Radio frequency energy coupling to high-pressure optically pumped nonequilibrium plasmas

Abstract

This article presents an experimental demonstration of a high-pressure unconditionally stable nonequilibrium molecular plasma sustained by a combination of a continuous wave CO laser and a sub-breakdown radio frequency ͑rf͒ electric field. The plasma is sustained in a CO/N 2 mixture containing trace amounts of NO or O 2 at pressures of Pϭ0.4-1.2 atm. The initial ionization of the gases is produced by an associative ionization mechanism in collisions of two CO molecules excited to high vibrational levels by resonance absorption of the CO laser radiation with subsequent vibration-vibration (V-V) pumping. Further vibrational excitation of both CO and N 2 is produced by free electrons heated by the applied rf field, which in turn produces additional ionization of these species by the associative ionization mechanism. In the present experiments, the reduced electric field, E/N, is sufficiently low to preclude field-induced electron impact ionization. Unconditional stability of the resultant cold molecular plasma is enabled by the negative feedback between gas heating and the associative ionization rate. Trace amounts of nitric oxide or oxygen added to the baseline CO/N 2 gas mixture considerably reduce the electron-ion dissociative recombination rate and thereby significantly increase the initial electron density. This allows triggering of the rf power coupling to the vibrational energy modes of the gas mixture. Vibrational level populations of CO and N 2 are monitored by infrared emission spectroscopy and spontaneous Raman spectroscopy. The experiments demonstrate that the use of a sub-breakdown rf field in addition to the CO laser allows an increase of the plasma volume by about an order of magnitude. Also, CO infrared emission spectra show that with the rf voltage turned on the number of vibrationally excited CO molecules along the line of sight increase by a factor of 3-7. Finally, spontaneous Raman spectra of N 2 show that with the rf voltage the vibrational temperature of nitrogen increases by up to 30%. This novel energy efficient approach allows sustaining large-volume high-pressure molecular plasmas without the use of a high-power CO laser. This opens a possibility of using the present technique for high-yield plasma chemical synthesis and plasma material processing.

Coherent anti-Stokes Raman scattering and spontaneous Raman scattering diagnostics of nonequilibrium plasmas and flows

“…These results exhibited markedly non-Boltzmann distribution of vibrational level populations, with first level vibrational temperature of T vib = 2500 K. More recently, spatially resolved measurements of nitrogen vibrational populations, N 2 (v = 0-8), have been performed in a short-lived afterglow of a microwave discharge in nitrogen [86], exhibiting a slowly evolving V-V pumped plateau in N 2 vibrational distribution. Spontaneous Raman measurements of vibrational level populations of three diatomic species, N 2 , O 2 , and CO, have been made in high-pressure (up to 1 bar) mixtures of these gases where CO in relatively low vibrational levels, v < 10, was vibrationally excited by resonance absorption of the CO laser radiation (10-20 W c.w [87][88][89]. ),…”

Selective generation of excited species in ns pulse/RF hybrid plasmas for plasma chemistry applications

“…The advantages of this method include the use of a single pair of electrodes, which can be external to the plasma chemical reactor, as well as better stability at high pressures and input powers, since the RF discharge may remain non-self-sustained in the entire gap. Previously, this approach has been used to generate an RF-enhanced optically pumped CO-N 2 plasma at pressures of up to 1 atm, sustained by resonance absorption of CO laser radiation resulting in associative ionization of highly vibrationally excited CO molecules [6,7]. RF energy coupling to the plasma increases CO vibrational level populations by up to an order of magnitude [6], while N 2 vibrational temperature, measured by spontaneous Raman scattering, increases by up to a factor of two [7].…”

“…Previously, this approach has been used to generate an RF-enhanced optically pumped CO-N 2 plasma at pressures of up to 1 atm, sustained by resonance absorption of CO laser radiation resulting in associative ionization of highly vibrationally excited CO molecules [6,7]. RF energy coupling to the plasma increases CO vibrational level populations by up to an order of magnitude [6], while N 2 vibrational temperature, measured by spontaneous Raman scattering, increases by up to a factor of two [7]. This method (i.e.…”