  The Debye sheath is known to vanish completely in magnetised plasmas for a
sufficiently small electron gyroradius and small angle between the magnetic
field and the wall. This angle depends on the current onto the wall. When the
Debye sheath vanishes, there is still a potential drop between the wall and the
plasma across the magnetic presheath. The magnetic field angle corresponding to
the predicted sheath collapse is shown to be much smaller than previous
estimates, scaling with the electron-ion mass ratio and not with the square
root of the mass ratio. This is shown to be a consequence of the kinetic
electron and finite ion orbit width effects, which are not captured by fluid
models. The wall potential with respect to the bulk plasma at which the Debye
sheath vanishes is calculated. Above this wall potential, it is possible that
the Debye sheath will invert.
