Laser initiated, RF sustained air plasmas

Summary form only given. Measurements and analysis of air breakdown processes by focusing a 193 nm, 200 mJ, 10 MW high power UV laser radiation to 20-60 ??m spot sizes that produce maximum laser intensities of 10¹²-10¹³ W/cm² are presented. Both classical quantum (multi-photon) and (collisional cascade) ionization processes affect the breakdown and plasma formation. Observations are made for a wide range of pressures from 50 Torr to 5 atmospheres. Multi-photon ionization (n = 3) processes play a substantial role for lower pressures at 193 nm due to the high photon energy (6.4 eV). An extensive range of optical and spectroscopic diagnostics with 5 ns time scale gating and 13 ??m ICCD resolution is utilized to characterize the plasma. The spatial and temporal evolution of the laser focused plasma is measured using shadowgraphy and two-color laser interferometry techniques with Abel inversion. The plasma temperatures are obtained by measuring the velocity of the shock wave front and by using optical emission spectroscopy. Optical emission spectroscopy is performed to diagnose the plasma temperature using nitrogen and oxygen emission lines. Measurements of the core laser plasma density (ne= 8x10¹⁷/cc) and electron temperature (10 eV) decay are made. A longer 18 cm focal length lens and its ionizing shock wave front are utilized to produce air seed plasma to instantaneously initiate a larger volume (500 cc) RF sustainment discharge coupled by means of a helical coil at up to 10 kW power levels. The RF system is matched to the inductive plasma impedance by a set of dual capacitors that can be switched during the rf pulse to obtain an optimal match. Observations of the plasma densities and temperatures, plasma dynamics, RF matching and gas heating during the sustainment pulse utilizing mm wave interferometry and optical spectroscopy will be presented.