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

Oks, Efim

Hardcover

ISBN 10: 3527406344 ISBN 13: 9783527406340

Publisher: Wiley-VCH, 2006

Synopsis

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.

About this title

About the Author

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.

From the Back Cover

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.

Excerpt. © Reprinted by permission. All rights reserved.

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 anode is approached. The discharge system geometry considerably affects the conditions under which the discharge plasma is generated, and consequently the discharge parameters. For efficient oscillation of fast electrons, one should either decrease the exit aperture of the cathode cavity or increase the cathode dimensions. It was shown in that decreasing the ratio of the exit aperture area S_a (in most cases equal to anode area) to the area of the inner surface S_c of the cathode, S_a/S_c, significantly decreases the lower limit to the operating pressure. Moreover, the operating pressure p in this region is directly proportional to S_a/S_c. As S_a/S_c is reduced, the discharge operating voltage rises steeply in response to the decrease in pressure. At a specified operating voltage, the lower limiting pressure and the operating pressure also show an abrupt increase, and a double electrostatic sheath across which U_s =10–40 V is formed in the region of the exit aperture.

Since the cathode cavity is an electrostatic trap for fast electrons which, oscillating chaotically, can escape only through the exit aperture, the energy of a primary electron expended in ionization depends on the ratio A/L. (Here A is the relaxation length of the electron: the average distance over which its initial energy decreases to the ionization potential U_i of the working gas, and L is the average distance traversed by an electron inside the cathode cavity before it leaves through the aperture.) For the case where the energy lost by a fast electron is determined only by inelastic collisions with gas molecules, A is approximately equal to the ionization relaxation length A_i, which, according to, is estimated as

A_i = (U_c/U_i)λ_i, (1.2)

where λ_i is the mean free path of the electron between two successive ionization events. For S_a/S_c << 1, the spatial distribution of primary electrons is near-uniform and isotropic. Under these conditions, the S_a/ S_c dependence of L can be obtained assuming the oscillating primary electrons to move with equal probability toward all parts of the cathode surface. It has been shown by the use of expressions for the probability of an electron leaving the cavity and for the average length of a single electron transit that

L = 4VaS_a, (1.3)

where V is the volume of the cathode cavity.

For a hollow-cathode glow discharge, the energy of a primary fast electron in the operating pressure range is determined by the cathode fall potential, which depends on the ratio of the area of the exit aperture to the area of the inner surface of the cathode. The fast electron energy is fully expended in ionization in the cathode cavity only for the case where A. At pressures approaching the lower limiting pressure (~5 x 10^-2 Pa), the electron mean free path at an energy of 300–600 eV is ~2 m, which is 10–100 times greater than the commonly used cathode cavity diameters. Consequently, the loss of primary electrons due to their absorption by the cathode surface does not affect the discharge parameters, whereas electron losses through the exit aperture of the cavity are critical. As the exit aperture of the cathode cavity is reduced, an electrostatic double sheath may form in the region of the exit aperture where the potential jump is localized. The criterion for the formation of this sheath follows from the equality of the discharge cathode current and anode current. The author of assumed the anode, of rather large surface area, to be negatively charged with respect to the plasma. In this case, the potential difference that results in electron reflection vanishes for S_a/S_c [approximately equals] (m_e/M_i)^1/2. As the ratio S_a/S_c is further decreased, the condition for current passage in the discharge can be fulfilled only if a double sheath with a surface area greater than S_a is formed inside the cathode cavity in the region of the exit aperture. The electrons accelerated in the double sheath are focused and, passing through the (small) exit aperture, ensure equality of the current through the aperture to the anode and the cathode current. Thus the condition for the formation of a double sheath in the region of the exit aperture of the cathode cavity takes the form

S_a/S_c < (m_e/M_i)^1/2. (1.4)

Condition (1.4) agrees well with the experimental data for argon reported in, where it is demonstrated that with an optimal ratio S_a/S_c a glow discharge can exist in the high-current (2 mA cm^-2) low-voltage (below 1000 V) form at pressures of up to 0.03 Pa.

