Multipole Resonance Probe – Function

Plasma diagnostics with real-time-capable electronics and evaluation should provide insights in the plasma process .
The multipole resonance probe (MRP) was developed for this purpose at the Ruhr University Bochum. It provides measurement data on
electron density
collision frequency, as well as
electron temperature – the most important internal plasma parameters.
The measuring time with the MRP is therefore short and, thus, the probe can be used in real-time applications. Furthermore, it provides direct plasma parameters in the evaluation in contrast to the indirect, external parameters which are often used and therefore only allow indirect control of processes. The probe is available in different hardware versions as well as with software solutions to fulfill all requirements.

The measuring principle is based on active plasma resonance spectroscopy (APRS). The probe is used to couple a high-frequency signal in the megahertz to gigahertz range via a dielectric into the plasma. At a frequency close to the electron plasma frequency, the plasma absorbs the energy of the signal and resonates. The response of the plasma-probe-system – the reflection value S11 – is picked up by the probe and transmitted to an evaluation unit.
The MRP is a special realization of the APRS, which is characterized by a high geometric and electrical symmetry. The probe head consists of two symmetrically arranged electrodes designed as hemispheres, which are controlled electrically symmetrically. Due to this symmetry, the behavior of the probe can be analyzed mathematically transparent and a formulaic relationship between the resonance frequency fres and the electron density of the plasma can be specified:

fres ~ wpe ~ sqrt(ne)