By combining two different approaches to plasma stabilization, physicists in the US and Germany have developed a new technique for suppressing instabilities in tokamak fusion reactors. The team, led by Qiming Hu at Princeton Plasma Physics Laboratory, hopes its computer-modelling results could be an important step towards making nuclear fusion a viable source of energy.
Tokamak fusion reactors use intense magnetic fields to confine and heat hydrogen plasma within their doughnut-shaped interiors. At suitably high temperatures, the hydrogen nuclei will gain enough energy to overcome their mutual repulsion and fuse together to form helium nuclei, releasing energy in the process.
If more energy is released in the reaction than is fed into the tokamak, it would provide an abundant source of clean energy. This has been a goal of researchers since fusion was first created in the laboratory in the 1930s.
Stubborn roadblock
One of the most stubborn roadblocks to achieving sustained fusion is the emergence of periodic plasma instabilities called edge-localized modes (ELMs). These originate in the outer regions of the plasma and result in energy leaking into the tokamak’s walls. If left unchecked, this will cause the fusion reaction to fizzle out, and it can even damage the tokamak.
One of the most promising approaches for suppressing ELMs is the use of resonant magnetic perturbations (RMPs). These are controlled ripples in the confining magnetic field that create closed loops of magnetic fields to form inside the plasma.
Dubbed magnetic islands, these loops do not always have a desirable influence. If they are too large, they risk destabilizing the plasma even further. But by carefully engineering RMPs to generate islands with just the right size, it should be possible to redistribute the pressure inside the plasma, suppressing the growth of ELMs.
In their study, Hu’s team introduced an extra step to this process, which would enable them to better control the parameters of RMPs to generate magnetic islands of just the right size.
Spiralling electrons
This involved injecting the plasma with high-frequency microwaves in a method called edge-localized electron cyclotron current drive (ECCD). Inside the plasma, these waves cause energetic electrons to spiral along the direction of the confining magnetic field lines, generating local currents which run parallel to the field lines.
In previous experiments, ECCD microwaves were most often injected into the core of the plasma. But in their simulations, the Hu and colleagues instead directed them to the edge.
“Usually, people think applying localized ECCD at the plasma edge is risky because the microwaves may damage in-vessel components,” Hu explains. “We’ve shown that it’s doable, and we’ve demonstrated the flexibility of the approach.”
Tight control
In simulated tokamak reactors, the team found that their new approach can lower the amount of current necessary to generate RMPs, while also providing tight control over the sizes of magnetic islands as they formed in the plasma.
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“Our simulation refines our understanding of the interactions in play,” Hu continues. “When the ECCD was added in the same direction as the current in the plasma, the width of the island decreased, and the pedestal pressure increased.”
The pedestal pressure refers to the region close to the edge of the plasma where the pressure peaks, before dropping off steeply towards the plasma boundary. “Applying the ECCD in the opposite direction produced opposite results, with island width increasing and pedestal pressure dropping or facilitating island opening,” explains Hu.
These simulation results could provide important guidance for physicists running tokamaks – including ITER experiment, which should begin operation in late 2025. If the same results can be replicated in real plasma it could bring the long-awaited goal of sustained nuclear fusion a step closer.
The research is described in Nuclear Fusion.
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- Source: https://physicsworld.com/a/magnetic-islands-stabilize-fusion-plasma-simulations-suggest/
