Getters and getter-ion vacuum pumps
 9783718656684, 371865668X

Table of contents :
Content: Series Preface, Preface, Symbols and Abbreviations, Chapter 1: Basic Characteristics of Electrophysical Pumps, 1.1. Operating Principle and Classification of Electrophysical Pumps, 1.2. Comparative Characteristics of Different Types of Electrophysical Pumps, 1.3. Kinetics of Sorption of Gases by Nonrenewable Getter Films, 1.4. Sorption Characteristics of Titanium and Renewable Titanium Films, Chapter 2: Principles of the Planning and Optimization of the Geometric Structure of Electrophysical Pumps, 2.1. Basic Planning and Optimization of Surface-Action Pumps, 2.2. Mathematical Model and System of Generalized Criteria for Optimizing the Geometric Structure of Electrophysical Pumps, 2.3. Basic Characteristics of Electrophysical Pumps, 2.4. Performance Parameters and Structural Characteristics of Pumps of Various Design, 2.5. Algorithm for Planning and Optimizing the Performance of Electrophysical Pumps, Chapter 3: Evaporative Getter and Getter-Ion Pumps with Thermal Deposition of Getter Films, 3.1. Design and Operating Characteristics of Evaporators, 3.2. Engineering of Evaporation Pumps, 3.3. Evaporation Getter Pumps, 3.4. Evaporation Getter-Ion Pumps, Chapter 4: Electrophysical Pumps with Plasma Sources of Getter Films, 4.1. Physical Features of Plasma Sources of Getter Films, 4.2. Magnetic Control of an Arc Discharge, 4.3. Design and Operating Characteristics of Plasma Sources of Getter Films, 4.4. Getter and Getter-Ion Pumps with Plasma Evaporators, Chapter 5: Sputter-Ion Pumps. Physical Processes, 5.1.Gas Discharges, 5.2. Reflection and Capture of Gaseous Particles, 5.3. Sputtering of the Cathode Plates, 5.4. Special Features of the Pumping of Different Gases, Chapter 6: Sputter-Ion and Combined Pumps. Calculation, Design, and Operation, 6.1. Engineering Calculations and Design of Sputter-Ion Pumps, 6.2. Industrial Sputter-Ion Pumps, 6.3. Combined Getter-Ion Pumps, 6.4. Integrated Vacuum Systems Based on Sputter-Ion Pumps, 6.5. Operation of Sputter-Ion Pumps, Chapter 7: Nonevaporable Getters and Pumping Devices Based on Them, 7.1. Nonevaporable Getters, 7.2. Kinetics of Sorption-Desorption Processes, 7.3. Vacuum-Physical and Operating Features of Nonevaporable Getters, 7.4. Nonevaporable Getter Pumps, Chapter 8: Principles of the Creation of Nontraditional Electrophysical Pumps, 8.1. Implantation of Fast Gaseous Particles in Condensed Media, 8.2. Implantation Pumping Devices, 8.3. Membrane and Catalytic Pumps, 8.4. Barrier Model of the Interaction of Gaseous Particles with Condensed Media, References, Index

Citation preview

Getter and Getter-Ion Vacuum Pumps

THE PHYSICS AND TECHNOLOGY OF PARTICLE AND PHOTON BEAMS (formerly ACCELERATORS AND STORAGE RINGS) A series o f monographs edited by Swapan Chattopadhyay, Lawrence Berkeley Laboratory, California, USA

VOLUME 1

The Microtron, S.P. Kapitza and V.N. Melekhin VOLUME 2

Collective Methods of Acceleration, N.Rostoker and M. Reiser VOLUME 3

Recirculating Electron Accelerators, Roy E. Rand VOLUME 4

Particle Accelerators and their Uses, Waldemar Scharf VOLUME 5

Theory of Resonance Linear Accelerators, I.M. Kapchinskiy VOLUME 6

The Optics of Charged Particle Beams, David C. Carey VOLUME 7

Getter and Getter-Ion Vacuum Pumps, G.L. Saksaganskii

In Preparation; VOLUME 8

Storage Ring Dynamics: A Modern Formalism, E. Forest

This book is part o f a series. The publisher will accept continuation orders which may be cancelled at any time and which provide for automatic billing and shipping of each title in the series upon publication. Please write for details.

Getter and Getter-Ion Vacuum Pumps

Georgii Leonidovich Saksaganskii D. V. Efremov Scientific Research Institute of Electrophysical Apparatus, St. Petersburg, Russia

Translated by P. Shelnitz

Revised and expanded edition of 3neKTpo4111'1earne originally published in Russian in 1988.

BaKyyM111,1e 11acoc1,1

First published 1994 by Harwood Academic Publishers Published 2019 by Routledge 2 Park Square, Milton Park, Abingdon, Oxon OXl 4 4RN 52 Vanderbilt Avenue, New York, NY 10017

Routledge is an imprint ofthe Taylor & Francis Group, an informa business Copyright© 1994 Taylor & Francis.

