An atmospheric-pressure air microplasma is ignited and sustained in a 25μm wide discharge gap formed between two co-planar gold electrodes.
These electrodes are the two ends of a microstrip transmission line that is
microfabricated on an Al2O3 substrate in the shape of a split-ring resonator
operating with a resonant frequency of 895 MHz. At resonance, the device
creates a peak gap voltage of ~390V with an input power of 3W, which is
sufficient to initiate a plasma in atmospheric pressure air. Optical emission
from the discharge is primarily in the ultraviolet region. In spite of an
arc-like appearance, the discharge is not in thermal equilibrium as the N2
rotational temperature is 500–700 K. The intrinsic heating of the Al2O3
substrate (to 100°C) causes a downward shift in the resonant frequency of
the device due to thermal expansion. The temperature rise also results in a
slight decrease in the quality factor (142 > Q > 134) of the resonator. By
decreasing the power supply frequency or using a heat sink, the
microplasma is sustained in air. Microscopic inspection of the discharge gap
shows no plasma-induced erosion after 50 h of use