Two-dimensional particle simulations of the sheath in front of a flat Langmuir probe mounted into a particle absorbing plate are performed to study the influence of a strong magnetic field (relation between Larmor radii and Debye length: rho(e) less-than-or-equal-to lambda(D), rho(i) much-greater-than lambda(D)) which is oriented obliquely to the probe surface. Ion-attracting probes are considered and the sheath is assumed to be collisionless. The full particle orbits in the homogeneous magnetic field and the self-consistent electric potential are calculated with a particle-in-cell (PIC) code with two spatial coordinates and three velocity components (2d, 3v). The main results are: In the sheath the ion trajectories are bent towards the normal to the probe surface so that the ion flow is focused to the edges of the probe. This leads to an enhancement of the ion current as compared to the current flowing in the flux tube subtended by the probe. As a consequence the current does not saturate at large (negative) probe voltage, because the thickness of the Debye sheath, and thus the effective probe size, grows with increasing probe voltage. This effect is particularly strong if the sheath thickness is about as large, or larger than, the projection of the probe size along the magnetic field lines. These results can help to explain the ion-current nonsaturation found in recent measurements with Langmuir probes in the boundary layer of magnetically confined plasmas.
