Transient energy losses from I-mode plasmas in future fusion reactors

A collaboration between IPP and MIT (USA) brought new insights into relaxation events at the edge of I-mode plasmas.

Relative loss of plasma energy as a function of collisionality at the plasma edge. The energy losses by PREs are significantly lower than those by ELMs. They increase with decreasing edge collisionality. The predicted values for fusion power plants are in the range of 2-3 % plasma energy content.

Graph: IPP

Relative loss of plasma energy as a function of collisionality at the plasma edge. The energy losses by PREs are significantly lower than those by ELMs. They increase with decreasing edge collisionality. The predicted values for fusion power plants are in the range of 2-3 % plasma energy content.

Graph: IPP

For the highest possible fusion power, future fusion power plants must be operated with the best possible plasma confinement, as achieved in the H-mode ("H" for "high confinement"). However, this is accompanied by an edge instability, the edge localised modes, ELMs, which periodically lead to high wall loading by high-energy plasma and damage the components. Therefore, ELMs must be avoided.

An alternative confinement regime is the I-mode ("I" for "improved"). Here, good plasma confinement is achieved without ELMs. Under certain conditions, however, smaller relaxations can occur at the plasma edge, which have been baptised PREs, "pedestal relaxation events".

To predict how large the energy loss due to such PREs will be in a fusion reactor, scientists from IPP and MIT (USA) analysed data from two tokamaks, ASDEX Upgrade (IPP) and Alcator C-Mod (MIT).

They found that the energy losses from PREs correlate with the collisionality at the plasma edge. This allowed a prediction of the expected PRE-induced energy losses in the DEMO and ARC power plants developed in Europe and the USA, respectively. Furthermore, this study showed that PREs only occur near the H-mode transition in both plants, paving the way for a stable I-mode plasma without transient energy losses.

This work was recently published in the journal Nuclear Fusion:

Silvagni et al. 2022 Nucl. Fusion https://doi.org/10.1088/1741-4326/ac4296