Kinetic modeling of Active Plasma Resonance Spectroscopy: Why ``Drude'' is not enough

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 $\omega_{\mathrm{pe}}$: a radio frequent signal (in the GHz range) is coupled into the plasma via a probe, the spectral response is recorded, and a mathematical model is used to determine parameters such as the plasma density or the electron temperature. This contribution discusses the importance of choosing the proper physical theory for that mathematical model. The so-called Drude model (also known as cold plasma model) provides a relatively simple qualitative description, but is not suited for a quantitatively correct analysis. The physically superior kinetic theory is, unfortunately, mathematically more complex. The contribution discusses the difference between the two approaches and shows how the mathematical problems of the kinetic description can be overcome.