Skip to content
Try: plasma ITER tokamak
↑↓ to navigate Enter to select Esc to close

H-mode

H-mode (high-confinement mode) is an operating regime in which, once a tokamak plasma is heated above a certain level, a transport barrier spontaneously forms at the plasma edge and confinement performance suddenly improves.

Simple Definition (High School)

Section titled “Simple Definition (High School)”

As you heat a plasma, the moment you exceed a certain heating strength, the edge region of the plasma abruptly switches into a state where heat and particles escape much less easily. This is H-mode. It is like an insulating layer of a vacuum flask forming naturally at the plasma edge, so that with the same heating the central temperature can be kept far higher. It was first discovered in 1982 on the German device ASDEX, and because the confinement time is roughly doubled compared with the previous ordinary state (L-mode, low-confinement mode), it dramatically changed the course of fusion research.

Precise Definition (Undergraduate and Above)

Section titled “Precise Definition (Undergraduate and Above)”

H-mode (High-confinement mode) is a state in which, when the heating power exceeds a threshold (the L-H transition power), a narrow transport barrier (edge transport barrier) forms at the outermost region of the plasma. The plateau-like structure where temperature and density rise steeply just inside this barrier is called the pedestal. The barrier is understood to be maintained because the shear in the radial electric field ErE_r that arises at the plasma edge tears apart turbulent eddies and suppresses turbulent transport. Because the height of the pedestal raises the entire core temperature, it is an important indicator that governs confinement performance. On the other hand, when the pressure gradient and current in the pedestal region become too large, periodic instabilities occur at the edge.

Role in Fusion Research

Section titled “Role in Fusion Research”

Because H-mode can achieve higher temperature and density for the same heating input, it is an operating scenario directly tied to reaching the Lawson criterion. Next-generation tokamaks, starting with ITER, are designed with H-mode as the standard operating regime. However, in H-mode a periodic instability called the ELM (Edge Localized Mode) occurs, releasing the energy stored in the pedestal over a short time, and the released heat locally damages the divertor. How to reconcile the trade-off between the benefit of enhanced confinement and the heat load caused by ELMs is an important current research topic.

Learn More

Section titled “Learn More”
Section titled “Related Terms”