© 1988 by Energoatomizdat, Moscow, Russia All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. British Library Cataloguing in Publication Data Saksaganskii, G.L. Getter and Getter-Ion Vacuum Pumps. (Physics of Particle & Photon Beams, ISSN 0272-5088; Vol.7) I. Title II. Series

621.55

ISBN 13: 978-3-7186-5668-4 (hbk)

Contents

ix xi

Series Preface Preface Symbols and Abbreviations

XV

Chapter 1: Basic Characteristics of Electrophysical Pumps 1. 1. Operating Principle and Classification of Electrophysical Pumps 1.2. Comparative Characteristics of Different Types of Electrophysical Pumps 1.3. Kinetics of Sorption of Gases by Nonrenewable Getter Films 1.4. Sorption Characteristics o f Titanium and Renewable Titanium Films

1 5 10 18

Chapter 2: Principles o f the Planning and Optimization of the Geometric Structure of Electrophysical Pumps 2 . 1. Basic Planning and Optimization of Surface-Action Pumps

30

2. 2. Mathematical Model and System of Generalized Criteria for Optimizing the Geometric Structure of Electrophysical Pumps

34

2. 3. Basic Characteristics of Electrophysical Pumps

44

2.4. Performance Parameters and Structural Characteristics o f Pumps o f Various Design 2. 5. Algorithm for Planning and Optimizing the Performance of Electrophysical Pumps

52 72

Chapter 3: Evaporative Getter and Getter-Ion Pumps with Thermal Deposition of Getter Films 3 . 1. Design and Operating Characteristics o f Evaporators 3.2. Engineering of Evaporation Pumps v

81 89

vi

CONTENTS

3. 3. Evaporation Getter Pumps

103

3.4. Evaporation Getter-Ion Pumps

116

Chapter 4: Electrophysical Pumps with Plasma Sources of Getter Films 4 . 1. Physical Features of Plasma Sources of Getter Films

128

4. 2. Magnetic Control of an Arc Discharge 4 . 3. Design and Operating Characteristics of Plasma Sources of Getter Films 4. 4. Getter and Getter-Ion Pumps with Plasma Evaporators

137 143 160

Chapter 5: Sputter-Ion Pumps. Physical Processes 5 . 1.Gas Discharges 5.2. Reflection and Capture of Gaseous Particles

177 190

5. 3. Sputtering of the Cathode Plates

194

5.4. Special Features of the Pumping of Different Gases

197

Chapter 6: Sputter-Ion and Combined Pumps. Calculation, Design, and Operation 6 . 1. Engineering Calculations and Design of Sputter-Ion Pumps 6. 2. Industrial Sputter-Ion Pumps

205 215

6. 3. Combined Getter-Ion Pumps 6.4. Integrated Vacuum Systems Based on Sputter-Ion Pumps

231 239

6. 5. Operation of Sputter-Ion Pumps

250

Chapter 7: Nonevaporable Getters and Pumping Devices Based on Them 7 . 1. Nonevaporable Getters

254

7. 2. Kinetics of Sorption-Desorption Processes 7. 3. Vacuum-Physical and Operating Features o f Nonevaporable Getters 7.4. Nonevaporable Getter Pumps

259

Chapter 8: Principles of the Creation o f Nontraditional Electrophysical Pumps 8 . 1. Implantation of Fast Gaseous Particles in Condensed Media

269 286

302

CONTENTS

vii

8.2. Implantation Pumping Devices 8. 3. Membrane and Catalytic Pumps 8.4. Barrier Model o f the Interaction of Gaseous Particles with Condensed Media

317 329

References Index

342 362

335

Series Preface

Vacuum plays one of the most elemental and essential roles in any enterprise dealing with the production, manipulation, storage and control of particle and photon beams. The interaction o f particles and photons in a beam (via various forms of scattering, absorption, emission, adsorption, etc.) with the re­ sidual gas molecules very often determines such fundamental characteristics of a beam as quality, lifetime, stability in propagation and storage. Recently, the role of vacuum has taken on heightened importance with the advent of high brightness synchrotron radiation sources and such high luminosity stor­ age ring colliders as B- and Phi-meson factories, where one pays a steep pre­ mium in photon beam spectral quality and collision luminosity, respectively, with inadequate high vacuum. Accordingly, the physics, principles and tech­ nology o f vacuum and vacuum pumps are more relevant today than ever to practicing beam physicists and engineers. Vacuum physics and engineering is a specialty in its own right. Practi­ tioners o f this art bring a formidable arsenal o f theoretical atomic physics, kinetic gas theory, surface physics and chemistry, diffusive processes, mech­ anical and electrical design and computation to the process of designing ap­ propriate vacuum systems. Dr G.L. Saksaganskii is one such leading practitioner. In this book, he has set down the basic physical, geometric and operational principles and characteristics o f electrophysical pumps, more commonly known in the West as Getter and Getter-Ion pumps, in a way that has never been attempted in the past. In addition to being complete and pro­ viding the reader with the necessary tools for the design, computation and operation of these pumps, this book offers deep insights into the basic func­ tional principles and limitations that would allow practitioners to go beyond the state-of-the-art to novel applications. This book can be read, understood and appreciated by scientists and en­ gineers alike. In it the reader will find a wealth of material on this very special vacuum technology that simply does not exist anywhere else in the lit­ erature. It is a pleasure to present this book to our audience. Let us welcome ix

