Using a direct current (DC) glow discharge electrode as a non-invasive impedance probe for measuring electron density

A small RF signal is applied to the anode of a low pressure (P ≤ 200 mTorr), low temperature (T ≤ 3 eV) direct current (DC) glow discharge operating at an electron density of n ∼ 10⁶ cm^-3. The discharge is modeled as a collection of capacitive, resistive, and inductive circuit elements that have resonances at particular frequencies, much like an RLC circuit. The location of these resonances in frequency space provides information about the plasma parameters. In this work, an electrode that is used to sustain a DC glow discharge is also used to probe the impedance of the discharge. The benefit of this approach is that it is not necessary to insert a physical probe that could introduce perturbations or contaminate the discharge. Experiments were performed to demonstrate this non-invasive impedance probing method for extracting the plasma discharge density at various neutral gas pressures and discharge voltages and currents from changes to the input impedance of the anode. Comparisons between densities extracted with this method and Langmuir probe measurements showed overall good agreement.