van der B Bart Sijde

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.

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 theoretical study and experimental investigation of non-LTE phenomena in an inductively-coupled argon plasma-I. Characterization of the discharge

Abstract In the past considerable attention has been paid to the problem or explaining analyte and argon excitation in inductively-coupled plasmas. Deviations from Local Thermal Equilibrium (LTE) have been frequently reported. In this paper a theoretical framework is constructed to explain quantitatively analyte and argon excited level densities. An essentially new description of the discharge is proposed in terms of partial LTE and the saturation regime. Models and their applications to an ICP discharge are thoroughly discussed and compared with former LTE models.

Approaches for clarifying excitation mechanisms in spectrochemical excitation sources

Abstract It is shown that the state of an ICP plasma can be adequately described by a measured electron density distribution. Even though the plasma is not in full local thermal equilibrium (LTE), the electron temperature can be deduced from the density with the LTE relation with more accuracy than direct measurements permit. From energy and mass balances considerations it is argued that the deviations from LTE are sufficiently small; the ground state of the argon atom is underpopulated with respect to LTE. Spatially resolved measurements of excited state densities of argon neutrals and analyte neutrals and -ions are in good agreement with this “ close enough to LTE ”-concept. Only the levels of excited analyte ions which are resonant with the argon ion ground level show a significant overpopulation with respect to LTE. This is shown to be caused by the charge transfer process.

On the charge transfer in an inductively coupled argon plasma

Absolute population densities for several excited states of magnesium are obtained for several locations in an inductively coupled plasma (ICP). They were used to construct Boltzmann-Saha plots for these positions and show that magnesium is close-to-LTE. The deviations from LTE are mainly limited to the levels sensitive to charge exchange with argon ions. These measured deviations can be explained by a simple model which shows that, although charge transfer is a dominant excitation and ionization mechanism in an ICP, the associated LTE deviations are limited in magnitude.

Temperature determination in non-LTE plasmas

Abstract The temperature determination of plasmas in LTE, or in reality close-to-LTE, from the ratio of line intensities is a straightforward method which in principle depends only on the accuracy of line intensity measurements, the accuracy of the values of transition probabilities of the lines and the energy differences of upper levels in relation to the temperature. Even in these “simple” circumstances it turns out that the use of the “Boltzmann plot” method leads to large practical problems. Methods that include in some way the ground state density or ion density are much more favorable. Errors or uncertainties in the (electron) temperature may lead to a misinterpretration of the plasma state. In inductively coupled plasmas (ICP), close-to-LTE, it is possible to determine the electron temperature from the Saha equation with measurements (by Hβ broadening) of the electron density and of the densities of highly excited states. In non-LTE circumstances it is improper to use the “Boltzmann plot” method. One has to account for the stage of departure from equilibrium of the plasma. This stage is determined by the dominant excitation equilibrium and can be classified into the corona, excitation saturation, capture radiative cascade or partial LTE regime. In all these circumstances it is possible to obtain information from line intensities, since these are determined by the electron temperature and density, as well as the density of the ground and/or ion state. It is clear, however, that a determination of all these parameters asks for more information than is provided by line intensity measurements. In the excitation saturation regime it is sometimes possible to determine the electron temperature by taking into account the specific dependency of the density of levels on the principal quantum number of the levels.

Spectroscopic determination of electron density and temperature profiles in an inductively-coupled argon plasma

Abstract We have measured local profiles of the electron density and temperature in an atmospheric plasma using absolute intensities of highly excited argon states. The electron-density profile has been determined independently by spatially resolved measurements of the Stark broadening of the hydrogen-β line. By comparing these two methods, deviations from local thermal equilibrium may be characterized. Experimental results from an inductively-coupled argon plasma are presented and indicate that the plasma is close to LTE.

Non equilibrium characterization and spectroscopic analysis of an inductively coupled argon plasma

Abstract We have measured local electron density and temperature profiles in an inductively coupled plasma using absolute intensities of highly excited argon states. The electron density profiles are compared with the results of spatially resolved Stark broadening of the H-β line. By this comparison ionizing and recombining parts of the inductively coupled plasma can be distinguished from each other. The density profile is further investigated for different operating conditions.

Experimental evidence for the complete saturation phase in the argon neutral system

Abstract An experimental study of the population densities of excited states confirms the existence of the complete excitation saturation phase in the argon neutral system. Collisional radiative coefficients r (1) m are independent of n e and decrease with decreasing ionization energy. At higher n e -values the levels are observed to come consecutively into Saha equilibrium (PLTE).

An experimental study of the ion energy balance of a magnetized plasma

The authors report on an experimental study of the ion energy balance of the magnetized and current-driven plasma of a hollow cathode discharge. The balance appears to be classical. At the axis of the plasma column the electron-ion Coulomb interaction is in equilibrium with the ion-neutral interaction. The authors find no significant influence on the energy balance by the spontaneously appearing plasma turbulence.