Pjw Peter Vankan

Flow dynamics and invasion by background gas of a supersonically expanding thermal plasma

The transport of neutral argon atoms in an expanding thermal argon/hydrogen plasma is studied by means of laser-induced fluorescence spectroscopy around 811 nm, on the long living Ar[4s] atoms. Although the Doppler shifted laser-induced fluorescence measurements are performed on argon atoms in the metastable Ar*(3P2) and resonant Ar*(3P1) states, it is argued that in the plasma jet the velocity distribution function of these Ar[4s] atoms images the velocity distribution functions of the ground-state argon atoms. From the results it is inferred that the velocity behaviour of the supersonically expanding argon gas can be predicted from the momentum balance, and the temperature from the adiabatic relation between density and temperature. However, the adiabatic constant is found to be 1.4±0.1, smaller than the adiabatic constant of a neutral argon gas expansion which is (5/3). Both in the axial and in the radial directions the velocity distributions measured in the shock region show clear departures from thermodynamic equilibrium. From the radial velocity distribution it is concluded that background gas invades the supersonic part of the expanding plasma jet. The results on temperature and velocity in the subsonic region show that the radius of the plasma jet hardly increases after the stationary shock front, indicating that the flow pattern is geometrically determined.

Density and temperature of N atoms in the afterglow of a microwave discharge measured by a two-photon laser-induced fluorescence technique

Both the axial density and temperature profiles of ground-state nitrogen atoms have been measured in a microwave discharge and its afterglow in the presence of the so-called short-lived afterglow by means of two-photon absorption laser-induced fluorescence (TALIF). The temperature is obtained from the Doppler broadening of the spectral profile, after deconvolution with the laser profile. The N atom temperature decreases from about 1400 K in the end of the discharge zone to about 300 K in the downstream part of the afterglow. The sharp temperature decrease immediately behind the discharge zone can reasonably be explained by heat transfer to the flow tube wall. The absolute N atom density is obtained by calibrating the fluorescence yield with a TALIF signal from krypton atoms. The N density increases from 1.5×1021 m-3 in the discharge zone to about 3.5×1021 m-3 in the late afterglow. However, the N atom flux is conserved along the flow tube, indicating negligible consumption or production of N atoms in the short-lived afterglow.

Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

We report on quantitative, spatially resolved density, temperature, and velocity measurements on ground-state atomic hydrogen in an expanding thermal Ar–H plasma using two-photon excitation laser-induced fluorescence (LIF). The method’s diagnostic value for application in this plasma is assessed by identifying and evaluating the possibly disturbing factors on the interpretation of the LIF signal in terms of density, temperature, and velocity. In order to obtain quantitative density numbers, the LIF setup is calibrated for H measurements using two different methods. A commonly applied calibration method, in which the LIF signal from a, by titration, known amount of H generated by a flow-tube reactor is used as a reference, is compared to a rather new calibration method, in which the H density in the plasma jet is derived from a measurement of the two-photon LIF signal generated from krypton at a well-known pressure, using a known Kr to H detection sensitivity ratio. The two methods yield nearly the same re...

Two-photon laser induced fluorescence spectroscopy performed on free nitrogen plasma jets

The properties of free nitrogen plasma jets are examined by studying the ground-state nitrogen atom flow characteristics. The plasma is created by a cascaded arc and subsequently it expands freely into a low pressure vessel. In such a way supersonic flows with high Mach number are achieved. N(4S) atoms are locally probed by means of two-photon absorption laser induced fluorescence spectroscopy. Axial and radial N(4S) atom temperature and velocity profiles present the shape predicted by the neutral gas supersonic expansion theory. The adiabatic exponent γ is equal to 1.45 in the supersonic domain. A Mach number M of 4.4 is measured ahead of the normal shock wave. In contrast, density profiles reveal a departure from the classical expansion picture. Too small density jumps over the shock region indicate a non-conservation of the N(4S) atom forward flux. Moreover the partial N(4S) atom static pressure decreases in the subsonic domain. Loss of nitrogen atoms during the plasma expansion is a direct consequence of plasma–wall interactions. However, losses are limited because of the relatively high N atom mass and because of a low surface recombination probability of N atoms. The dissociation degree at the arc exit is around 40%. Under such circumstances N2(A) molecules cannot survive in the jet. The local electron density is estimated from a measure of the radiative lifetime of the nitrogen atom excited state.

Absolute density measurements of ammonia produced via plasma-activated catalysis

The generation of ammonia from atomic hydrogen and nitrogen has been demonstrated by means of cavity enhanced absorption spectroscopy. The atomic species are produced in a thermal plasma source in which plasma is created from mixtures of hydrogen and nitrogen. It is shown that for large atomic flux conditions, 2% of the hydrogen and nitrogen can be converted to ammonia. The process in which the ammonia molecules are formed from atomic radicals at the fully covered surface is called plasma-activated catalysis.

