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Title:Radical probe system for in-situ measurements of radical densities of hydrogen, oxygen and nitrogen
Author(s):Qerimi, Dren
Advisor(s):Ruzic, David
Contributor(s):Andruczyk, Daniel
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Hydrogen, Oxygen, Nitrogen, Radical Density, Langmuir Probe, Electron Temperature, Electron Density
Abstract:The current state-of-the-art methods to identify presence of radical species in vacuum chambers are optical methods, which suffer from the lack of spatial resolution and require expensive optical equipment. In this study Center for Plasma Material Interactions (CPMI) at the University of Illinois developed an in-situ catalytic radical probe system together with Ra-Den software platform to measure concentrations of reactive species in low temperature plasma with high spatial resolution. Radical probes as plasma diagnostic tool can be used to determine radical densities of hydrogen, nitrogen and oxygen but not limited to in any continuous plasma source in vacuum environment. The basic principle and advantage of a probe array is the capability to distinguish between different gas species due to several sensitive elements acting as recombination catalysts [1] [2]. The catalytic coatings cover an area of several square millimeters on the tip of a sheathed thermocouple. The catalytic probe surface provides efficient recombination of active species with subsequent energy release as a heat. All the probes are exposed to the same background plasma heating/cooling mechanisms, but the temperatures are not the same due to the fact that different catalytic materials have different recombination coefficients, therefore a temperature difference between probes is generated. The system consists of two additional probes, first to obtain the overall heat flux on probe array, and the second is a reference probe with surface chemically active to all gases. Radical densities of hydrogen, nitrogen and oxygen were measured in HARP helicon chamber by sweeping power from 300W up to 1100W and by sweeping pressure from 10mT up to 100mT. Generally, it is observed that radical densities increase with respect to pressure and power, accordingly. Additionally, plasma is parametrized by electron density and electron temperature such that electron density increased with increasing power and pressure and electron temperature increased with increasing power but it did decrease with increasing pressure. Further tests showed that radical probe system is capable is detecting specific gases when a mixture of gases is present in the chamber based on known recombination coefficient of the given gas on catalytic surface. Ionization and radical density percentages of all three gases are compared to total ionization cross-sections of each gas, agreement is observed for each corresponding gas. The array of several probes is capable to distinguish between different gas species with sub centimeter spatial resolution. The probes give accurate results in a broad range of reactive species concentrations from about 10^12 cm−3 to 10^14 cm−3.
Issue Date:2019-10-29
Rights Information:Copyright 2019 Dren Qerimi
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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