Edge Localised Modes
Fusion plasmas can develop various kinds of instabilities. Edge Localised Modes (ELMs) are considered as potentially severe because they can harm the inner lining of the vessel wall (first wall). ELMs are repetitive bursts of the edge plasma. Because of their periodicity (albeit irregular), one way to imagine the ELM phenomenon is to picture a single ELM cycle. The most rapid changes occur during an ELM crash which is usually significantly shorter than the time between the ELMs. ELM activity may evolve as a short, intense heat load on the plates and causes erosion of the divertor materials. During the instability, the edge pressure gradient is reduced until the plasma becomes stable again. Then the pressure gradient starts recovering to the level where it reaches the stability limit so that another ELM occurs. If the conditions stay constant, the cycle can continue indefinitely. Depending on the ELM type and the details of a plasma device, each ELM removes 1 – 7 % of the plasma energy and particles.
The figure shows the plasma cross-section and the radial plasma pressure profile at four different time points during an ELM crash. First the plasma is stable and has a steep pressure gradient at the edge. The gradient is maintained by the edge transport barrier that is always associated with the high confinement mode (H-mode) of tokamak operation. Then, pressure builds up at the plasma edge. The onset of an ELM can be imagined as an onset of many small turbulent eddies at the edge. The pressure collapses and the plasma is lost to the Scrape-Off Layer (SOL) where it flows along the magnetic field lines towards the divertor, whose plates produce a distinctive peak in the D-alpha radiation (visible light emitted by excited atoms of deuterium fuel).
In the presence of the edge transport barrier, i.e. in the tokamak H-mode operation, ELMs are instrumental
for maintaining a stable density of confined plasma. In other words, without ELMs the plasma density
in the H-mode increases above the overall stability limit, leading to sudden loss of the plasma confinement
in a major instability called plasma disruption. However, two ELM-free operating
modes with stable density have been observed in high confinement mode (H-mode) of tokamak operation.
In order to decrease the divertor erosion and, at the same time, maintain a good control of the pressure
profile, several methods of ELM suppression are considered at present. The two most promising approaches
are the following
- pace making of ELMs by injecting small pellets of frozen fusion fuel into the plasma edge at
a high frequency, see e.g. JET’s capabilities in support of ITER
- plasma edge ergodisation by resonant perturbations of the magnetic field. Studies at the DIII-D tokamak demonstrated
an unexpectedly strong ELM suppression via resonant magnetic field perturbations. This is considered
to be a very promising result for a reactor-relevant operation. Similar observations have been made at the ASDEX Upgrade tokamak.