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.