Admittance of a spherical probe in a warm electron-ion plasma

Two-fluid, scalar-pressure theory with an absorptive boundary condition is used to calculate the input admittance of a spherical probe in a warm electron–ion plasma. In addition to the usual resonances at the electron plasma frequency and at the sheath–plasma "series resonance" frequency, the theory predicts a rather broad resonance just below the ion plasma frequency. In the zero-frequency limit, the predicted input conductance is essentially the slope of a Langmuir probe current–voltage curve. The prediction of an ion plasma resonance is in qualitative agreement with experiments reported elsewhere.