How big is a small Langmuir probe?

The area of the sheath around a thin, disk-shaped electrode that is biased below the plasma potential has been computed using a hybrid simulation with cold, collisionless ions and Boltzmann electrons. That is, the "collecting area" of a double-sided, planar Langmuir probe has been determined for the ion saturation current regime. Sheath areas are calculated for probe radii from 10 to 45 electron Debye lengths and for probe biases from -5 to -30 times the electron temperature. The dependence of the sheath area on probe radius and bias is parameterized using simple empirical formulas.