Optical Emission Measurements of Laser Initiated, RF Sustained High Pressure Seeded Plasmas

Summary form only given. High density ~10¹²-10¹⁴ cm^-3, large (500-2500 cc) volume, high-pressure air and air constituent atmospheric (50-760 torr) plasmas are of great interest in variety of scientific and industrial applications such as material processing, biological decontamination, radar and stealth antennas. We present a technique for producing these plasmas and measuring their temperature, density and recombination rates by means of optical emission and millimeter wave interferometry. The plasma is produced by utilizing a seed (15 mtorr) organic gas, tetrakis (dimethyl-amino) ethylene (TMAE), in high-pressure air and air component gases is pre-ionized by a 393 nm excimer laser (300 mJ), 20 ns pulse width. The seed plasma is then sustained by the efficient absorption of the pulsed 13.56 MHz radio frequency (RF) power (1-25 kW) through inductive coupling of the wave fields, which reduces the RF power requirement for plasma initiation to a great extent. High-resolution optical emission spectroscopic (OES) diagnostics are carried out to measure the density and temperature profiles of the plasma. We use a 0.5-m monochromator with a 1200-groove/mm grating with entrance and exit slits of 10 mum to obtain high resolution. The monochromator is calibrated for spectral intensities between 200-800 nm with a Hg-Ar light source. H_beta (486.132 nm) and H_alpha (656.28 nm) line broadening techniques have been used to measure the FWHM, which in turn used to measure the density and temperature of the plasma. A small amount of H₂ (2% mole fraction) is mixed with the working gas. The background emission spectrum, due to the N₂ first positive system B³Pi_g-A³Sigma_u ⁺ is also measured by turning off hydrogen flow. For the temperature (0.4-0.6 eV) and density (10¹²-10¹⁴ cm ^-3) range of the plasma generated, the line wi- th is dominated by Stark broadening. The spectroscopic measurements are used to obtain density and collisional rates are compared with those using a 105 GHz interferometer. The spectroscopic measurements can also provide highly localized measurements of the plasma conditions in regions that are less accessible to interferometer measurements