Sheath Potential Reversal at Shallow Magnetic Field Angles

To minimize heat fluxes onto the walls of fusion reactors, the magnetic field is inclined at a shallow angle relative to the wall. Due to their lighter mass, electrons typically reach the wall first, setting up an electron-repelling Debye sheath and magnetic presheath. In recent years, gyrokinetic models for shallow-angle incidence [1], [2] have been developed to self-consistently calculate the electrostatic potential in the kinetic solution to the Debye sheath and the magnetic presheath. These models are fast, but they assume the potential and its derivatives are monotonic. It has been observed that these models do not converge to any solution below certain critical angles, implying that monotonic potential profiles do not exist in this regime. In this work, we will improve the accuracy of these fast sheath and presheath models by correcting the lowest order orbits, and we characterize the failure to obtain monotonic potential profiles by studying small deviations to the potential profile.

[1] A. Geraldini, F. I. Parra, and F. Militello, “Solution to a collisionless shallow-angle magnetic presheath with kinetic ions,” Plasma Physics and Controlled Fusion, vol. 60, no. 12, p. 125 002, 2018.

[2] R. J. Ewart, F. I Parra, and A. Geraldini, “Sheath collapse at critical shallow angle due to kinetic effects,” Plasma Physics and Controlled Fusion, vol. 64, no. 1, p. 015 010, 2021.