Zenon Zakrzewski

An atmospheric pressure waveguide-fed microwave plasma torch: the TIA design

We report theoretical and experimental investigations of an atmospheric pressure, microwave plasma torch with axial gas injection (torche a injection axiale design). It is a waveguide-based structure comprising both waveguide and coaxial elements serving the purpose of wavemode conversion and impedance-matching. The device includes features common to various waveguide-fed torches disclosed previously by other authors, but not yet modelled. Our paper provides a simple equivalent circuit description of the torch operation that accounts for its impedance-matching, power transfer to plasma and tuning characteristics, as verified experimentally. From the outcome of the model and using our experimental results, we introduce new features in the torch design that enable one to optimize its performance. We also examine ways of simplifying its structure and operation.

The theory and characteristics of an efficient surface wave launcher (surfatron) producing long plasma columns

Long plasma columns, in many instances preferable to the positive columns of DC discharges, can be obtained by means of a UHF surface wave propagating along the column. This is possible through the use of a novel wave-launching structure, called a surfatron. The launcher is compact and located on the outside of the dielectric tube containing the plasma. The surfatron is described and analysed in terms of an equivalent circuit theory. Guidelines for its design and operation are given.

Plasma sources using long linear microwave field applicators: main features, classification and modelling

Microwave plasma sources using elongated field applicators that extend over distances large in comparison with the free-space wavelength are in demand for many applications, mainly for the processing of large surfaces of materials that can be moved transversely with respect to the applicator. Various long applicators have been presented in the literature but no unified description is available to assist the potential user in selecting the one best suited for his or her particular needs. This paper is intended to meet this need. All applicators of this kind provide an electromagnetic wave from which originates the electric field sustaining the discharge. However, they differ in some essential aspects which reflect on their performance and on the modelling of the resulting plasma. We propose to classify these applicators first into two main categories according to the way the power flows to the plasma: (i) transmission-line applicators; and (ii) antenna applicators. Then, in each category, we need to distinguish two possible wave characters: travelling and standing. This leaves us with four systems to examine. For each of these systems, we briefly approach discharge modelling, mainly to examine the longitudinal distribution of plasma properties and establish guidelines for the methodical design and operation of these plasma sources. Finally, we evaluate the advantages and disadvantages of the four systems.

The Development and Use of Surface-Wave Sustained Discharges for Applications

Laboratory investigations as well as practical applications involving plasmas most generally call for a plasma source with specific characteristics. As a rule, this source should be stable, reproducible, quiescent and eventually free from contamination. Ideally, it should also be simple to build, easy to operate and inexpensive. Surface-wave sustained discharges (SWDs) have been developed along these guidelines since their inception in the early seventies [1–5]. In addition, and this is an original feature of SWDs, these discharges have also led to major and innovative contributions to the understanding of high frequency (HF) discharges in general, facilitating their application in science and technology. As far as microwave plasma generation is specifically concerned, turning to SWDs was the occasion for major breakthroughs, which include increased plasma volume through long tubular discharges and, more recently, the advent of large-area configurations, mostly planar, which are particularly well suited for surface treatment.

The electromagnetic performance of a surfatron-based coaxial microwave plasma torch

Abstract Low power microwave plasma torches are of particular interest to analytical chemists. The torch design investigated herein, called TPS, is based on the known surfatron structure to which a coaxial section is added consisting of an impedance transformer followed by the metallic nozzle at the tip of which the discharge occurs. A series of experiments illustrate the main electromagnetic features and performance of this novel coaxial microwave plasma torch operating at 2.45 GHz and with input power in the range 10–180 W. A specially devised slotted coaxial line with a movable probe arrangement can be inserted into the torch in place of the transformer section to provide in situ measurements of the plasma impedance. Analyzing these results, we show that the shape of the torch tuning characteristics can be controlled to improve the power transfer to the plasma and stability of operation with respect to changes in discharge conditions; under these conditions, the design of the device can be simplified. The procedures presented have a general character and can be applied to various torch configurations.

On the supply and measurement of power in microwave induced plasmas

Abstract Power feeding and power measurement with microwave induced plasmas (MIP) pose specific problems as compared to rf produced plasmas. This note presents a concise review of these problems and indicates possible solutions. It concentrates on two main aspects. It recommends the ways to assure a stable discharge and the efficient power transfer to the plasma. It discusses the most common mistakes in power measurements and indicates the proper means and procedures that should be followed.

Compact Waveguide-Based Power Divider Feeding Independently Any Number of Coaxial Lines

The device described in this paper has been designed to enable the feeding of many individual plasma sources from a single microwave generator, providing a noninterfering and constant supply of power to each coaxial line driving these plasma sources. The power coming from the generator flows through a waveguide under standing-wave conditions provided by the presence of a conducting plane located at the waveguide end opposite that linked to the generator. Power is extracted from the waveguide, at the maximum of intensity of the E-field standing wave, by a waveguide-to-coaxial-line transition designated as a probe. One or two probes can be set at each such maximum of field intensity (and this on both sides of the waveguide wide wall), yielding a compact power divider. Each coaxial line feeds a microwave field applicator, sustaining plasma, through a matching circuit comprising a tuning means and a ferrite isolator (circulator with a matched load), the latter ensuring that whatever happens to the plasma source, the other feeding lines are not affected. The conditions required for a perfect match of the microwave generator to the power divider are elaborated and examples of actual designs are presented

Spatial distributions of electron density and electric field in discharges sustained within microwave circuits

Overcritical density discharges sustained in cylindrical vessels placed within microwave circuits (e.g. cavities, waveguides) and controlled by diffusion are theoretically investigated. In such discharges, the mutual interaction between the plasma and the microwave electric field leads to a redistribution of the imposed (plasma-free) field. The authors predict the resulting spatial distributions of electron density and electric field. In the common case where the length of the active zone exceeds at least a few times its diameter, it is found that (1) the electron density and the electric field tend to be axially uniform and (2) the maintenance field intensity is the same as in an underdense discharge.

5 – Surface Wave Plasma Sources

As a first approach to presenting surface wave (SW) plasma sources, let us consider their distinctive features with respect to the other plasma sources described in the book: 1. The discharge can be sustained far away from the active zone of the field applicator . This is because the electric field supporting the discharge is provided by a wave that carries away the power from the applicator. It is an electromagnetic surface wave whose sole guiding structure is the plasma column that it sustains and the dielectric tube enclosing it 1. , 2. xa0andxa0 3. . This is, thus, a non-cumbersome method for producing long plasma columns; plasma columns up to 6 meters in length have been achieved in our laboratory while launching the wave with a field applicator that surrounded the discharge tube over a few centimeters in length only. 4. xa0andxa0 5.

Surface-Wave Plasma Sources

The phenomenon of electromagnetic surface wave propagation along the interface between a preexisting plasma column and its surrounding dielectrics has been recognized and described in detail decades ago.1?2 Later, in the early seventies, these waves began to be used to sustain plasma columns.3,4 These so-called surface-wave discharges (SWD) exhibit many advantageous features: long, stable plasma columns can be sustained over large domains of wave frequencies (from less than 1 MHz up to approximately 10 GHz*) and gas pressures (from approximately 10−5 torr up to a few times atmospheric pressure). SWD setups are simple, compact, easy to handle and ensure an efficient energy transfer from the microwave power source to the plasma.5