Divertor transport

Divertors in stellarators are inherently three-dimensional (3D) and the magnetic fields at the edge usually exhibit certain types of stochastic behaviour. Typical examples are the island divertor for the low-shear stellarator W7-family and the helical divertor for the heliotron-type device LHD. Understanding the plasma exhaust processes in stellarators therefore necessitate 3D transport models that are capable of dealing with transport processes in a stochastic field where closed flux surfaces do not exist. For this, a 3D edge Monte Carlo code EMC3 [1] has been developed in close interaction with the W7-AS island divertor experiment. It is based on fluid models for both the plasma bulk and impurities. Coupled with Eirene [2], the EMC3-Eirene code package is presently the only 3D numerical tool that is capable of self-consistently treating the plasma, impurity and neutral transport in realistic 3D scrape-off-layers (SOLs) of complex field and divertor geometries. Currently, the code is being employed for studying 3D edge transport problems encountered in both stellarators and tokamaks and plays a bridge role between tokamaks and stellarators in exploring the optimal plasma-exhaust concepts for fusion devices on a broader configuration basis [3].

Understanding the plasma transport, exhaust and plasma-surface interaction processes in the island divertor, optimizing the island divertor concept, and identifying the potentially critical issues on the way to a reactor are the most important tasks of the EMC3-Eirene applications at W7-X. In addition, international research on optimizing divertor concepts including tokamaks and stellarators is enriching the divertor database and will improve the code prediction capability for next-step devices. There are close collaborations in this field with the international groups at LHD, AUG, TEXTOR, JET, DIII-D, NSTX and ITER.

The EMC3-Eirene code needs to be further developed towards more complete physics. A time-dependent solver will extend the code application range to cover dynamic processes of interest. The “trace”-impurity model needs to be further developed towards a multi-fluid model. Additionally, EMC3 is well compatible with kinetic impurity transport models. The physics governing the potential and currents in general 3D SOLs has to be clarified and drift effects need to be self-consistently implemented in the code. It is also planned to introduce more plasma-surface-interaction physics.

[1] Feng Y et al. 2004 Contrib. Plasma. Phys. 44 57
[2] Reiter D et al 2005 Fusion Science and Technology 47 172
[3] Feng Y et al 2011 Plasma Phys. Control. Fusion 53 024009

Fig.1: basic features of stable partial detachment of the W7-AS island divertor. From left to right: carbon radiation and hydrogen ionization distributions predicted by the EMC3-EIRENE code and the calculated power load in comparison with thermography measurement.
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