needed, and the less power is subtracted from the gain budget. Bootstrap is the mechanism that makes steady, gain-positive operation plausible in a compact ST.

The catch

Bootstrap current is tied to the pressure profile, so it cannot be dialed independently of the plasma state — the current profile and pressure profile are coupled, and that coupling can be unstable if mismanaged. Designing a self-consistent operating point where a high bootstrap fraction, a stable current profile, and the target pressure all coexist is a simulation problem the program treats as open.

• Self-generated current from the plasma pressure gradient
• Large in high-beta spherical tokamaks
• Coupled to the pressure profile — must be kept self-consistent and stable

This page describes a design-and-simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. No hardware net-gain is claimed before FOAK.