X

SERIES PREFACE

such a timely and critical contribution to the field of beam physics and tech­ nology. Swapan Chattopadhyay

Preface

This is the second, revised, and greatly expanded edition of a book devoted to high- and ultrahigh-vacuum electrophysical pumps with about a six-year sep­ aration from the first Russian edition. A few words regarding the title o f the book are in order. The first edition was called Electrophysical Vacuum Pumps. The Russian form of this term fully defines the content o f the book and is widely used in the Russian lit­ erature on vacuum technology. Nevertheless, many English-speaking readers may be unfamiliar with this term, and it may even seem excessively strange or foreign. Therefore, the title o f this edition has been changed to Getter and Getter-Ion Vacuum Pumps. However, since the meaning o f the word electro­ physical is clear and can be very useful, it is employed throughout this edition as well in referring to the broader class o f vacuum pumps that includes all the more specific types o f pumps discussed. It should also be kept in mind that the scope of the book is considerably broader than the framework tradition­ ally allocated by the terms in the title in English-language technical literature. Electrophysical pumps include surface-action pumps, in which an electric discharge, resistive or electron-beam heating, a flux of charge particles, or electromagnetic radiation is employed to create and activate the sorbing sur­ faces. The focus on their use is due to the possibility o f obtaining virtually un­ restricted pumping speeds and a spectrum of residual gases that is free of heavy hydrocarbons. Electrophysical pumps are characterized by the utiliza­ tion o f a great variety o f basic principles o f applied physics. Some types are highly economic and capable o f self-regulation and have a considerable ser­ vice life. These virtues predetermined the high caliber and efficiency o f such pumps, as well as their role in solving a number of important technological problems. The wide use o f electrophysical pumps has transformed many areas where vacuum systems are utilized for industrial and technological purposes, as well as scientific instrumentation.

xi

xii

PREFACE

The specific characteristics of the physical and chemical mechanisms underlying electrophysical pumps called for a reexamination of even the very principles of the construction of high- and ultrahigh-vacuum systems. For example, in Russia, integrated vacuum systems were proposed at the begin­ ning o f the sixties for colliders and storage rings and subsequently became widely employed. The electrode systems of the electrophysical pumps are built directly into the vacuum chamber, and they operate in the intrinsic elec­ tromagnetic fields of the machines being pumped. As a result, the effect of the limiting conductance of the chamber is eliminated, and the residual pressure can often be lowered by two to three orders o f magnitude; the cost and size of the vacuum system can also be sharply reduced. Such an effect is generally unattainable by other means and technological procedures. There are many similar examples in other areas where high- and ultrahighvacuum technology is employed. The history of electrophysical pumps goes back to Penning’s work on the physics of gas discharges in a magnetic field. A powerful stimulus for further research in this area was provided by investigations on controlled thermonu­ clear fusion, electronics, and high-energy physics, as well as the development of space technology. The noteworthy milestones in the perfection of these pumps are associated with the names o f Gurevich and Westendorp (1954), Reichrudel (1956), Hall (1958), and Jepsen (1959). Industrial models of elec­ trophysical pumps were created in Russia by a team of experts led by Aca­ demician S.A. Vekshinskii. Different versions of specialized pumps were developed at the I. V. Kurchatov Institute of Atomic Energy (Moscow), the In­ stitute of Nuclear Physics (Novosibirsk), the D.V. Efremov Scientific Re­ search Institute of Electrophysical Apparatus (St. Petersburg), the Khar’kov Institute of Physics and Technology, and several other scientific centers. Electrophysical pumps have been mass-produced and are employed in the manufacture of vacuum tubes, semiconductors, optical instruments, microe­ lectronics, particle accelerators, colliders, thermonuclear facilities, devices for radiation vacuum and space materials science, precision metallurgy, and the manufacture of other industrial and scientific instruments. Fundamental research and investigations in. the area of applied physics have had a strong stimulating influence on the development of new principles and means of oil free pumping, particularly electrophysical pumps. Therefore, new or previously unutilized physical and chemical effects are constantly being integrated into vacuum technology. The largest group of such effects is associated with the interaction of fast atoms, ion and electron beams, plasmas, and electromagnetic radiation with matter in the solid and liquid state. Several new methods for pumping gases, predominantly hydrogen, have been pro­ posed on this basis. None of the ideas advanced has yet gone beyond the stage

