![]() In a tokamak, magnetic-field lines wind helically around the plasma. When the pressure gradient becomes the main instability driver, Harrer and colleagues demonstrate that the instability threshold (the magnitude of the pressure gradient needed to produce ELMs) can then be changed by adjusting the system’s magnetic topology. Moreover, predictions indicate that smaller ELMs could remove spent fusion fuel (helium ash) from the plasma core, preventing the core from being contaminated with nonfusible particles. These smaller ELMs lack the destructive impact of their larger counterparts. The result is a reduction in the size of the ELMs and an increase in their frequency. The density increase reduces the local plasma current, such that the pressure gradient at the plasma edge is the dominant instability driver. The researchers show that large ELMs can be avoided by first increasing the plasma density and then tailoring the magnetic topology. Using additional diagnostics, the team compared the observed ELM onset with theoretical predictions. They measured these ELM properties using a filterscope-an optical device that detects the visible light produced when a plasma interacts with gas in the divertor. The researchers investigated how the topology of the confining magnetic field influenced the size and frequency of the resulting ELMs. 1), located at the Max Planck Institute for Plasma Physics in Garching, Germany. Harrer and colleagues performed experiments on the ASDEX Upgrade tokamak (Fig. Fundamental advances in the understanding of plasma dynamics are required to extrapolate findings from current devices to fusion reactors because reactor-relevant conditions cannot be replicated in present-day setups. For this reason, researchers are actively working on ways to avoid or mitigate large ELMs in tokamaks. In a fusion reactor, these ELMs could be even more destructive because the plasma core’s stored energy-whose magnitude determines the energy deposited per ELM in the divertor-will be hundreds to thousands of times higher than that in current devices. ![]() In the largest operating tokamak, the UK-based Joint European Torus, large ELMs have contributed to the melting of the tungsten tiles used in the device’s divertor. Their size, expressed as a percentage of the energy stored in the plasma core, strongly influences how much heat and how many particles will be deposited by each ELM in the divertor. ELMs come in various sizes and frequencies (repetition rates). ELMs transport heat and particles along magnetic-field lines, moving them from the well-confined plasma core (the “filling” of the donut) to the divertor-a region of the tokamak’s walls. Instabilities that originate at the plasma edge (the “glaze” of the donut) are called edge-localized modes (ELMs). ![]() The researchers’ findings offer a fresh approach to running future fusion reactors.Ī tokamak uses a powerful magnetic field to confine fusion fuel in the form of a plasma (a highly ionized gas) that is shaped like a ring donut. Now Georg Harrer at the Vienna University of Technology and his colleagues have shown how these destructive instabilities can be avoided by adjusting the properties of the plasma and its confining magnetic field. Scientists have been looking for ways to prevent instabilities in a tokamak-a leading candidate for a fusion reactor-because the instabilities can cause substantial damage to the tokamak’s walls. ×Īll magnetically confined plasmas naturally develop instabilities, regions where small perturbations grow rapidly. The plasma’s edge is directed onto divertor plates located at the vessel’s base. View of the donut-shaped plasma (pink) confined in this vessel (right). Max Planck Institute for Plasma Physics (IPP) Figure 1: The plasma vessel of the ASDEX Upgrade tokamak (left).
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