van der Jjam Joost Mullen

A combined Thomson-Rayleigh scattering diagnostic using an intensified photodiode array

A combined Thomson–Rayleigh scattering device is discussed. It consists of a Nd:YAG laser as a light source in combination with a multichannel detection technique consisting of a gated light amplifier in combination with an optical multichannel analyzer. Special attention is focused on the analysis of the measured spectra. Including convolution methods and taking into account weak coherent effects increases the dynamic range and the accuracy of the measured electron density ne and temperature Te and neutral particle density n0. Accuracies of 1%–4% for ne, 2%–6% for Te, and 10%–50% for n0 depending on the plasma condition are obtained. The dynamic range for ne is 7×1017–1021 m−3, for n0 is 1020–1023 m−3 and for Te is 1000–50 000 K.

Laser scattering on an atmospheric pressure plasma jet: disentangling Rayleigh, Raman and Thomson scattering

Laser scattering provides a very direct method for measuring the local densities and temperatures inside a plasma. We present new experimental results of laser scattering on an argon atmospheric pressure microwave plasma jet operating in an air environment. The plasma is very small so a high spatial resolution is required to study the effect of the penetration of air molecules into the plasma. The scattering signal has three overlapping contributions: Rayleigh scattering from heavy particles, Thomson scattering from free electrons and Raman scattering from molecules. The Rayleigh scattering signal is filtered out optically with a triple grating spectrometer. The disentanglement of the Thomson and Raman signals is done with a newly designed fitting method. With a single measurement we determine profiles of the electron temperature, electron density, gas temperature, partial air pressure and the N2/O2 ratio, with a spatial resolution of 50 µm, and including absolute calibration. (Some figures may appear in colour only in the online journal)

Instantaneous and delayed responses of line intensities to interruption of the RF power in an argon inductively coupled plasma

Abstract Instantaneous and delayed responses of line intensities to the sudden interruption of the RF power have been studied in an argon inductively coupled plasma (ICP). The instantaneous responses are caused by equilibrium shifts in the balances of elementary processes that control the populations of the excited states. It has been found that excited levels of Ar and H are predominantly populated by recombination of free electrons with ionic species, while most levels of metals such as Mg, Cd, Na, Fe, Al and Cu are populated by excitation from the ground state atom. Also charge transfer between Mg 1 and Ar 1 has been observed in the temporal behaviour of line intensities of two Mg + states, quasi-resonant for charge transfer. Furthermore, we observed that in the inner part of the plasma the temperature remains constant during the recombination decay time, after an initial cooling of the electrons to the heavy particle temperature. When the power is switched on again, the electron temperature seems to increase temporarily to a value that is higher than the steady state value. The delayed responses are caused by disturbances created in the expansion zone of the plasma during and after the interruption. It was found that these disturbances travel through the plasma with a velocity of 12 m s −1 .

A model study of propagation of the first ionization wave during breakdown in a straight tube containing argon

The mechanisms responsible for the propagation of the first anode directed ionization wave that occurs in a straight discharge tube during breakdown are studied by means of a fluid model. The discharge tube contains argon at a pressure of a few Torr and is operated at a dc voltage with the cathode heated to thermal electron emission temperatures. The two-dimensional model incorporates continuity and momentum equations for the electrons, for several effective excited states and for the ions, a balance equation for the electron energy and the Poisson equation. The model is capable of describing the first ionization front in a way that is qualitatively consistent with observations made in experiments. The mechanisms behind the breakdown evolution are investigated by considering the temporal and spatial evolution of the quantities described by the model. Previously, researchers have described this breakdown evolution in terms of an RC-line circuit. The validity of this picture is surveyed by considering the distribution of charges within the lamp. The effect of control parameters on the breakdown process and the assumptions that affect the validity of the model for later stages in breakdown are considered.

Deviations from the local field approximation in negative streamer heads

Negative streamer ionization fronts in nitrogen under normal conditions are investigated both in a particle model and in a fluid model in local field approximation. The parameter functions for the fluid model are derived from swarm experiments in the particle model. The front structure on the inner scale is investigated in a one-dimensional setting, allowing reasonable run time and memory consumption and high numerical accuracy without introducing superparticles. If the reduced electric field immediately before the front is ⩽50kV∕(cmbar), solutions of fluid and particle model agree very well. If the field increases up to 200kV∕(cmbar), the solutions of particle and fluid model deviate, in particular, the ionization level behind the front becomes up to 60% higher in the particle model while the velocity is rather insensitive. Particle and fluid model deviate because electrons with high energies do not yet fully run away from the front, but are somewhat ahead. This leads to increasing ionization rates in th...

