Plasma Impedance Probes (PIPs)

Measures

1. Plasma frequency [Blackwell2005]
2. Electron density (by measuring plasma frequency) [Blackwell2005]
3. Plasma potential
4. Collision frequency

PIPs can be operated in a number of methods

1. Actively driven RF sweeps [Blackwell2005]. Relate the peaks and zero intercepts of Re(Z) and Im(Z) to circuit models and dispersion relationships to get the plasma frequency.
2. Plasma oscillation (excite a two stream instability) [Blackwell2005]. Drive weak beam of electrons into the plasma to excite a two stream instability. Under correct conditions, its frequency is close to the plasma frequency. An FFT of passive PIP measurements reveals the frequency.

• [Blackwell2005] An improvement over Langmuir probes because contamination of the probe has a larger impact on its measurements than on the PIP.
• Better resolution and accuracy than traditional Langmuir probes [citation]

• More complicated. Requires careful setup/calibration and the use of an expensive network analyzer.
• (Check this) Cannot be used in real-time because the network analyzer data can only be post-processed.

• [Blackwell2005] recommends “choosing sufficiently shielded coax cables so that the impedance characteristics are not changed as cables are moved around during the experiment. ”
• [Blackwell2005] Nearby objects to the probe reflected radiation
• [Blackwell2005] At frequencies related to certain wavelengths, standing waves can form in the chamber.

• Planar probes
• Spherical probes - leads to a simpler analysis [Blackwell2005]
• Monopole antenna [Bilen1999]

Blackwell2005 discusses a PIP in a cold (cold ions) and unmagnetized plasma using a spherical probes. Provides a derivation of the circuit model (I reproduce part of it below) that captures the impedance of the sheath and the plasma.

Derivation of Eq. 2. Also citing Piel2009.

A calibration is common to remove the impedance of the non-plasma components (coax feedlines, network analyzer, etc).

• [Blackwell2005] does this by attaching a 50ohm load and then a short at the end of the feedline with the other end attached to a network analyzer. “the calibration consists of programming the analyzer to add phase shifts to the value of G to cancel out the phase shift from the cable.”