Measurements on the electron-energy distribution function in a low-pressure cesium discharge

Electron-energy distribution functions have been determined from the second derivatives of Langmuir-probe characteristics in the positive column of a discharge in cesium vapour. To explain second-derivative curves at energies below 0.5 eV we assumed that reflection of electrons on the probe surface is important in this region. We determined the reflection coefficient as a function of the energy from acomparison of second-derivative curves measured at large discharge currents with the Maxwellian distribution functions that can be expected in this case. We compared the second derivative with distribution functions corrected for this reflection coefficient under circumstances where we cannot expect a Maxwellian distribution function. We obtained good agreement at high energy; at low energy we remain uncertain about the correctness of the theoretical distribution functions. A conclusion from the work is that in the case of reflection of electrons on the probe surface the plasma potential in general is not connected to the energy at which the second derivative of the probe current is zero or where it has a minimum or a maximum value.