Charles H. Kruger

Optical diagnostics of atmospheric pressure air plasmas

Atmospheric pressure air plasmas are often thought to be in local thermodynamic equilibrium owing to fast interspecies collisional exchange at high pressure. This assumption cannot be relied upon, particularly with respect to optical diagnostics. Velocity gradients in flowing plasmas and/or elevated electron temperatures created by electrical discharges can result in large departures from chemical and thermal equilibrium. This paper reviews diagnostic techniques based on optical emission spectroscopy and cavity ring-down spectroscopy that we have found useful for making temperature and concentration measurements in atmospheric pressure plasmas under conditions ranging from thermal and chemical equilibrium to thermochemical nonequilibrium.

Arrays of radiative transition probabilities for the N2 first and second positive, no beta and gamma, N+2 first negative, and O2 Schumann-Runge band systems

Abstract Extensive arrays of Einstein A coefficients, band oscillator strengths, sums of the squared electronic-vibrational transition moments, and Franck-Condon factors are presented for the N2 first and second positive, NO beta and gamma, N+2 first negative, and O2 Schumann-Runge band systems. These results are based upon recent ab initio calculations of electronic transition moments, and potential curves obtained with updated spectroscopic constants.

Methyl radical measurement by cavity ring-down spectroscopy

Abstract Cavity ring-down spectroscopy has been used to measure the absorbance of methyl radicals near 216 nm. The methyl radicals are generated by a hot tungsten filament heated to 2300 K in a mixture of 0.5% CH 4 in H 2 slowly flowing through the reactor at 20 Torr total pressure. CH 3 absorbances are detected with a noise-equivalent sensitivity of two parts in 10 5 using a narrow pencil of UV light 0.5 mm in diameter, which allows measurement of spatial profiles of CH 3 for column densities of 3×10 12 radicals/cm 2 (3×10 12 radicals/cm 3 ×1 cm absorption pathlength).

Shock tube determination of the rate coefficient for the reaction N2+O→NO+N

The rate coefficient for the reaction N 2 + O → k 1 NO + N has been measured in the temperature range 2384–3850 K using a shock tube technique. Test gas mixtures consisting of N 2 , O 2 , N 2 O and Kr were heated by incident shock waves, and the concentration of NO in the post-shock region was monitored using two independent spectroscopic techniques: IR emission at 5.3 μm, and absorption of CO laser radiation at 5.17 μm. The two methods yielded excellent agreement. The importance of interferences from other reactions was minimized by careful tailoring of the test mixture composition, in particular by using N 2 O rather than O 2 as a source of 0 atoms. The rate coefficient k 1 was determined by comparing experimental and calculated NO profiles using a computer simulation in which k 1 was an adjustable parameter. The results of 20 experiments indicate that k 1 =1.84×10 14 exp (−76,250/RT) cm 3 /mol×s with an uncertainty of approximately ±35% in the temperature range investigated.

Nonequilibrium in Confined‐Arc Plasmas

The nonequilibrium behavior of argon and helium confined‐arc plasmas at atmospheric pressure is analyzed in terms of a two‐temperature ambipolar‐diffusion model. Examination of spectroscopic data for the nonquilibrium arc shows that the free and bound electrons are mutually in equilibrium at a common electron temperature but not in equilibrium with the translational degrees of freedom of the heavy particles; the electron density in the outer portions of the arc may be as much as several orders of magnitude greater than its local thermodynamic‐equilibrium value. Integration of the electron continuity equation and the electron and heavy‐particle energy equations, with calculated transport properties and recombination coefficients, yields a satisfactory comparison with the measurements.

Nonequilibrium discharges in air and nitrogen plasmas at atmospheric pressure

Diffuse glow discharges were produced in low temperature (<2000 K) atmospheric pressure air and nitrogen plasmas with electron number densities in excess of 1012 cm3, more than six orders of magnitude higher than in thermally heated air at 2000 K. The measured discharge characteristics compare well with the predictions of a two-temperature kinetic model. Experimental and modeling results show that the steady-state electron number density exhibits an S-shaped dependence on the electron temperature, a behavior resulting from competition between ionization and charge-transfer reactions. Non-Maxwellian effects are shown to be unimportant for the prediction of steady-state electron number densities. The power requirements of DC discharges at atmospheric pressure can be reduced by several orders of magnitude using short repetitive high-voltage pulses. Between consecutive pulses, the plasma is sustained by the finite rate of electron recombination. Repetitive discharges with a 100-kHz, 12-kV, 10-ns pulse generator were demonstrated to produce over 1012 electrons/cm3 with an average power of 12 W/cm3, 250 times smaller than a DC discharge at 1012 cm3.

Direct-current glow discharges in atmospheric pressure air plasmas

Investigations have been conducted to experimentally validate the mechanisms of ionization in two-temperature atmospheric pressure air plasmas in which the electron temperature is elevated with respect to the gas temperature. To test a predicted S-shaped dependence of steady-state electron number density on the electron temperature and its macroscopic interpretation in terms of current density versus electric field, direct-current (dc) glow discharge experiments have been conducted in flowing low temperature, atmospheric pressure air plasmas. These experiments show that it is feasible to create stable diffuse glow discharges with electron number densities in excess of 1012 cm−3 in atmospheric pressure air plasmas. Electrical characteristics were measured and the thermodynamic parameters of the discharge were obtained by spectroscopic measurements. The measured gas temperature is not noticeably affected by whether or not the dc discharge is applied. The discharge area was determined from spatially resolved...

Measurement of the methyl radical concentration profile in a hot‐filament reactor

The spatial profile of methyl radical concentration in a hot‐filament reactor has been measured using cavity ring‐down spectroscopy (CRDS) at a wavelength of 213.9 nm for which the CH3 absorption cross section has been shown to be nearly independent of temperature. Methyl radicals are generated with a 25 mm long tungsten filament heated to 2400 K in a slowly flowing mixture of 0.6% CH4 in H2 (20 Torr total pressure). CRDS is employed to measure CH3 absorbance as a function of a distance perpendicular to the axis of the filament. The CH3 absorbance profiles do not change when the direction of the process gas flow through the reactor is reversed, which indicates cylindrical symmetry of the CH3 distribution about the filament. Consequently, the radial CH3 concentration in the reactor is determined by Abel inversion of the CH3 absorbance profile. The CH3 concentration peaks ∼4 mm from the filament (1.04×1014 molecules/cm3). Methyl radicals decay rapidly as a function of a distance from the filament and disapp...

Boundary layer profiles in plasma chemical vapor deposition

A nonlinear optical spectroscopy based on degenerate four-wave mixing has made possible direct measurements of species temperature and concentration profiles through the boundary layer of a reactive plasma at atmospheric pressure. Spectra were obtained for CH and C2 radicals over a range of conditions including those for the plasma chemical vapor deposition of diamond films. Numerical simulations based on a one-dimensional stagnation-point flow model are in good agreement with the measurements. The CH mole fraction is shown to rise and fall as a function of distance from the substrate, which is compelling experimental evidence for the complex chemistry that is occurring in the plasma boundary layer.

Measurement of absolute CH3 concentration in a hot-filament reactor using cavity ring-down spectroscopy

Abstract Methyl radicals were generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (length, 20 mm) in a slowly flowing mixture of 0.5% CH 4 in H 2 . The UV absorbance of CH 3 was measured during deposition using a new line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements were carried out at 213.9 nm, a wavelength at which the CH 3 absorption cross-section has been shown by others to be independent of the temperature over a large range. We observed that the CH 3 absolute concentration varied strongly as a function of the position between the substrate and the filament, and its value was strongly influenced by the substrate temperature.