Der Afterglow von Edelgasplasmen: Rekombination und Erzeugung angeregter Zustände
Zusammenfassung der Projektergebnisse
The afterglow phase of a plasma is characterized by a number of interconnected processes whose description requires a multi-physics approach for its proper description. The initial fast cooling of the electrons and their diffusion are phenomena from the realm of plasma physics. The capture – either radiatively or in a ternary collision – of slow electrons by ions to form excited atoms and the following collisional and radiative cascades to form an atom in the ground state require expertise in atomic physics. And the dynamics of the long-lived metastable atoms, that are also formed in the recombination process, is shared by both fields. Therefore, up to now the different aspects have been targeted separately, mostly neglecting the interplay between them. In this project a unified theoretical description has been achieved which has been benchmarked against a number of experimental results. This helped to identify several interesting aspects. One of the most prominent ones is the electron heating by the recombination process. As the electrons recombine, their energy is taken up by the remaining electrons increasing their thermal movement and slowing down the further capture of electrons. Until now this effect has been only roughly estimated or directly ignored. In this project the effect was quantified and included in the treatment of the recombination, showing a significant influence and much better agreement with experiments. The electron heating by the intensive recombination can be so strong that their cooling abruptly slows down and their temperature remains in the vicinity of the temperature of the background gas. It has been proposed that this could be used as diagnostics for the measurement of the gas temperature. For the realization of this method only the temporal evolution of the electron density would be needed. It can be used to infer the recombination rate of the electrons and from there the electron temperature could be obtained. Provided that the conditions are suitable and the electron and gas temperature are close to each other, this will be a direct measure for the gas temperature. The work on the project included also investigations on collisional-radiative models and the way the radiation trapping should be handled. These investigations showed that the commonly used escape factor concept is in many of the common cases inapplicable and can give misleading results. The correct way of handling the radiation trapping is to consider the radiation transport leading to elaborate models and complex calculations. These investigations further opened new opportunities for measuring the absolute density and the spatial profile of the metastable atoms. This method would require spatially resolved measurements of the emission intensities of selected lines. A fitting procedure then would deliver the quantities sought. The afterglows in electronegative gases show also interesting aspects that have been investigated in this project. Depending on the duration of the afterglow, different behavior of the electron density is observed. It has been explained based on a numerical model of an oxygen afterglow. Depending on whether the negative ions have been destroyed by detachment and if a significant amount of the positive ions have been lost by diffusion and recombination, the electron density can exhibit either an overshoot or an undershoot at the beginning of the next pulse. It has been proposed to use this effect as an indirect diagnostics for the density of the negative ions. Finally, a seemingly unrelated, but nevertheless fundamentally important aspect has emerged from the work on the project. It turns out that the distribution in energies of the ions reaching the walls is closely related to the atomic characteristics of the gas and the spatial profiles of the plasma parameters. Furthermore, it turns out that knowing these values it is possible to reconstruct the distribution of the ions. Knowing the distributions in energies of ions and of electrons allows further an experimental test of a fundamental concept in plasma physics – the Bohm criterion. This is a relation, prescribing the minimal velocity that the ions need to have when leaving the plasma. It is often used as boundary condition in simulations and in estimates and is central for the field of plasma physics. Therefore, the emerging possibilities to experimentally benchmark this theoretical concept is of paramount importance for the community and is an interesting topic for future studies.
Projektbezogene Publikationen (Auswahl)
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“The glow in a three-body recombination dominated afterglow” IEEE Trans. Plasma Sci. 42 (2014) 2388
Ts. V. Tsankov, S. Siepa, P. S. Böhm, D. Luggenhölscher and U. Czarnetzki
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“2D collisional-radiative model for non-uniform argon plasmas: with or without ‘escape factor”’ J. Phys. D: Appl. Phys. 48 (2015) 085201
X.-M. Zhu, Ts. V. Tsankov, D. Luggenhölscher and U. Czarnetzki
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“Rydberg state, metastable, and electron dynamics in the low-pressure argon afterglow” Plasma Sources Sci. Technol. (2015) Volume 24, Number 6, 065001
Ts. V. Tsankov, R. Johnsen and U. Czarnetzki
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“Self-absorption method in combination with an optical probe: a possibility to determine the radial density profile of rare-gas metastables in low-temperature plasmas” Plasma Sources Sci. Technol. 24 (2015) 035023
X.-M. Zhu, Ts. V. Tsankov and U. Czarnetzki
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“The electron density evolution in pulsed 60MHz capacitively coupled oxygen discharges” J. Phys. D: Appl. Phys. 48 (2015) 035206
F.-X. Liu, Ts. V. Tsankov and Y.-K. Pu
