Island Divertor

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The Island Divertor is a concept in magnetic confinement fusion devices that utilizes inherent low-order magnetic islands to manage power and particle exhaust. Developed for advanced low-shear stellarators in the Wendelstein-7 family, the island divertor was first tested on W7-AS before its shutdown in 2002[1]. The concept has since been investigated in more detail and at a larger scale in Wendelstein 7-X (W7-X).

One major challenge magnetic confinement fusion devices face is managing power and particle exhaust. In future reactors, hundreds of MWs of power will stream out from the confined plasma region (core) and must be dissipated before reaching the plasma-facing components (PFCs). Excessive heat and erosion can lead to short lifetimes of the PFCs, as well as the release of impurities and subsequent contamination of the confined plasma.

Divertors are dedicated plasma-wall interaction zones where particles and heat stream to, moving parallel to the open magnetic field lines in the scrape-off layer (SOL). However, the fast parallel heat transport leads to localized heat deposition on the targets. In stellarators, several edge topologies have been proposed and used to form a divertor for particle and heat exhaust (e.g., helical divertor, non-resonant divertor).[2] The island divertor is one such concept, using intrinsic magnetic islands in the SOL to set up a divertor volume.

The first W7-X island divertor experiments and 3D modeling studies with EMC3-EIRENE have found a strong dependence of the divertor heat fluxes on the magnetic configurations and island geometry. Local heat load profiles showed offsets and varying peak fluxes, complicating the matching between experiments and 3D modeling[3][4].

The island divertor has shown great success in accessing and stabilizing detached scenarios[5]. During the first island divertor operation at W7-X, a stable operation regime had been achieved with reduced heat load on all divertor targets. This regime was maintained over several energy confinement times, and the plasma scenario proved reproducible and robust under various conditions. The plasma radiation, primarily due to oxygen, was located at the plasma edge[6]. Island divertor detachment has been achieved since then for different plasma parameters and magnetic configurations[7][8][9].

A particular feature of the island divertor topology is the existence of multiple, adjacent counter-streaming flow regions at the plasma edge[10]. Strong counter-streaming flows can lead to frictional dissipation of momentum, causing a reduction of the flow speed parallel to the magnetic field lines. This is likely to have played a role in substantial heat flux mitigation at the targets.

Radiative power exhaust by impurity seeding was demonstrated during the first island divertor experiments at the Wendelstein 7-X stellarator[11]. Stable plasma operation was shown during seeding with both neon (Ne) and nitrogen (N2). High radiative power losses (80%) were found to reduce the divertor heat loads globally by 2/3 with both seeding gases injected at a single toroidal location into one of five magnetic islands.

The island divertor concept has shown reliable heat flux control with hydrogen gas puffing and impurity seeding, making it a promising solution for future detachment control in high-performance scenarios and upgrades towards a metal divertor. Feedback-controlled divertor detachment has been achieved with hydrogen gas injection in W7-X[12] and may be extended to impurity seeding in the future.

The edge magnetic structure in helically symmetric stellarators, such as the Helically Symmetric eXperiment (HSX) and Wendelstein 7X (W7-X), has been shown to have a significant impact on particle fueling and exhaust of the main plasma species (hydrogen) and impurity helium. The magnetic island chain in the plasma edge can control the plasma fueling from the recycling source and active gas injection, a basic requirement for a divertor system[13].