Confinement Scaling Laws (IPB98)

Because confinement can't be computed exactly, the field uses empirical scaling laws. Kronos quotes its requirement as H98 over the IPB98(y,2) standard.

What

Empirical formulas predicting confinement time

Standard

IPB98(y,2), from a multi-machine database

Kronos metrics

H98 = 1.84; also ST-specific H_ST

Caveat

Extrapolation must be checked by gyrokinetics

Because energy confinement time cannot yet be predicted from first principles for a reactor, the fusion field relies on empirical scaling laws — formulas fitted to databases of many machines that predict confinement from size, field, current, and power. The most widely used is IPB98(y,2), and performance is expressed as a multiplier H98 over it.

Kronos MetroVolt quotes its confinement requirement as H98 = 1.84, and also tracks a spherical-tokamak-specific scaling (H_ST) in prediction P1. Scaling laws are powerful but they are extrapolations: applying a database built mostly on positive-triangularity H-mode plasmas to a negative-triangularity machine must be checked, which is exactly why the nonlinear gyrokinetic study and the G1 testbed exist.