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Achieving high plasma density near the divertor target in magnetically confined fusion devices is beneficial for two main operational objectives: firstly, the high densities lead to high neutral densities near the pumping duct, allowing for efficient particle exhaust. Secondly, it allows for significant radiation (heat exhaust) in the SOL with minimal impurity concentration ($P_{rad}∝n_e^2 c_{imp}$). In W7-X, the island divertor is being tested as a possible exhaust solution for a future stellarator reactor. The divertor in its current form is an open divertor. It is optimized for high configuration flexibility rather than for any specific divertor configuration. Therefore, its performance may not be optimal in every configuration.
According to the stellarator two-point model [2], larger field-line pitch increases the weight of parallel transport, which is hypothesized to have a beneficial effect on the density build-up. To verify this, three different island divertor configurations (low-iota, standard and high-iota) in W7-X, with different field-line pitches ($Θ_{i,avg}=.002,.0038,.0073$ respectively) were simulated with EMC3-Eirene. While the low-iota configuration exhibited the expected behavior with field-line pitch, showing the poorest density build-up, no changes in achievable downstream densities were observed when the field-line pitch was further increased by a factor of about two from the standard to the high-iota configuration.
The reason for this deviation from the expected scaling was determined to be the close proximity of the X-point to a region of a vertical divertor plate, which introduces a limiter-like component into the high-iota divertor configuration in that the neutral particles recycled there can easily penetrate the confined region. Although the vertical target only receives approximately 10% of the total recycling flux, the deeper location of the ionization source leads to convective transport, reducing the maximum achievable density in the divertor to a degree that compensates for the field-line pitch effect. When the vertical target was removed from the simulation, thus removing this core source of particles (and therefore more closely matching the assumptions made in two-point models), the expected scaling with the field line pitch was recovered. This work highlights the importance of the island’s neutral screening efficiency on the density build-up capability of the island divertor and possible consequences of external gas puffing in the private flux region in the divertor region. For example, purposefully injecting gas near the X-point, as an attempt to induce X-point radiators, may be detrimental in the island divertor in terms of pumping.