Modelling of volume- and surface wave based plasma resonance spectroscopy

Summary form only given. The term "plasma resonance spectroscopy" denotes a family of diagnostic methods which exploit the natural ability of a plasma to resonate on or near the plasma frequency: A radio-frequency signal in the range of 100MHz - 10 GHz is coupled to the plasma via an antenna or an immersed probe. The measured absorption or transmission spectrum S(ω) shows characteristic resonances from which parameters like electron density n_e or electron temperature T_e can be derived. This general concept has found numerous realizations in the last decades. To classify the approaches, one may make the following fundamental distinction: Volume wave based methods observe resonances that are present already in vacuum; the plasma density is then given by the shift of the resonance [1]. Surface wave based methods, in contrast, utilize resonances which are not present in vacuum and deduce the plasma parameters from the absolute value of the resonance frequency [2,3]. As an example, we study the behavior of two symmetrically RF-driven hemispheres surrounded by a plasma sheath and located in the center of a bounded spherical plasma. The analysis reveals two families of resonances, one below the plasma frequency ω_pe and one above. These families correlate with the distinction introduced above. We clarify the differences in the modeling of the two regimes and their advantages and disadvantages for diagnostics purposes.