Bootstrap Fraction
The bootstrap fraction (f_BS) is the share of plasma current the plasma drives itself — reducing the external current-drive power a fusion plant must pay for.
- Symbol
- f_BS
- MetroVolt values
- 0.59 (baseline) / 0.78 (aligned high-β_p)
- Why it matters
- Less external current drive → higher net electric
- Related
- non-inductive fraction f_NI
In a tokamak, a toroidal plasma current is needed to confine and heat the plasma. Driving that current externally costs power, which subtracts from the plant's net output. The bootstrap current is a current the plasma generates for itself out of its own pressure gradient. The bootstrap fraction (fBS) is the share of the total current supplied this way.
A high bootstrap fraction is one of the largest levers on fusion economics: every ampere the plasma self-drives is an ampere the plant does not have to power. The Kronos MetroVolt design point carries a bootstrap fraction of 0.59 at baseline and 0.78 in the aligned high-βp posture, with a non-inductive fraction of 0.61–0.81 at 62 MW of current-drive power.
Negative triangularity and the high poloidal beta of the design work together to raise the bootstrap fraction, which is why the current-drive frontier is priced explicitly in the design series rather than assumed away.