The behavior of molecules in microwave-induced plasmas studied by optical emission spectroscopy. 1. Plasmas at atmospheric pressure

The emission of various low-pressure microwave-induced plasmas created and sustained by a surfatron or by a Beenakker cavity has been studied after the introduction of molecular species (i.e. N2, CO2, SF6 and SO2). Only nitrogen yielded observable emission from the non-dissociated molecule (first and second positive system). Using other gases only, emission of dissociation and association products has been observed (i.e. atomic species, CN, C2, CO, OH and NH). Studies of these intensities have been performed as functions of gas composition, pressure and position in the plasma and have provided an insight into molecular processes such as dissociation and association occurring in the plasma. It is found that parameters such as pressure and gas composition play a very important role with respect to these processes. Since no unambiguous relationship between the observed emission of dissociation or association products and the injected molecules has been found, it is established that it will be difficult to use microwave plasmas at reduced pressure as analytical excitation sources for molecular gas analysis.

Numerical description of discharge characteristics of the plasma needle

The plasma needle is a small atmospheric, nonthermal, radio-frequency discharge, generated at the tip of a needle, which can be used for localized disinfection of biological tissues. Although several experiments have characterized various qualities of the plasma needle, discharge characteristics and electrical properties are still not well known. In order to provide initial estimates on electrical properties and quantities such as particle densities, we employed a two-dimensional, time-dependent fluid model to describe the plasma needle. In this model the balance equation is solved in the drift-diffusion approach for various species and the electron energy, as well as Poisson’s equation. We found that the plasma production occurs in the sheath region and results in a steady flux of reactive species outwards. Even at small (<0.1%) admixtures of N2 to the He background, N2+ is the dominant ion. The electron density is typically 1011cm−3 and the dissipated power is in the order of 10mW. These results are con...

The argon-hydrogen expanding plasma: model and experiments

An argon expanding cascaded arc plasma, with small amounts (0-10 vol.%) of hydrogen added to the flow, is investigated by means of Thomson-Rayleigh scattering and optical emission spectroscopy. The results, especially the electron density behaviour as a function of the distance from the onset of the expansion, are interpreted by comparison with results of a quasi one-dimensional model. The associative charge exchange reaction between Ar+ ions and H2 molecules plays a dominant role in the model. Assuming that H2 molecules from the wall enter the plasma in the shock region, the large ionization loss can be explained. Good agreement between model and experiment is found for the electron and neutral density and the electron temperature behaviour. This makes plausible the existence of a recirculation flow inside the vacuum vessel, which transports wall-associated hydrogen molecules towards the plasma.

An easy way to determine simultaneously the electron density and temperature in high-pressure plasmas by using Stark broadening

This paper discusses the possibility of determining, at the same time, both the electron density and temperature in a discharge produced at atmospheric pressure using the Stark broadening of lines spontaneously emitted by a plasma. This direct method allows us to obtain experimental results that are in good agreement with others previously obtained for the same type of discharge. Its advantages and disadvantages compared to other direct methods of diagnostics, namely Thomson scattering, are also discussed.

Thomson scattering on a low-pressure, inductively-coupled gas discharge lamp

Excitation and light production processes in gas discharge lamps are the result of inelastic collisions between atoms and free electrons in the plasma. Therefore, knowledge of the electron density ne and temperature Te is essential for a proper understanding of such plasmas. In this paper, an experimental system for laser Thomson scattering on a low-pressure, inductively-coupled gas discharge lamp and measurements of ne and Te in this lamp are presented. The experimental system is suitable for low electron temperatures (down to below 0.2?eV) and employs a triple grating spectrograph for a high stray light rejection, or equivalently a low stray light redistribution (Reff?7?10-9?nm-1 at 0.5?nm from the laser wavelength). The electron density detection limit of the system is ne?1016?m-3. The modifications to the lamp that were necessary for the measurements are described, and results are presented and compared to previous work and trends expected from the electron particle and energy balances. The electron density and temperature are about ne?1019?m-3 and Te?1?eV in the most active part of the plasma; the exact values depend on the argon filling pressure, the mercury pressure and the position in the lamp.