  A solid target in contact with a plasma charges (negatively) to reflect the
more mobile species (electrons) and thus keep the bulk plasma quasineutral. To
shield the bulk plasma from the charged target, there is an oppositely
(positively) charged sheath with a sharp electrostatic potential variation on
the Debye length scale $\lambda_{\rm D}$. In magnetised plasmas where the
magnetic field is inclined at an oblique angle $\alpha$ with the target, some
of the sheath potential variation occurs also on the ion sound gyroradius
length scale $\rho_{\rm S} \cos \alpha$, caused by finite ion gyro-orbit
distortion and losses. We consider a collisionless and steady-state magnetised
plasma sheath whose thickness $l_{\rm ms} \sim \max (\lambda_{\rm D}, \rho_{\rm
S} \cos \alpha)$ is smaller than the characteristic length scale $L$ of spatial
fluctuations in the bulk plasma, such that the limit $l_{\rm ms} / L
\rightarrow 0$ is appropriate. Spatial structures are assumed to be magnetic
field-aligned. In the case of small magnetic field angle $ \alpha \sim \delta
\equiv \rho_{\rm S} / L \ll 1$, electric fields tangential to the target
transport ions towards the target via ExB drifts at a rate comparable to the
one from parallel streaming. A generalised form of the kinetic Bohm-Chodura
criterion at the sheath entrance is derived by requiring that the sheath
electric field have a monotonic spatial decay far from the target. The
criterion depends on tangential gradients of potential and ion distribution
function, with additional nontrivial conditions.
