Kinetic Simulation of Recycling Bifurcation in an Expander Divertor

An expander divertor can be used to expand the Scrape-Off-Layer flux-tube of a linear device to arbitrary size, providing (i) reduced energy flux to the wall, (ii) increased pumping capacity, (iii) formation of a pre-sheath for electrostatic confinement of electrons, and (iv) large surface area for electrode biasing.  The magnetic field and plasma density can vary spatially by a factor ~30 or more in the expander, with the result that the ion gyroradius and ion-neutral mean free path may be simultaneously short and long in different regions.  To our knowledge this combination of features is not well represented by existing SOL transport models, so we have developed a new kinetic particle transport model where atomic and molecular ions and neutrals co-evolve, with each species acting as the target background for Monte Carlo Collisions with the other species.  Magnetic fields are static, with electric fields determined by electron pressure balance and electrode biasing.  Using this model a bifurcation from low to high recycling regime has been found which may help to explain operational boundaries of the C-2W experiment [1].

[1] H. Gota et al., Nucl. Fusion 59, 112009 (2019).