Active plasma resonance spectroscopy: Eigenfunction solutions in   spherical geometry
  The term Active Plasma Resonance Spectroscopy (APRS) denotes a class of related techniques which utilize, for diagnostic purposes, the natural ability of plasmas to resonate on or near the electron plasma frequency: A radio frequent signal (in the GHz range) is coupled into the plasma via an antenna or probe, the spectral response is recorded, and a mathematical model is used to determine plasma parameters like the electron density or the electron temperature. Based on the cold plasma model, this manuscript provides the general analytic expression of the electrical admittance of a spherical shaped probe immersed into a plasma. It is derived from the matrix representation of an appropriate operator, which describes the dynamical behavior of the probe-plasma system. This dynamical operator can be split into a conservative operator and a dissipative operator. It can be shown that the eigenvalues of the conservative operator represent the resonance frequencies of the probe-plasma system which are simply connected to the electron density. As an example, the result is applied to the spherical impedance probe and the multipole resonance probe. 
Jens Oberrath and Ralf Peter Brinkmann
physics.plasm-ph