Focused laser initiated RF sustained high pressure air plasmas

Measurements and analysis of air breakdown processes and plasma production by focusing 193 nm, 300 mJ, 15 MW high power laser radiation inside a 6 cm diameter helical RF coil. We observe quantum resonant multi-photon (REMPI)¹ and collisional cascade laser ionization processes that produce high density (n_e~5*10¹⁵/cm³) cylindrical seed plasmas at 20 Torr. The focused laser and associated shock wave produces a seed plasma for sustaining by the RF (1-10 kW, 0.5-1.5 s) pulse. Measurements of the helical RF antenna plasma-loaded impedance are obtained by measuring the complex reflection coefficient with and without the laser pulse. Additional diagnostics are done with a 105 GHz (mm wave) interferometer to measure plasma density and collision frequency and electron temperature. Spectroscopic measurements of the plasma and comparison with the SPECAIR code are made to determine rotational and vibrational neutral gas temperatures. The results demonstrate that the laser formed seed plasma allows RF sustainment at higher initial air pressures (20 Torr) than with RF only initiation (15 Torr).