PREFACE

xiii

o f laboratory investigations. Some experimental results have not yet been convincingly explained or even contradict one another. Nevertheless, some of the proposed methods (the implantation method, for example) already appear to be promising alternatives to the traditional techniques and methods - at least for solving special problems in vacuum physics. Hopefully, their de­ tailed description in this book will attract the attention of engineers working on the creation o f industrial means for high-vacuum pumping. The list of publications on electrophysical pumps and the pumping sys­ tems based on them includes more than 700 articles in scientific periodicals, volumes of collected works, and proceedings of specialized congresses, con­ ferences, and seminars. The list of patents contains hundreds of items. This book was also written for the purpose of systematizing and generalizing the experience which has been gained in this area. Attention was focused primar­ ily on industrial pumps, the calculation of their vacuum parameters, optimiza­ tion methods, and operating characteristics. Some emphasis was also placed on problems directly related to my own professional interests as a participant in the complex development of high- and ultrahigh-vacuum systems for par­ ticle accelerators, colliders, thermonuclear facilities, and reactors. Since ex­ perimental physics and vacuum technology have traditionally been interrelated and have constantly been enriching one another, such emphasis seems to be perfectly natural. The bibliography presented in the book by no means exhausts the abun­ dance o f scientific information characterizing the present stage of the devel­ opment and utilization of electrophysical pumps. It includes only monographs and articles which comprehensively cover certain topics, as well as a few publications of a historical nature. Such an approach forced me to refrain from standard reference to primary sources, which the reader can find by referring to the reviews presented. Only the publications devoted to the new means o f pumping, which have not yet become widely known, viz. nonevaporable getters and implantation, membrane, and catalytic pumps, are repre­ sented more completely (Chapters 7 and 8). I thank all the experts who have supplied information or gave advice re­ garding the content of the book. The contributions of V.V. Anashin, E.I. Kontor, A.I. Livshits, the kind interest of B.N. Yablokov, whose critical remarks greatly helped to shape the scientific and literary style of the book, and the support o f Academician V.A. Glukhikh are especially significant. I also highly value my discussions with L.S. Gurevich, B.D. Ershov, D. A. Karpov, V . V. Nazarov, V.V. Ryabov, D . V. Serebrennikov, S.I. Ukolov, and L.V. Fi­ lippova for the many years of creative collaboration on the development of electrophysical pumps for accelerators and thermonuclear facilities. The inter­ est of Mr M. Hablanian, Professor J. O ’Hanlon, and experts from CERN, as

Xiv

PREFACE

well as the invaluable help of Ms L.V. Belokosova was a decisive factor in the republication o f the book in English. Thanks also to O.S. Rastoropova who greatly assisted in preparing the manu­ script for press. I would welcome and sincerely appreciate any critical remarks and sugges­ tions from readers. G. L. Saksaganskii

SYMBOLS

B D D ,d

E E

e F G

Go I H

M m N

No n P

Po P

Pp Pop

Q 3, W(z) «

In the case of bulk sorption, we may restrict ourselves to the first term in the series in (1.14a). Then

(U 6) where h = S0p /C qD exp (—AE/R0T) and a tan (a5) = h. The numerical solution of the equations obtained and a comparison of the results with the experimental data show that the adsorption—diffusion model just described gives a completely adequate interpretation of the kinetics of the sorption of hydrogen and other gases by getter films.*

♦Here it would be appropriate to mention a fact which is beyond the scope of the gettering of gases proper and is of more general practical interest. As can be seen from Eqs. (1.3a)-(1.3c), the kinetics of the gas pressure in a chamber, the coverage of a surface with an adsorbate, the thermal desorption fluxes from the walls, and other gas-kinetic characteristics reflect the entire set of adsorption—diffusion processes occurring in a vacuum system. They are sensitive to some degree to any change in the temperature, the state of the surface, and other external parameters. After rapid changes in the parameters, a new equilibrium state is established with a finite rate, which may be lower than the rate of the changes themselves. For example, depending

18

G. L. SAKSAGANSKII

A fter similar treatment of the existing experimental information for different gas—getter-film systems, it is possible to determine the diffusion and sticking coefficients, the solubility, and other constants, as well as their temperature and pressure dependence. The coefficient p0 is determined from the initial pumping speed. The slope of the parabolic portion of the kinetic curve permits calculation of the argument of the function V in the relation (1. 15) or the exponent in (1.16); the time limits of the linear portion of the curve give another combination of constants. The condition for a transition from surface to bulk sorption (1.17) makes it possible to determine the diffusion coefficient. An analysis of this type reveals, in particular the weak dependence of the sticking coefficient on the pressure due to the large number of adsorbed states with different binding energies. This dependence can be adequately approximated by the function Po ~ pnt where the value of n for different gas—metal pairs varies from —0. 25 to —0. 33. The gas-diffusion constants obtained and formulas for the rate of sorption are given in Table 1.8. We shall now study the characteristics of the sorption of different gases in the example case of titanium getter films and solid samples of titanium. 1.4. Sorption Characteristics of Titanium and Renewable Titanium Films The getter properties of metals depend strongly on the temperature and the method employed to form the sorbing films. We shall first study the sorption characteristics of a solid sample of titanium and preformed titanium films at medium and high temperatures (see also Tables 1.8-1.10 and Figs. 1.2, 1.4-1.7). Hydrogen. Strips 40 pm thick are characterized by the surface sorption of hydrogen at a temperature of 473 K; at 7 = 573 K the sorption process already has a clearly expressed bulk character. The temperature dependence of the sticking coefficient is nonmonotonic: below 400 K p0 increases as the temp­ erature is lowered. The width of the linear portion of the kinetic sorption curve along the time scale is about 60 sec; the critical coverage with the adsorbate r y < 2.5 X 10~3.

on the direction of evolution of the parameters in the transition state, the chamber wall can be either a vast source of gas or a fairly large sink for gas. This fact should not be ignored in vacuum measurements under pulsed regimes and in the analysis of fast gas-dynamic and electrophysical processes, especially at ultralow pressures.

