The Bootstrap-Current Benchmark

The self-generated bootstrap current is estimated with neoclassical theory and cross-checked against the spherical-tokamak record, because low aspect ratio favors a high bootstrap fraction.

Current the plasma drives itself

Part of a tokamak's plasma current arises from the plasma's own pressure gradient — the bootstrap current. A high bootstrap fraction reduces the external current drive a steady machine must supply. Low-aspect-ratio spherical tokamaks favor a high bootstrap fraction, which is one reason the breeder (Hyperion) adopts the spherical geometry.

Method and cross-check

The bootstrap fraction is computed from neoclassical theory using the design pressure and current profiles, then compared to the fractions observed in NSTX/MAST-class experiments at similar aspect ratio. Agreement between theory and the ST database raises confidence; a discrepancy is flagged for FOAK measurement.

Why it matters for the gates

The bootstrap fraction feeds directly into whether the plasma current of 9.66 MA can be sustained without excessive recirculating power, which in turn affects the whole gain picture. The record reports the fraction with its uncertainty rather than assuming an optimistic value, and treats confirmation of the profile as a FOAK objective.

An optimistic bootstrap assumption is a common way a steady-state design flatters itself: overstate the self-driven fraction and the external current-drive burden shrinks on paper. The breeder analysis guards against this by carrying the neoclassical estimate with its interval and by refusing to credit a bootstrap fraction the ST database does not support at the design profiles.

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