Multipole Resonance Probe

Multipole Resonance Probe for electron density, collision frequency and temperature measurements

The Multipole Resonance Probe (MRP) is a plasma diagnostic that measures and evaluates live with real-time capable electronics. In low-pressure plasmas, it provides the electron density, the electron collision frequency and the electron temperature - in other words, the most important plasma parameters.

The measuring principle is based on so-called active plasma resonance spectroscopy. The measuring electronic generates a high-frequency signal which frequency is varied up to the gigahertz range. This signal is coupled into the plasma via a surrounding glass tube. The probe thus functions like an antenna that emits a signal and receives it again. At a certain resonance frequency, the electrons pick up the energy of the signal and oscillate. The system response of the plasma probe system (the reflectance value) is recorded and processed by the evaluation unit.

The MRP is a special form of active plasma resonance spectroscopy. It is characterized by geometric and electrical symmetry, which allows the behavior to be analyzed in a mathematically transparent way. The symmetry is created by the setup with two hemispheres acting as electrodes and a high-frequency optimized tapered balun. Thus, a formulaic relationship between the resonance frequency and the electron density in the plasma can be given:
fres ~ ωpe ~ sqrt(ne)
From the damping of the signal, the collision frequency and - if existing - the temperature of the electrons are calculated.

The MRP is available in different versions. It can be freely positioned but stationary in the chamber, or the glass tube is guided through the flange. It can be moved linearly in one direction from a flange using a stepper motor.

Last, the three-dimensional spherical symmetry was reduced to a two-dimensional disk symmetry. The so-called planar MRP (pMRP) can thus be integrated into the chamber wall.
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