T Pa

Ti-CO 650-1200

Ti-H2

450-1100

Characteristic

TABLE 1.8 Gas-diffusion Characteristics of Titanium

GETTER A N D GETTER-ION V A C U U M PUMPS 19

1 8 9 3 /5 exp (5450/r)

5.64/T03 exp (-6400/7)

1.3 X 10^/T03 exp (-1000/r)

C0, m3*Pa/m3

4 X 10“* exp (-19, 250/T)

1.8 X 10"6 exp (-6200/r)

1400

30,096

0.68/?07 exp (16,500/T)

\333q0 exp [~a2D(t -

Ti-CO

13.79 / 2 exp (615/r>

QJQr. em; minima of r r, r r. Similarity between the spatial distribution functions of the fluxes of the gas being pumped and the fluxes (concentration) of the active sites. System of generalized criteria (see § 2.2)

Systems containing sources of gas, radiant fluxes, fluxes of active sites, and sorbing surfaces

Layout; relative orientation of sources of molecular and radiant fluxes and fluxes of active sites; their spatial distribution; geometric propor­ tions

Minima of 50p» F^, Qr cm, Q r. em* Qr. abs* Qr. in

per unit of effective work of the system

The independent or regulatable optimization factors include the spatial distribution of the gas fluxes in the vacuum chamber being evacuated, the geometric structure of the electrophysical pump and the vacuum system as a whole, the rate of formation and spatial distribution of the active sites, and the properties of the sorbing surfaces. As is seen from Table 2. 2, some of the characterizing criteria require combined consideration of the molecular and radiative transfer in the electrophysical pumps being designed and the vacuum systems based on them. To illustrate the arguments just advanced, we shall describe the details of one of the criteria listed in Table 2.2 as applied to a getter pump. In an optimal­ ly designed pump the capture coefficient should have the maximum possible value for a particular gas and a particular sorbing surface. In addition, the degree of saturation of the getter layer should be the same in any region of the sorbing

34

G. L. SAKSAGANSKII

surface. When this condition is not fulfilled, local saturation of the getter layer will result in a decrease in the capture coefficient. Therefore, only a getter pump whose sorbing surfaces have the same degree of saturation at every point and at any moment during their service life may be regarded as a pump which satisfies the optimization criteria. The criterion formulated for a small region of a sorbing surface around a point f may be expressed quantitatively by the simple relation ( 2. 1)

where qcr is the critical degree of saturation of the sorbing surface, whose achievem ent is follow ed by a decrease in p. Relation (2 . 1) reflects the requirement formulated in Table 2.2 of similarity between the spatial distribution functions of the fluxes of impinging molecules and the fluxes of active sites as a criterion for optimizing the geometric structure of electrophysical pumps. Different modifications of electrophysical pumps will be analyzed from this point of view in § 2.4. Let us now proceed to the construction of a generalized mathematical model of an electrophysical pump. 2.2. Mathematical Model and System of Generalized Criteria for Optimizing the Geometric Structure of Electrophysical Pumps In addition to the characteristics of the sorbing surfaces t](r, f), fl(r, 0 .? as(^> 0* Nas(r, f), the flux densities of molecules incident to the sorbing surfaces qin(r t f), and the flux densities of molecules sorbed by them , i. e ., the spatial distribution of the molecular flux densities in the chamber being evacuated, to construct a model. This makes it possible to describe the rarefied gas outside o f the pump. T aking into account the differences between the mathematical descriptions of the processes of sorption of a gas in an electrophysical pump with continuous renewal of the getter film and in pumps w ith preformed sorbing surfaces, we shall perform the follow ing treatment separately for each of these types of electrophysical pumps.

Pumps with Continuous Renewal of the Getter Film. This category includes evaporation pumps operating with continuous evaporation (sublimation) of the getter and SIP’s. The active sites in them are sputtered atoms of the getter which have been deposited on the sorbing surfaces. With consideration of the notation introduced in § 1.4, it is convenient to write (2. 2)

GETTER A N D GETTER-ION VACUUM PUMPS

35

We shall define an optimal pump as one in which the flux of active sites is maintained at the minimal level necessary for sorption of the incident gas flow in any region of the sorbing surfaces. The quantitatively formulated condition is written in the form (2.3) Relation (2.3) may, of course, not hold in the case of a real pump. Under the condition «(r)^in(r) > , Kq, A.as(r), and \ x(r). The latter are usually assigned or can easily be determined (see § 2.4). Function (2. 5), which relates the capture coefficient T to the distribution of the molecular flux densities in the chamber being evacuated, the properties of the gas being pumped, the characteristics of the source of active sites, and the geometric structure of the pump, is the mathematical model of an electrophysical pump sought. Let us dwell in greater detail on its components.

