New insights into I-mode plasmas: pedestal relaxation events

In ASDEX Upgrade pedestal relaxation events present in the I-mode confinement regime have been investigated for the first time

One of the major challenges on the way to a fusion power plant is to find a plasma scenario with good confinement and at the same time low divertor power loads.

The H-mode has the best energy confinement, but loads the divertor target plates with high power from type-I ELMs (edge localized modes). The I-mode, on the other hand, is free of Type I ELMs while still providing good confinement. This makes it an attractive regime for fusion power plants. However, even in the I-Mode pedestal relaxation events of the plasma edge named PREs put strain on the divertor targets. Experiments on ASDEX Upgrade have now shown that PREs only occur near the transition from I-mode to H-mode. This means that they can be avoided if the plasma is operated with heating powers below the threshold for the transition to H-mode. An analysis of infrared data showed that PREs stress the divertor plates much less than large ELMs. This applies to both the energy ejected from the plasma and to the heat flow into the divertor. Without sufficient control, the PREs load on the target material of a fusion power plant could still be too high.

This work has recently been published in Nuclear Fusion.