Optical diagnostics of laser initiated, RF sustained high pressure seeded plasmas

Summary form only given. High-pressure, inductively-coupled plasmas (ICP) have been used for variety of scientific and industrial applications over a large gas pressure range from 50-760 torr. High density ~10¹¹-10¹⁴ cm^-3, large (500-2500 cc) volume, atmospheric plasmas are of great interest in applications such as material processing, biological decontamination, radar and stealth antennas. We present a technique for producing these plasmas and measuring their excitation temperature, density and plasma evolution from a seed gas 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 argon or nitrogen gas is pre-ionized by a 193 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. Optical emission spectroscopy is used to characterize the temporal evolution of the plasma and measuring the excitation temperature. The optically emitted spectral lines illustrate the temporal plasma evolution from a TMAE seed plasma initiated by the laser and early RF power to the steady-state RF plasma of neutral background gases such as argon and nitrogen. A three channel, wide band (200-850 nm) ST2000 Ocean optics spectrometer is used to record the plasma spectral emission perpendicular to the plasma column axis. Each channel is connected to a separate grating spectrometer (1200 lines/mm, with an optical resolution of 0.3 nm), which counts photons using a linear CCD array (2048 pixels). Samples are taken over the following wavelength ranges; 200-500 nm, 400-700 nm, and 600-850 nm. The results show that the laser ionization of the TMAE seed gas and transition to argon plasma is readily accomplished with our system. The spectroscopic measurements can also provide highly localized measurements of the plasma conditions in regions that are less accessible to interferometer measurements. We also present initial spectroscopic measurements of a high power UV laser focused in air near a dielectric microwave window