36

G. L. SAKSAGANSKII

The function \ in(r), which depends on O, Kq, q&s0, \ x(r), and A.as(r ), describes the distribution of the relative flux density of molecules impinging on the sorbing surfaces. The function \* (r ) describes the field of parameters of the interaction of the gas flows with the surfaces. When the temperature of the sorbing surface is uniform, \*(r) is usually equal to unity. The function \ as(r) describes the distribution of the flux density of active sites, which are sputtered getter atoms in the present case, and is determined by the type and operating conditions of their source. The parameter k0 characterizes the conditions for sorption of the gas in the zeroth zone of the sorbing surfaces. The requirements with respect to the intensity of the source of active sites for this zone (A0 = 1 ) follow directly from condition (2.3): ( 2 . 6)

We shall henceforth assume that condition (2.6) is unconditionally fulfilled. The function A (r) reflects the conditions of sorption of the gas on the sorbing surfaces compared with the optimal conditions. Therefore, the function defined by it (2.7) may be regarded as the local (for a small region around the point r) criterion of the similarity of a real pump to an optimal pump. For the latter we have ( 2 .8)

We shall now introduce several additional functions, which describe the details of the mathematical model of an electrophysical pump. The function (2.9)

which we shall always calculate with the use of the smaller of the two possible values of the integrand, gives a numerical characteristic of the structural perfection of a pump as a whole and may, therefore, be taken as the integral criterion of the similarity of a real pump to an optimal pump. The function (2 . 10)

numerically characterizes a pump as a whole from the point of view of the extent to which the flux of active sites is utilized and may, therefore, be regarded as the integral coefficient of similarity of a real pump to an optimal pump with respect

GETTER A N D GETTER-ION VACUUM PUMPS

37

to the criterion for minimizing the flux of active sites (the energy expenditures). The function (2 . 11)

serves as a numerical characteristic of a pump from the point of view of the achievement of the maximum possible capture coefficient and may, therefore, be regarded as the integral coefficient of similarity of a real pump to an optimal pump with respect to the criterion of the extent of utilization of the sorbing surfaces. In calculating the functions Acn and AM, we shall restrict ourselves to the value of the integrand which is equal to or less than unity. Knowledge of the distribution of the sorbed gas on the sorbing surfaces l/x(r)]: (2. 13) In the former case, substituting (2.12) into (2.4), we obtain (2. 14) where

Here, as before, the optimization conditions are (2.15) We shall next devise a series of coefficients and similarity criteria, which are equivalent in meaning to those previously considered, but have a different argument. Here we have

Thus, when condition (2 . 12) is fulfilled, both systems for the mathematical description of electrophysical pumps are identical.

38

G. L. SAKSAGANSKII

In the latter case, it is impossible, in principle, to quantitatively evaluate a pump in the sense of requirement (2. 3), since there is no unique relationship between qabs(f) and qin(r) under these conditions. One more universal optimization criterion, viz., the integral throughput

coefficient of an electrophysical pump with continuous renewal of the getter film , will be introduced and analyzed in detail in § 2.5. Pum ps with Preform ed Sorbing Surfaces. This category includes adsorption, chemisorption, and implantation electrophysical pumps, as well evaporation pumps, operating under the conditions of periodic renewal of the getter films. A mechanism for the self-regulation of the flow of neutralized active sites operates in all these pumps. In contrast to the pumps with continuous renewal of the getter film, these pumps always satisfy the condition qin(r) ^ ^ s(0 - If is convenient to assume that (2. 16) where p(f, t) is the probability of the irreversible sorption of a gaseous molecule at a certain point on a sorbing surface. The flux density of neutralized active sites is (2.17) An optimal pump is one in which the relative change in the surface or bulk concentration of active sites as a result of the sorption of molecules is identical in all regions of the sorbing surfaces. When this condition is not fulfilled, some regions will be saturated more rapidly than others, and the capture coefficient will consequently begin to decrease from its optimal theoretical value. The quantitatively formulated condition may be written in the form (2.18) where e(f) is a function of time, which is identical for all points on the sorbing surfaces. Before proceeding to an analysis of Eq. ( 2 . 18), we shall make one stipulation. Pumps with preformed sorbing surfaces generally operate under nonstationary conditions, since the parameters of the elementary interaction acts between a molecule and a sorbing surface (the coefficient p in the present case) and thus the spatial distribution of the molecular fluxes in the cavity of a pump are subject to some regulating factor. Therefore, a set of time-dependent characteristics, rather than a single characteristic, is needed for the rigorous description of a pump. The comparative characteristics of pumps with different geometries, however, scarcely depend on the absolute values of their integral characteristics. This allows us to restrict ourselves to consideration of only the

