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.^[1] 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.

Physical properties

L-H transition

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.

Edge transport barrier

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

Edge-localized modes

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.

Energy confinement scaling

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.^[2] 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 ${\displaystyle {\text{MW}}}$