Spectral Methods of Control of Impurities, Their Flows and Localization in an Equilibrium Low-Temperature Plasma of Low Pressure

Abstract

The state-of-art of studies of processes involving small fractions of particles formed as a result of chemical transformations of the source gas or appearing from the outside as an impurity is discussed. Various high-sensitivity methods for measuring the low concentrations of particles are considered. The main focus is on spectral methods, both classical and laser ones. The main modern fields of the application of non-equilibrium plasma, features, and achievements of the diagnostic tools used in them are highlighted. The emphasis is on spectroscopic studies of plasma purity in the low-temperature near-wall layers of thermonuclear reactors. In this case, the issue of the penetration of water molecules into the plasma chamber from the cooling circuits is of particular importance. In view of the limited accessibility to the reactors, new remote versions of spectral emission methods are being developed that are focused on the diagnostics of the ITER near-wall plasma. A new method of multispectral optical actinometry is discussed, in which intermediate chemically unstable particles can play the role of actinometers. This broadens the range of possibilities for quantitative measurements of concentrations of particles of different kinds, and increases the sensitivity. Effective actinometric pairs were found for the objects under consideration, e.g., hydrogen and deuterium atoms. The schemes are worked out on laboratory facilities, and kinetic models of plasma-chemical processes are developed. Scaling of the results of model experiments makes it possible to predict the measurement of the impurity flows at the level (10–9–10–13) Pa m3 s–1 with the localization of sources determined by the spatial resolution of the optical system.

Measurements and interpretation of EEDF in a discharge with a hollow cathode in helium: effect of the measuring probe and the anode on the form of the distribution function

“…The gas temperature in the discharge plasma was measured in separate experiments in which the discharge was ignited in a mixture of helium with small additions of water vapor. It was identified with the rotational temperature of the 'cold' group in the hydroxyl spectrum [15]. Under the conditions of present experiments, the gas temperature is T g ≈ 340 K, and the corresponding gas number density is N ≈ 2.13 × 10 16 cm −3 .…”

“…The experiments were carried out at the 'leak' setup, a detailed description of which is given in [15]. The scheme of the setup is shown in figure 1(a).…”

Electron sink on the Langmuir probe in plasma, its influence on measurements and methods to account for it

“…In the literature, deviations of the EEPF from Maxwellian ones in the high-energy region are usually discussed [16] and are associated with collisions of the second kind with heating of electrons by excited atoms and molecules. We will consider the issue of deviations in the region of low energies, associating them, as indicated in the introduction, with the possible influence of the electron sink on measurements, which is not taken into account by the main relation (1). Based on the structure of this relationship, it should be assumed that the problem exists due to the influence of the sink on the shape of the IVC and, as a consequence, on the second derivative d 2 I e /dV 2 .…”

“…The experiments were carried out on the Tech setup [1,13], designed to study the composition of the plasma under conditions near the walls of the ITER reactor by quantitative emission spectroscopy. A vacuum chamber (22 L, residual pressure ∼5 × 10 −7 mbar, ability to heat the walls up to 500 K) is used, there are several independent channels for gas inlet.…”

“…The substantiation of various diagnostic methods is also related with this information. For example, in optical spectra studies using the EEDF, the rate constants of excitation of various energy levels are determined, both for the plasmaforming gas and its ions, and for small impurities, include optical actinometers [1]. EEDF measurements in low-temperature plasma are carried out mainly with the help of probes introduced into the plasma [2][3][4][5].…”