Plasma Cathode Electron Sources: Physics, Technology, Applications - Hardcover

Oks, Efim

Hardcover

ISBN 10: 3527406344 ISBN 13: 9783527406340

Verlag: Wiley-VCH, 2006

Inhaltsangabe

This book fills the gap for a textbook describing this kind of electron beam source in a systematic and thorough manner: from physical processes of electron emission to examples of real plasma electron sources and their applications.

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Über die Autorin bzw. den Autor

Efim Oks is head scientist of the Plasma Sources Department at the High Current Electronics Institute, Russian Academy of Sciences, Russia. His work focuses on the twin areas of plasma cathode electron beam sources and vacuum arc ion beam sources and was awarded prestigiously. Professor Oks has established numerous collaborative scientific research programs with researchers in the United States and Europe. He thus has become a significant international plasma physicist, having authored numerous papers in international journals.

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An up-to-date review and summary of this important subfield of applied plasma physics. Concentrating equally on providing a physical understanding of the basic processes involved in plasma electron emission and on the design and applications of plasma cathode electron beam sources, this monograph is of interest to designers of electron sources as well as to scientists and engineers using electron beams in research and industry. It will also be of benefit to both undergraduate and postgraduate students involved in vacuum and plasma electronics, the generation of charged-particle beams, and their applications.

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Plasma Cathode Electron Sources

By Efim Oks

John Wiley & Sons

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
All right reserved.
ISBN: 978-3-527-40634-0

Chapter One

Low-Pressure Discharges for Plasma Electron Sources

Two conflicting requirements occur in the design of plasma-cathode electron sources, both of which need to be met simultaneously. In order to ensure the required emission current density, adequate plasma density must be attained, for which efficient ionization in the plasma near the emission boundary must be provided. On the other hand, accelerating the electron beam to the required energy calls for the application of high voltage in the region of electron-beam formation and acceleration; this in turn necessitates decreasing the ionization processes that can cause breakdown within the acceleration gap. High electric field in the acceleration gap is needed to provide the electron energy, but this same high field can cause breakdown in the gap. This problem can be solved by establishing a pressure difference between the plasma generation region and the electron extraction region. This is possible, however, only for the case of a relatively small plasma emission surface area, e.g., for small-area focused electron beams. For large-cross-section electron beams or electron beams generated at fore-vacuum pressures, it is difficult or almost impossible to produce such a pressure difference. In this case the choice of an appropriate discharge system that is capable of providing conditions for efficient generation of electrons in the plasma and their stable extraction is likely to be the only way for successful operation of a plasma-cathode electron source.

The discharge employed in plasma-cathode electron sources must provide generation of dense plasma in the region of electron extraction, at the lowest possible pressure. From this standpoint the most suitable kinds of plasma sources are the hollow-cathode glow discharge, discharges in crossed electric and magnetic fields, such as Penning or cylindrical magnetron discharges, the constricted arc discharge, and the vacuum arc. Note that for most plasma cathodes, two different discharge systems are combined into a single device. For instance, one of the discharges (the main discharge) is used to produce the emissive plasma and the other (the auxiliary discharge) is employed to initiate and sustain the main discharge. Let us briefly consider the peculiarities of each of the discharge systems that are most commonly employed in plasma-cathode electron sources.

1.1 Hollow-Cathode Discharge

The hollow-cathode discharge is widely used in various plasma devices, including plasma electron sources. A characteristic feature of this kind of discharge is the oscillation of fast electrons emitted from the inner walls of the cathode cavity and accelerated into the cathode sheath. Unlike reflex discharges in crossed electric and magnetic fields where electrons are confined by the magnetic field (see Section 1.2), in the hollow-cathode glow discharge the fast electrons reside within the plasma for a long period of time, being repeatedly reflected in the cathode fall region. There are a number of different hollow-cathode configurations that can provide electron oscillation. In plasma electron sources, the cathode cavity is normally a hollow cylinder with a central hole in one of its faces (see Fig. 1.1). The characteristic dimensions of the cavity vary from several millimeters to tens of centimeters, depending on the required plasma emission parameters. The optimal ratio of the cavity length l_cav to the cavity diameter d_cav lies in the range l_cav/d_cav [approximately equals] 7–10. The diameter of the hole in the open face of the cavity d_o is typically several times smaller than d_cav. Electrostatic confinement of electrons in the cathode cavity is responsible for the so-called hollow-cathode effect, which shows itself as an abrupt decrease in discharge operating voltage and an increase in discharge current (see Fig. 1.2), and as an extension of the operating pressure range toward lower pressures. Note that the hollow-cathode effect occurs only when the electron mean free path exceeds the characteristic dimensions of the cathode cavity. The type of hollow-cathode discharge is determined by the mechanism of electron emission from the cathode surface. In this connection, one can distinguish arc discharges with cold and hot hollow cathodes, including a self-heating cathode, and also high-voltage and low-voltage hollow-cathode glow discharges.

A low-voltage discharge with a "cold" hollow cathode is rather easily produced; it is characterized by time stability and spatial uniformity of the plasma parameters. This kind of discharge is quite commonly employed for producing plasmas in plasma-cathode electron sources. Under steady-state conditions, the discharge current I_d in such systems is, as a rule, no greater than 1 A at a discharge operating voltage U_d =400–600 V, yet it can be increased by about an order of magnitude provided that the formation of cathode spots is precluded.

In pulsed mode, it is possible to realize a diffuse form of a hollow-cathode discharge in the microsecond range with a current of hundreds of amperes. In this kind of discharge, the plasma electron temperature T_e is generally several electronvolts. The plasma density n_e is determined by the discharge current density to the cathode (from several milliamperes to several amperes per square centimeter) and typically lies in the range n_e ~ 10¹⁰ -10¹³ cm^-3.

In studies of the low-voltage hollow-cathode discharge, the suggestion was made that UV radiation from the bulk plasma may result in additional electron emission from the cathode surface. However, the authors came to recognize that photoelectron emission can be of only secondary importance. They also suggested that the main factor responsible for the development of the hollow-cathode effect is multiplication of electrons in the cathode potential fall region. The contribution from this factor becomes less significant with increasing discharge current and decreasing operating pressure, when the thickness of the cathode fall region decreases compared to the dimensions of the cathode cavity, and the electron mean free path λ_e becomes much greater than the characteristic width of the discharge gap.

The thickness of the cathode sheath (region of potential fall at the cathode) l_s can be determined by solving simultaneously the well-known Child-Langmuir and Bohm equations:

ls [approximately equals] (ε₀/n_i) ^1/2(U_c)^3/4/(ekT_e) ^1/4. (1.1)

Here e is the electron charge, U_c is the cathode fall potential, n_i is the plasma ion density, and T_e is the electron temperature.

The uniformity of the ion current density distribution over the hollow-cathode surface depends on both the cathode geometry and the operating pressure. In a long and narrow cathode cavity, the plasma density, and hence also the ion current density to the cathode, increases as the exit aperture facing the...

Details zur Ausgabe

• Verlag: Wiley-VCH
• Erscheinungsdatum: 2006
• Sprache: Englisch
• ISBN 10: 3527406344
• ISBN 13: 9783527406340
• Einband: Gebundene Ausgabe
• Auflage: 1
• Anzahl der Seiten: 181
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: Nicht verfügbar