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When an unmagnetized plasma comes in contact with a material surface, thedifference in mobility between the electrons and the ions creates a nonneutrallayer known as the Debye sheath (DS). However, in magnetic fusion devices, theopen magnetic field lines intersect the structural elements of the device withnear grazing incidence angles. The magnetic field tends to align the particleflow along its own field lines, thus counteracting the mechanism that leads tothe formation of the DS. Recent work using a fluid model [P. Stangeby, Nucl.Fusion {\bf 52}, 083012 (2012)] showed that the DS disappears when theincidence angle is smaller than a critical value (around $5^\circ$ forITER-like parameters). Here, we study this transition by means of numericalsimulations of a kinetic model both in the collisionless and weakly collisionalregimes. We show that the main features observed in the fluid model arepreserved: for grazing incidence, the space charge density near the wall isreduced, the ion flow is subsonic, and the electric field and plasma densityprofiles are spread out over several ion Larmor radii instead of a few Debyelengths as in the unmagnetized case. As there is no singularity at the DSentrance in the kinetic model, this phenomenon depends smoothly on the magneticfield incidence angle and no particular critical angle arises. The simulationresults and the predictions of the fluid model are in good agreement, althoughsome discrepancies subsist, mainly due to the assumptions of isothermal closureand diagonality of the pressure tensor in the fluid model.

Chodura and Debye sheaths for magnetic fields with grazing incidence -- kinetic simulations