H98

$H_{98}$ is a metric for plasma confinement quality. It is defined as the ratio of the energy confinement time to the confinement time predicted by the IPB98(y,2) scaling law.

$H_{98}={\frac {\tau _{E}}{\tau _{E,98y2}}}$

where

$\tau _{E,98y2}=0.0562\;I_{p}^{0.93}\;B_{T}^{0.15}\;\langle n_{e}\rangle ^{0.41}\;P_{SOL}^{-0.69}\;R_{geo}^{1.97}\;\kappa _{a}^{0.78}\;\epsilon ^{0.58}\;M^{0.19}$

$\tau _{E}={\frac {W}{P-dW/dt}}$

and $I_{p}$ is the plasma current, $B_{T}$ is the toroidal magnetic field at $R_{geo}$, $\langle n_{e}\rangle$ is the average density, $P_{SOL}$ is the loss power across the LCFS into the SOL, $R_{geo}$ is the geometric major radius (average of maximum and minimum $R$ of the LCFS) of the plasma, $\kappa _{a}$ is the elongation, defined unusually in this case as $\kappa _{a}=Area_{CX}/\pi \;a^{2}$, $\epsilon =a/R_{geo}$ is the inverse aspect ratio, $M$ is the ion mass, and $a$ is the minor radius.

For a basic, predictable H-mode scenario, $H_{98}\approx 1.0$. L-mode will have $H_{98}$ significantly below 1, and some regimes such as super H-mode can have $H_{98}$ significantly above 1.0.