Thus a decrease in S_a/S_c has a beneficial effect on the parameters of the hollow-cathode discharge, involving a decrease in operating voltage and in lower limiting pressure (see Fig. 1.3), only to the point determined by inequality (1.4). Further decrease of this ratio leads to the reverse effect because of the electrostatic double sheath formed in the anode region of the discharge.

For the optimal operating conditions of a hollow-cathode discharge, the lifetime of the electrons is sufficient for them to lose almost all their energy in ionization. Nevertheless, with a magnetic field produced in the cathode region, the discharge operating voltage decreases by 100–150 V. This clearly indicates that the addition of a magnetic field to the hollow-cathode configuration leads to enhanced ionization in the plasma. A drop in discharge operating voltage in this case is accompanied by fluctuations of the ion current density to a probe. The frequency of these fluctuations lies in the range 5–50 kHz, increasing with increasing magnetic field. The influence of a magnetic field on the operation of a hollow-cathode discharge may be associated with collective instabilities arising in the plasma. However, this problem calls for further investigations.

In conclusion, it should be noted that, despite the wide use of the hollow-cathode glow discharge in plasma electron sources, the operating pressure of this type of discharge is somewhat higher than the pressure required for stable electron emission from the plasma. Therefore, a reduction of the operating pressure of a hollow-cathode discharge and its operating voltage is still an urgent problem whose solution is critical for the development of plasma-cathode electron sources based on this kind of discharge.

A number of other aspects of the operation of hollow-cathode discharges as applied to their use in plasma electron sources are considered in.

1.2 Discharges in Crossed Electric and Magnetic Fields

Penning- and magnetron-type discharges qualify as glow discharges in crossed E x B fields. These types of discharge are well known and widely employed in various gas discharge devices (ion pumps, gas discharge pressure gauges, ion sources, sputtering systems, etc.). Although the electrode systems of Penning and magnetron discharges are different, the conditions for plasma generation and current passage are so much alike that they can be treated as two kinds of one and the same discharge in a magnetic field. Discharges in crossed fields, because of electron oscillation, are easily established at low and ultralow pressures and may exist in the high-current, low-voltage form in the operating pressure ranges of plasma electron sources, affording the required electron-beam current. It is significant that, in plasma electron emitters based on discharges in crossed E x B fields, no problem arises in matching the cathode and the external magnetic field, which can be used to focus and/or transport the accelerated electron beam.

Simple schematics of the electrode systems of Penning and magnetron discharges are shown in Fig. 1.4a and b, respectively. The electrons accelerated in the cathode fall region are confined by the magnetic field, moving in crossed E x B fields along closed trajectories, reciprocally in a Penning discharge and along a cycloid path in a magnetron discharge. Fast electrons can escape from the discharge system and reach the anode only when almost all their energy is lost in repeated collisions. These conditions provide a high degree of ionization of the working gas up to a gas pressure of 10^-2 Pa, which is somewhat lower than the pressure required for a hollow-cathode glow discharge.

Interest in magnetron-type discharges with cylindrical electrodes (see Fig. 1.4b) stems from the feasibility of a tubular (otherwise termed "annular") electron-beam source. Such an electrode system, if used in an inverted magnetron type of configuration (anode 2 inside cathode 1 and facing electrodes 3 at cathode potential), ensures more efficient electron confinement. Experiments with plasma-cathode electron sources have shown that over the operating pressure range an ignition voltage U_ig =1.5–2.0 kV and a magnetic field B [approximately equals] 0.01 T are sufficient for stable initiation of a discharge in the "inverted magnetron" system. The discharge operating voltage falls within U_d =400–600 V and the current slowly increases with discharge voltage.

(Continues...)

Excerpted from Plasma Cathode Electron Sourcesby Efim Oks Copyright © 2006 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Excerpted by permission of John Wiley & Sons. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Edition details

• Publisher: Wiley-VCH
• Publication date: 2006
• Language: English
• ISBN 10: 3527406344
• ISBN 13: 9783527406340
• Binding: Hardcover
• Edition number: 1
• Number of pages: 181