GETTER A N D GETTER-ION VACUUM PUMPS

39

initial state of an electrophysical pump and to reduce the problem to the stationary case. W ith consideration of the stipulation made, condition (2 . 18) may be rewritten in the form (2. 19) where

and c is a constant. Of course, some violation of relation (2.19) may be observed in a real pump, implying a decrease in the capture coefficient in comparison to the maximum possible value. T herefore, in each region of the sorbing surfaces of an electrophysical pump, the quantity ( 2 . 20)

may take any value from zero to infinity, and the degree to which relation (2.19) is satisfied may be regarded as a measure of the structural perfection of a pump in the sense of condition (2.18). The function (2 .21)

may be regarded as a mathematical model of a pump. Let us dwell on the features of this model. The function \p(r) defines the field of physical parameters of the interaction of gaseous molecules with sorbing surfaces. In the case of implantation pumps with a uniform distribution of the temperature on the sorbing surfaces, X.p(r) is usually equal to unity. In the case of evaporation and chemisorption pumps with significant nonuniformity of the thickness of the getter film, especially when the temperature of the getter is elevated, it is possible that K^(r) # 1. The function \ as(r) describes the distribution of the relative surface or bulk concentration of the active sites. The parameter \ 0 characterizes the conditions for the sorption of gaseous molecules in a fixed zone. It directly specifies the requirements placed on the concentration of active sites in this zone as a function of the flux density of the incident gas at an assigned rate of lowering of the sorption capacity and, conversely, permits the determination of this rate, if the concentration of the active sites is assigned. The function A(r) reflects the degree of closeness of the kinetic character­ istics of the sorption of gaseous molecules at an arbitrary point on the sorbing

40

G. L. SAKSAGANSKII

surfaces to the analogous characteristics in a fixed zone. Therefore, the function defined by it (2 .22)

may be regarded as the local criterion of the similarity of a real pump to an optimal pump. For the latter we have (2.23) We shall now introduce several derived functions. The function (2.24)

characterizes the structural perfection of a pump as a whole from the point of view of requirement (2. 18) and may, therefore, be interpreted as the integral criterion of the similarity of a real pump to an optimal pump. The function (2.25)

has a double meaning. First, it characterizes the nonuniformity of the saturation of the sorbing surfaces. Second, in the case of pumps whose sorbing surfaces are held at a temperature differing from the temperature of the housing, it maintains the physical meaning of the integral coefficient of similarity of a real pump to an optimal pump with respect to the criterion for minimizing the energy expenditures. In such electrophysical pumps the presence of “excess” (i. e., saturated less rapidly than the fixed zone) cooled (cryosorption pumps) or heated (implantation and chemisorption electrophysical pumps) zones results in the increased sorption or emission of radiant fluxes. The function (2.26)

has physical meaning as the integral coefficient of similarity of a real pump to an optimal pump with respect to the criterion of utilization of the sorbing surfaces : the rapid saturation of the zones with A (r) > 1 is equivalent to a decrease in their effective area in comparison to the nominal value. We shall now introduce a series of similarity coefficients and criteria with the use of the function qabs(r). In all electrophysical pumps of the type under

GETTER A N D GETTER-ION VACUUM PUMPS

41

consideration (2.27) Substituting (2.27) into (2.19), we obtain (2.28) where

Here the optimization conditions are (2.29) We can next devise a series of similarity coefficients and criteria, which are equivalent in meaning to those previously considered, but differ from them with respect to the argument. When A.p(r) = 1: A.in(r) = \ abs(f); A(r) = A'(r)'. A*(r) = A*'(r); A* = A*'; A^ = A'cn; A* = A '.. A universal optimization criterion, viz., the integral throughput coefficient of an electro physical pump with preformed sorbing surfaces♦ A ', will be introduced and analyzed in detail in § 2.5. The use of the similarity criteria and coefficients found makes it possible to determine specific methods to perfect the structural geometry of electrophysical pumps being developed and to improve the technical and economic characteristics of vacuum systems as a whole. The principles and criteria described are equally valid for surface-action pumps of other types, particularly condensation and cryosorption pumps. For example, a combined examination of the relationships T = f\(l/d9 p), A en = / 2 ^ /d , P), and A ss = / 3(//d, P)** for pumps with preformed sorbing surfaces makes it possible to select the value of l /d on a firm basis and to determine the utility of a transition to pumps of the slot-shaped and honeycomb types. Variations of the zone of a pump with a continuously renewed getter film in which the condition A0 = 1 is fulfilled make it possible to optimize the pump with respect to the capture coefficient or the integral similarity coefficients. The requirements placed on the design of the evaporator, which must provide for the mandatory law of spatial distribution of the fluxes of active sites, follow

♦The same criterion is, of course, valid for pumps with periodically renewed sorbing surfaces operating under a regime with pauses between successive formation cycles of the getter films. ♦♦Here / and d are the characteristic longitudinal and transverse dimensions of the pump.

42

G. L. SAKSAGANSKII

directly from condition (2.3). Condition (2. 19), in turn, uniquely specifies the required law of variation of the thickness of the getter layer along the length of the pump. As will be shown in § 2.5, multifactor optimization with respect to the criteria A and A' makes is possible to design an electrophysical pump with a maximum throughput. Examples illustrating the arguments just advanced will be presented in § 2.4 and § 2.5 and in the following chapter. Let us now turn to the construction of a system of criteria which can be used to optimize the geometric structure of electrophysical pumps and take into account the thermophysical processes accompanying pumping. For this purpose, we introduce the following concepts and notation: qTin(f) and 0

