  The available diagnostics for atmospheric micro-plasmas remain limited and
relatively complex to implement; so we present a radio-frequency technique for
diagnosing a key parameter here. The technique allows one to estimate the
dependencies of the electron density by measuring the RF-impedance of the
micro-plasma and analyzing it with an appropriate equivalent circuit. This
technique is inexpensive, can be used in real time and gives reasonable results
for argon and helium DC micro-plasmas in holes over a wide pressure range. The
electron density increases linearly with current in the expected range
consistent with normal glow discharge behavior.
