Custom Fabry-Perot transmission lines : A quantitative testbed for studying RF plasmas

A custom air-dielectric coaxial transmission line with airtight seals and a transparent copper-mesh outer conductor was designed and constructed to ignite and study radio-frequency (RF) plasma. RF power was capacitively coupled into one end of the coax while the opposite end was terminated by an open circuit. With both ends near open-circuited, voltage standing waves produced antinode regions of strong electric fields that can ignite and sustain a stable plasma. A preliminary test of the Fabry-Perot nature of coax cables was performed, and confirmed that these antinodes are feasible. Exploiting the fact that the air inside the transmission line changes the index of refraction for the RF waves, the relative permittivity of atmospheric air was determined to be 1.00035(10) ± 9.1 × 10−⁵ , based on the shift in resonant frequencies compared to the vacuum. Plasmas were successfully ignited in the antinode regions at 80, 164, and 245 MHz resonant frequencies. We measured the plasma ignition power as a function of the air pressure inside the transmission line to produce a Paschen curve. At high pressure, the mean free path of the electrons is restricted, and they are unable to ionize other air particles. On the other hand, at low pressure, the electrons rarely scatter with other air particles, so ionization is rare. At both extremes, a high power delivery is required. Finally, we developed a transmission line model with plasma inside it. The model allows us to determine the electrical conductivity and susceptibility of the plasma from S₁₁ data constructed from reflection coefficients produced using a vector network analyzer (VNA). Our results show that both the plasma conductivity and susceptibility increase as the delivered RF power is increased and as the air pressure is decreased from 900 to 200 mTorr.