Effects of Finite Probe Size in the Determination of Electron Energy Distribution Functions

The corrections which are necessary in the measurement, by means of a spherical Langmuir probe, of the energy distribution function f0(U) of the electrons in a plasma are analysed. The principal effect considered here is the disturbance of the plasma produced by the drain of electrons to the probe. This disturbance becomes increasingly serious as the gas pressure p and the probe radius a increase and as U decreases. It is shown that the true value of f0(U) differs by less than 25% from that determined from the simple theory neglecting the disturbance provided U > ½U and a < ½le where le is the electron mean free path.