Models for the Collecting Area of Langmuir Probes Comparable to the Ion Larmor Radius

An analytical model has been developed to estimate the collecting area of a \u201cgapped\u201d single Langmuir probe flush to a local limiter, when the probe characteristic dimension perpendicular to the magnetic field direction is comparable to the ion Larmor radius. The problem has been solved for angles between the magnetic field direction and the probe surface between 12\ub0 and 42\ub0, and for square and circular probe cross-sections. A numerical model, adopting a Montecarlo type approach, has also been developed in order to generalize the solution. Both models also estimate the dependence of the collecting area on the sheath thickness. The models are in good agreement and have been applied to the probes used on the RFX experiment