High-confinement mode

In plasma physics and magnetic confinement fusion, the high-confinement mode (H-mode) is a phenomenon observed in toroidal fusion plasmas such as tokamaks. In general, plasma energy confinement degrades as the applied heating power is increased. Above a certain characteristic power threshold, the plasma transitions from L-(low-confinement) to H-mode regime, where the particle and energy confinement is significantly enhanced.

The H-mode was discovered by Friedrich Wagner and team in 1982 on the ASDEX diverted tokamak. It has since been reproduced in all major toroidal confinement devices, and is foreseen to be the standard operational scenario of many future reactors, such as ITER.

Plasma confinement degrades as the applied heating power is increased (referred to as the low-confinement mode, or the L-mode). Above a critical power threshold that crosses the plasma boundary, the plasma transitions to H-mode where the confinement time approximately doubles.

In the H-mode, an edge transport barrier forms where turbulent transport is reduced and the pressure gradient is increased.

The steep pressure gradients in the edge pedestal region leads to a new type of magnetohydrodynamic instability called the edge-localized modes (ELMs), which appear as fast periodic bursts of particle and energy in the plasma edge.

H-mode is the foreseen operating regime for most future tokamak reactor designs. The physics basis of ITER rely on the empirical ELMy H-mode energy confinement time scaling. One such scaling named IPB98(y,2) reads:

${\displaystyle \tau _{E}^{\text{IPB98(y,2)}}=0.0562M^{0.19}I_{\text{P}}^{0.93}R^{1.97}\epsilon ^{0.58}\kappa ^{0.78}n^{0.41}B^{0.15}P^{-0.69}}$

where

• ${\displaystyle M}$is the hydrogen isotopic mass number
• ${\displaystyle I_{\text{P}}}$is the plasma current in ${\displaystyle {\text{MA}}}$
• ${\displaystyle R}$is the major radius in ${\displaystyle {\text{m}}}$
• ${\displaystyle \epsilon }$is the inverse aspect ratio
• ${\displaystyle \kappa }$is the plasma elongation
• ${\displaystyle n}$is the line-averaged plasma density in ${\displaystyle 10^{19}{\text{m}}^{-3}}$
• ${\displaystyle B}$is the toroidal magnetic field in ${\displaystyle {\text{T}}}$
• ${\displaystyle P}$is the total heating power in ${\displaystyle {\text{MW}}}$