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--- private:writeups:plasmaimpedanceprobes [2020/07/01 17:30]
admin created
+++ private:writeups:plasmaimpedanceprobes [2020/07/02 14:17] (current)
admin
@@ Line 3: / Line 3: @@
 === What it does ===
-=== Advantages/disadvantages ===
+Measures
+  - Plasma frequency [Blackwell2005]
+  - Electron density (by measuring plasma frequency) [Blackwell2005]
+  - Plasma potential
+  - Collision frequency
+=== Methods ===
+PIPs can be operated in a number of methods
+  - 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.
+  - 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.
+=== Advantages ===
+  * [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]
+=== Disadvantages ===
+  * 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.
+=== Sources of error ===
+  * [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.
 === Types of electrodes used ===
@@ Line 11: / Line 39: @@
   * Monopole antenna [Bilen1999]
+=== [Blackwell2005] ===
+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.
+{{ :private:writeups:img_20200701_140114.png?nolink&600 |}}
+=== Calibration ===
+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."

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