High-confinement mode

In plasma physics and magnetic confinement fusion, the high-confinement mode (H-mode) is a phenomenon observed in toroidal plasmas such as tokamaks where the particle and energy confinement is significantly enhanced. In H-mode plasmas, the plasma edge features reduced turbulent transport and enhanced thermodynamic gradients (such as density, temperature and pressure) over a narrow region, known as the edge transport barrier or the pedestal. The H-mode transition requires a heating power above the critical power threshold.

The H-mode phenomenon was discovered 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 baseline operational scenario of many future reactors, such as ITER and SPARC. The physical origin of H-mode is an open problem in plasma physics. The reduction of turbulence is thought to be caused by sheared poloidal flows at the plasma edge. The steep profile gradients at the edge also drive a particular form of magnetohydrodynamic instability termed edge localized modes.

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

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