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Fusion reactor instability can be fine-tuned by adjusting plasma density and magnetic field

An international team of researchers has found a way to control the magnitude of plasma instabilities in fusion reactors. Large instabilities can damage the reactor, while small instabilities can help remove waste helium from the plasma. Therefore, this finding may provide important clues about the operation of large fusion reactors.

Nuclear fusion of hydrogen nuclei in magnetically confined plasmas has the potential to produce vast amounts of green energy. However, controlling ultra-high temperature plasmas remains a major challenge. In the donut-shaped tokamak reactor, which is currently the most commonly used fusion experiment, the plasma is confined by a strong magnetic field. This creates a steep pressure gradient between the plasma edge and reactor wall. Excessive edge pressure gradients can lead to instabilities called Edge Localized Modes (ELM). These emit bursts of particles and energy that can severely damage reactor walls.

This latest research was led by his Georg Harrer at the Vienna University of Technology. To study the conditions under which ELMs form, the team conducted an ASDEX upgrade tokamak experiment at the Max Planck Institute for Plasma Physics in Germany.
Increased plasma density

They found that increasing the plasma density could avoid large ELMs, resulting in more frequent small ELMs. A small ELM not only reduces damage, but also helps remove excess helium from the plasma.

The team also found that ELM formation can be controlled by adjusting the topology of the magnetic field lines that confine the plasma when the plasma density is high. In a tokamak, these magnetic field lines swirl around the plasma, implying that the force they transmit changes direction depending on the pressure gradient. Forces counteract instabilities in some regions of the plasma, and promote instabilities in other regions. This tradeoff can be characterized by an instability threshold that defines the minimum pressure gradient required to generate an ELM.

Harrer and his colleagues found that increasing the helical winding of the magnetic field increased the instability threshold and thus decreased his ELM production. Moreover, increasing the magnetic shear at the edge of the plasma increases the instability threshold. Magnetic shear is the angle between two intersecting magnetic field lines.

Using a plasma with a large pressure gradient increases the fusion power gain of the fusion reactor, but with the trade-off of an increased risk of ELM damage. However, a small ELM could help with waste helium evacuation. Therefore, these phenomena must be finely balanced to optimize the operation of future fusion reactors. This latest study provides important insight into how this is achieved.

The team will report their findings in a physical review letter.


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