Bootstrap Current

A pressure-gradient-driven bootstrap current is the plasma partly sustaining its own current, easing the demand on external drive in a compact ST.

Current the plasma makes itself

When a plasma has a strong pressure gradient, kinetic theory predicts a self-generated toroidal current called the bootstrap current. It arises from trapped-particle orbits and grows with plasma pressure. Because it needs no external power, a large bootstrap fraction is highly valued: it lets the plasma carry part of its own current toward the 9.66 MA target.

Why it matters in a spherical tokamak

A spherical tokamak has limited central-solenoid flux, so it cannot rely on inductive drive to hold the current for long. External current drive consumes recirculating power. Bootstrap current fills the gap: the higher the bootstrap fraction, the less external drive is needed and the closer the machine comes to steady, efficient operation. Low aspect ratio and high beta both tend to favor bootstrap current, which suits Hyperion's design.

The balancing act

Bootstrap current is tied to the pressure profile, so it cannot be dialed independently: shaping the current profile means shaping the pressure profile, which also affects stability. The design must find an operating point where bootstrap, beam, and RF drive together give a stable 9.66 MA profile. Predicting the bootstrap fraction accurately at Hyperion's parameters is part of the confinement modeling the first-of-a-kind campaign will test.

• Self-generated current from the plasma pressure gradient
• Reduces external current-drive power
• Favored by low aspect ratio and high beta

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.