Divertor

History

The concept of a magnetic divertor was described by Lyman Spitzer in 1951. Initially envisioned for stellarators, the idea is to add special magnetic coils that create a magnetic separatrix to separate regions of "closed" and "open" field lines. Plasma particles and heat diffusing into the open field lines are transported rapidly along the magnetic field to material surfaces far form the confined core.

The divertor is now seen as an essential component of magnetic confinement devices. ITER's divertor will consist of a standard "single-null" magnetic divertor configuration and tungsten-armored plasma-facing components.

Tokamak divertors

A tokamak featuring a divertor is known as a divertor tokamak or divertor configuration tokamak. In this configuration, the particles escape through a magnetic "gap" (separatrix), which allows the energy absorbing part of the divertor to be placed outside the plasma. The divertor configuration also makes it easier to obtain a more stable H-mode of operation. The plasma facing material in the divertor faces significantly different stresses compared to the majority of the first wall.

Stellarator divertors

In stellarators, low-order magnetic islands can be used to form a divertor volume, the island divertor, for managing power and particle exhaust. The island divertor has shown success in accessing and stabilizing detached scenarios and has demonstrated reliable heat flux and detachment control with hydrogen gas injection, and impurity seeding in the W7-X stellarator. The magnetic island chain in the plasma edge can control plasma fueling. Despite some challenges, the island divertor concept has demonstrated great potential for managing power and particle exhaust in fusion reactors, and further research could lead to more efficient and reliable operation in the future.

The helical divertor, as employed in the Large Helical Device (LHD), utilizes large helical coils to create a diverting field. This design permits adjustment of the stochastic layer size, situated between the confined plasma volume and the field lines ending on the divertor plate. However, the compatibility of the Helical Divertor with stellarators optimized for neoclassical transport remains uncertain.

The non-resonant divertor provides an alternative design for optimized stellarators with significant bootstrap currents. This approach leverages sharp "ridges" on the plasma boundary to channel flux. The bootstrap currents modify the shape, not the location, of these ridges, providing an effective channeling mechanism. This design, although promising, has not been experimentally tested yet.

Given the complexity of the design of stellarator divertors, compared to their two-dimensional tokamak counterparts, a thorough understanding of their performance is crucial in stellarator optimization. The experiments with divertors in the W7-X and LHD have shown promising results and provide valuable insights for future improvements in shape and performance. Furthermore, the advent of non-resonant divertors offers an exciting path forward for quasi-symmetric stellarators and other configurations not optimized for minimizing plasma currents.

References

References

1. (1958). "The Stellarator Concept". The Physics of Fluids.
2. (1958). "The Divertor, a Device for Reducing the Impurity Level in a Stellarator". The Physics of Fluids.
3. (2006). "Physics of island divertors as highlighted by the example of W7-AS". Nucl. Fusion.
4. (2021). "Stable heat and particle flux detachment with efficient particle exhaust in the island divertor of Wendelstein 7-X". Nucl. Fusion.
5. (2019). "First demonstration of radiative power exhaust with impurity seeding in the island divertor at Wendelstein 7-X". Nucl. Fusion.
6. (2018). "Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators". Physics of Plasmas.
7. (2021). "Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation". Nucl. Fusion.
8. (2013). "Initial experiments towards edge plasma control with a closed helical divertor in LHD". Nucl. Fusion.
9. (2015). "Stellarator design". Journal of Plasma Physics.
10. Bader, Aaron. (December 6, 2018). "Progress in Divertor and Edge Transport Research for Stellarator Plasmas".