van de Mj Marco Sande

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.

Entrainment of ambient air into a spectrochemical inductively coupled argon plasma

Abstract A spectrochemical inductively coupled argon plasma (ICP) is normally operated in the open air. Therefore, it is suggested in the literature that entrainment of air molecules into such an ICP may cause loss of electrons, especially so at the plasmas edge. The present study discusses the significance of this effect. The density and temperature of electrons and nitrogen molecules around the edge of the plasma were measured by Thomson and rotational Raman scattering. A region where both electrons and nitrogen were present in detectable amounts (10 19 and 10 24 m −3 , respectively) could not be observed. Above the torch inner wall the nitrogen concentration drops rapidly towards the plasma. Measurements suggest that the nitrogen concentration at 1 mm from the plasma is only a few percent, and in the active zones of the plasma (far) below 0.1%. This is not enough to affect the plasma significantly. Moreover, electron loss due to diffusion of nitrogen into the plasma is calculated to be much slower than the loss observed in earlier studies. Hence, air entrainment is unlikely to play a significant role in the ICP. A possible alternative is the formation and destruction of molecular rare gas ions.

The relation between internal and external parameters of a spectrochemical inductively coupled plasma

The excitation kinetics in a spectrochemical plasma are governed by the electron density ne, electron temperature Te, and heavy particle (gas) temperature Th. Therefore, knowledge of these ‘internal’ plasma parameters is important for an understanding of the relation between the sample concentration in the plasma and light emission. Because of the small size of the plasma, the internal plasma parameters are related rather directly to the ‘external’ operational parameters of the plasma, such as the plasma dimensions, power density, and pressure. This relation is established by the various particle and energy balances, and can be used to estimate the internal plasma parameters and predict trends for a change in the operational parameters. In the present work, this approach was applied to spectrochemical inductively coupled plasmas under various gas-flow, gas-composition, and plasma-power conditions, and validated by Thomson scattering experiments. The measured values and trends of the internal plasma parameters are in close agreement with those expected on the basis of the operational parameters of the plasma.

Thomson scattering measurements on an atmospheric Ar dc discharge lamp

Thomson scattering (TS) experiments have been performed in the region near the electrodes of a dc powered model lamp filled with 1–2 bar argon gas. In order to suppress the false stray light and Rayleigh scattered photons, a triple grating spectrograph was used. In this way the electron density and electron temperature could be measured in the near-electrode region for different arc currents. In order to get the electron density and electron temperature out of the TS spectrum, both collective and non-collective TS fitting procedures were used for the data processing. It was found that the radial profile of the electron temperature is more or less flat in the anode region with a value of about 1 eV. The electron density increases with arc current and is of the order of 1021 m−3. From the result, we can see that the plasma regions investigated are not in local thermal equilibrium in the sense that, locally, the rate of ionization is much larger than that of recombination.

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

Abstract The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density ne and temperature Te in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of ne and Te can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.

Time resolved electron density and temperature measurements on a capacitively coupled helium RF discharge

Small capacitively coupled RF plasma sources operated with helium are popular as excitation sources for element detection, especially in combination with a gas chromatograph. The high mobility of helium and the use of relatively low RF frequencies lead to strong time dependences of the electron density and temperature, and hence excitation efficiency. In this study, the electron gas parameters of such a plasma were measured with temporal resolution. This was done by Thomson scattering using a triple-grating spectrograph for stray light rejection and by absolute spectral line intensity measurements. The electron density is measured to vary within a factor of two around 1.5×1019 m−3. The electron temperature found by Thomson scattering varies with the RF signal between 0.5 and 3.8 eV, whereas the temperature deduced from the absolute spectral line intensity ranges from 1.2 to 2.0 eV. This difference suggests the electron energy distribution to deviate from a Maxwellian equilibrium shape, as was observed in a number of Thomson spectra. A model is presented to explain the electron density behaviour on the basis of the measured electron temperatures.