The fascinating dance of magnetic fields: How to make plasma move gracefully in a tokamak?

In magnetically confined core fusion, a deivertor is a magnetic field configuration that directs heat and escaping particles to dedicated plasma surface components, thereby spatially separating the regions where the plasma interacts with the surface from the confined core . This process requires establishing a magnetic field configuration bounded by a separatrix, typically accomplished by using external coils to create pole field null points (X-points). As an important component of magnetically confined nuclear fusion equipment, deivetors were originally introduced in the 1950s by Lyman Spitzer for the stellarator concept.

The deivertor extracts the heat and ash generated by the fusion reaction while protecting the main chamber from thermal loading and reducing plasma contamination levels from sputtered impurities.

In tokamak, the deivertor configuration makes it easier to obtain the more stable high-constraint mode (H-mode). It is currently expected that future fusion power plants will generate deivertor heat loads that exceed the engineering limits of plasma surface assemblies. Therefore, finding mitigation strategies to reduce deivetor power exhaust challenges has become an important topic in nuclear fusion research.

Tokamak deivertor

A tokamak with a deivertor is called a deivertor tokamak or a deivertor configuration tokamak. In this configuration, particles escape through a magnetic "separatrix," which allows the energy-absorbing deivertor to be placed outside the plasma. This deivertor configuration also makes it easier to obtain more stable H-mode operation.

The plasma surface material in the deivertor faces significantly different stresses than most first walls.

Star device deivertor

In stellarators, low-order magnetic islands can be used to form a deivertor capacity, called an island deivertor, to manage power and particle emissions. The success of the island deivertor in acquiring and stabilizing desorption scenarios and demonstrating reliable heat flow and desorption control in the W7-X stellarator using hydrogen injection and impurity seeding.

The magnetic island chain can control the fuel supply of the plasma.

Despite some challenges, the island deivertor concept shows great potential in managing the power and particle emissions of nuclear fusion reactors, and further research may lead to more efficient, reliable operations in the future.

Large spiral device and non-resonant deivertor

Helical divertors, such as the large helical coils used in Large Helical Devices (LHD), use these coils to create a steering field. This design allows tuning of the random layer size located between the confined plasma volume and the field lines ending at the deivertor plate. However, it remains uncertain whether spiral deivetors are compatible with stellarators optimized for neoclassical transport.

The non-resonant deivertor provides an alternative design for optimized stellarators with significant self-generated currents.

This method uses sharp "ridges" on the plasma boundary to conduct magnetic flux. Self-generated currents modify the shape, but not the position, of these ridges, providing an efficient channeling mechanism. Although this design is promising, it has not yet been experimentally tested. Considering the higher design complexity of a stellarator deivertor compared to its 2D tokamak counterpart, a full understanding of its performance is crucial in stellarator optimization.

Experiments and future prospects

Deivertor experiments in W7-X and LHD show promising results and provide valuable insights for future improvements in shape and performance. Moreover, the emergence of nonresonant deivetors provides an exciting new path for quasisymmetric stellarators and other configurations that are not optimized to minimize plasma currents.

In this elegant dance of magnetic fields, can future nuclear fusion facilities overcome the challenges of deivertor and achieve the ideal energy solution?

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