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

${\tau }_{E,98y2}=0.0562I_p^{0.93}{B}_T^{0.15}⟨n_e{⟩}^{0.41}{P}_{SOL}^{-0.69}{R}_{geo}^{1.97}{\kappa }_a^{0.78}{ϵ}^{0.58}{M}^{0.19}$

${\tau }_E=\frac{W}{P_{input}-dW/dt}$

and $I_p$ is the plasma current, $B_T$ is the toroidal magnetic field at $R_{geo}$, $⟨n_e⟩$ 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 ${\kappa }_a=Are{a}_{CX}/\pi a^2$, $ϵ=a/R_{geo}$ is the inverse aspect ratio, $M$ is the ion mass, $a$ is the minor radius, $W$ is stored energy, and $P_{input}$ is the input heating power.

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.