Inflow and shock formation in supersonic, rarefied plasma expansions

In this paper the physics of plasma expansion in the rarefied regime is reviewed. Densities, temperatures, and velocity distributions in argon, hydrogen, and nitrogen expansions that have been measured using laser scattering and fluorescence techniques are compared. The velocity distributions in the region of the expansion where the density is below the background density show a bimodal character. It is interpreted in terms of a component expanding from the source and a component flowing into the plasma expansion from the periphery. Also in the shock of the expansion, bimodal velocity distributions are encountered. These distributions show the gradual change in the flow from supersonic to subsonic—the formation of the shock. From a comparison of the three expansions, a general view of the shock formation is derived. This new insight leads to a better understanding of how the chemical reactivity of the usually impenetrable, supersonic plasma can be used most efficiently.

Influence of surface chemistry on the transport of H atoms in a supersonic hydrogen plasma jet

The transport of ground-state hydrogen atoms in the expansion of a thermal hydrogen plasma created by a cascaded arc is studied by means of two-photon absorption laser induced fluorescence. The low-dissociation degree measured at the source exit implies that H atoms flow in a H2 environment. It is shown that the H atom expansion pattern is in disagreement with the neutral gas supersonic expansion theory. Indeed the transport of H atoms in the plasma jet is strongly influenced by surface-recombination processes. Because of the large density gradients between the core of the jet and its surroundings induced by the recombination of H atoms at the reactor walls, hydrogen atoms diffuse out of the plasma jet in the course of the expansion. When the surface loss probability is high, i.e., the combination of a large wall-recombination probability with a long residence time, the losses of radicals by diffusion cannot be avoided even when the mass of the carrier gas is close to the mass of the radical.

Plasma expansion: fundamentals and applications

The study of plasma expansion is interesting from a fundamental point of view as well as from a more applied point of view. We here give a short overview of the way properties like density, velocity and temperature behave in an expanding thermal plasma. Experimental data show that the basic phenomena of plasma expansion are to some extent similar to those of the expansion of a hot neutral gas. From the application point of view, we present first results on the use of an expanding thermal plasma in the plasma-activated catalysis of ammonia, from N2–H2 mixtures.

Relaxation behavior of rovibrationally excited H2 in a rarefied expansion

The evolution of the rotational and vibrational distributions of molecular hydrogen in a hydrogen plasma expansion is measured using laser induced fluorescence in the vacuum-UV range. The evolution of the distributions along the expansion axis shows the relaxation of the molecular hydrogen from the high temperature in the upstream region to the low ambient temperature in the downstream region. During the relaxation, the vibrational distribution, which has been recorded up to v = 6, is almost frozen in the expansion and resembles a Boltzmann distribution at T approximately 2200 K. However, the rotational distributions, which have been recorded up to J = 17 in v = 2 and up to J = 11 in v = 3, cannot be described with a single Boltzmann distribution. In the course of the expansion, the lower rotational levels (J < 5) adapt quickly to the ambient temperature ( approximately 500 K), while the distribution of the higher rotational levels (J > 7) is measured to be frozen in the expansion at a temperature between 2000 and 2500 K. A model based on rotation-translation energy transfer is used to describe the evolution of the rotational distribution of vibrational level v = 2 in the plasma expansion. The behavior of the low rotational levels (J < 5) is described satisfactory. However, the densities of the higher rotational levels decay faster than predicted.

Atomic and molecular hydrogen densities in a plasma expansion

We treat the kinetics of the three dominant species in a hydrogen plasma expansion, which was studied using various laser spectroscopic techniques. Whereas the ground-state H2 molecules expand like a normal gas, the behaviour of H atoms and ro-vibrationally excited molecules is considerably altered as a consequence of their reactivity. The hydrogen atoms are primarily lost via surface association. In the source, H atoms are lost on the surface of the nozzle, reducing the atomic flux, and in the expansion, the H atoms diffuse out due to large H atom concentration gradients between the plasma and the background that exists as a result of surface association on the vessel walls. The loss process of H atoms in the nozzle, surface association, is the main production process of H2 molecules with high rotational and vibrational excitation. The behaviour of these ro-vibrationally excited H2 molecules in the expansion shows the relaxation process from the high temperature in the upstream region to the low temperature of the downstream region. The lower rotational levels (J 7) do not because of the large energy spacing between the hydrogen rotational levels. The consequence of this high rotational excitation is demonstrated in the dissociative attachment of electrons to H2.