h

- 0 , 25

- = 0 , 1

-

- = 0 ,5

a

*0 — = 0 , 75

G e o m e tric p ro p o rtio n s

A g«ani

* = am

o

A goom

A gcom

A„

A jjj,,

A* = A m

W Po

A» ~

A* = A „

A. = A ^

a

Am -

o A* = A „

A* Am

C h a racteristics

0, 21

0 , 23

0, 26

0 , 20

0 , 22

0 , 24

1, 00

0 , 35

1, 00

0 , 19

10*

1, 00

0 , 20

0 , 20

1,00

0 , 19

1 , 1 ■ 10"4

0 , 20

3 ,8 -

0 , 21

1, * • 10"3

0, 42

3, 9 - 10~3

0 , 18

0 , 18

0 , 19

0 , 31

0 , 17

0 , 17

0 , 18

0 , 29

0 , 16

0 , 16

0 , 17

0 , 27

0, 99

0, 98

0, 98

0 , 46

0, 99

0, 53

0, 49

0 , 30 0 , 30

0 , 34 0 , 34

0 , 39 0 , 39

0, 45 0, 46

0, 52

0, 58 0 , 60

0 , 60 0, 62 1, 00

1,0

0,8

0,6

0.4

0,2

0,1

P= 0,05

TABLE 2.4 (continued)

GETTER A N D GETTER-ION V A C U U M PUMPS 69

XII

XI

f

= ,.

d0 ~d

d, - - 0 , 1

to ~d~

h

~d~

0 , 23

A* = A „

0 ,4 0

A*

0, 21

0 , 22

0, 93 0, 57

0, 56

0, 51

0, 5 0

0, 5 0

A*

0, 94

0, 49

0 , 32

0, 94 0, 54

0 , 74 0 , 29 1,00 0 , 96

0, 61

0, 92

0, 52

0, 67

5 , 9 • 10"2

0, 5 4 0 , 34

0 , 19

A« A » — hfeoa

0 , 75 0 , 19 1, 00 0, 96

3, 5 • 10*2

0, 55

0 , 36

0 , 35 0 , 38

0 , 76 0 , 17 1, 00 0, 96

1, 0 - 1 0 '2

0 , 19

0 ,2 0

2 , 0 • 10'3 0, 5 6

0 , 36

r

Aon A u — Ageocn

0 , 20

r

A pon

A.

0 , 21

0 , 21 0 , 21

r

Quo/Vo

Apom

A.

A* = AW1

r

TABLE 2.4 (continued)

0, 9 0 0 , 65

0 , 78 0, 57

0 , 89 0 , 70

0 , 59

0 , 85

0, 93 0, 24

0 , 17

0 , 85 0 , 26

0, 96

0 , 87

0, 96 0 , 16

0 , 18

0 , 88 0 , 17

0 , 88 0 , 77

0, 88 0, 65

0, 97 0, 21

0, 99 0 , 16

1, 00 0 , 15

70 G. L. SAKSAGANSKII

XIV

X III

D esign schem e

/

I T =uo

~d~ ~ 1,0

d0 T = 0,2

^0

do T = 1,0

~d

d\

G eo m etric p ro p o rtio n s

1, 00 0, 44

0 , 75

0 , 69

0 , 65

0, 63

A„ A*g _

0, 96

0, 97

0, 97

0, 98

0, 51

0, 51 1, 00

0, 46 1, 00 0 , 46

0, 54 0 , 34

0, 91 0 , 72 0, 97 0 , 76

0, 85 0, 66 0, 98 0, 68 0, 99 0, 59

0 , 65 0 , 76 0, 87 0 , 89 0, 93 0, 83

0 , 86

0, 96 0 , 80 0 , 94

0, 90

0, 96 0, 91

0, 98

0 , 66

0, 81 0 , 64

1,0

0 , 75

0 , 73 0 , 58

2 , 0 • 1 0 '2

0, 73 0 , 73

0, 67

0 , 64

0, 61

0 , 71 0, 98

0 , 78 0 , 70

0,8

0, 85 0 , 76

0, 87

10'2

0, 74

0 , 76

0, 68

0 , 71

0,6

0 , 80

0, 62 0, 62

0, 54

1,2

0 , 76 0 , 74

0, 45 0, 57

0 , 34

A* Ag^gg

0,4

3,6 ■ 10"2

0,2

0 , 20

0 , 70

0, 20 0, 44

r

A„ - A ^

Aeo

A*

r

0 , 76

0, 69

0 , 29 0, 54

0,1

0 , 77

0, 53

A*

A„ A „ ~ Agoom

0 , 17

P = 0,05

r

C ha racteristics

TABLE 2.4 (continued)

GETTER A N D GETTER-ION V A C U U M PUMPS 71

72

G. L. SAKSAGANSKII

for a point source with a uniform angular distribution of the emitted flux, (2.41b) These equations correspond to placement of the source on the continuation of the axis of the pump at a distance / from its inlet cross section; d is the diameter of the inlet cross section. TABLE 2.5. Integral Characteristics of the Geometric Structure of Pumps with a Renewable Getter Layer under an Evaporation Regime D esign schem e IX

G e o m etric p ro p o rtio n s

l/d

r

A*

A ,

A„

Ago«n

0 , 98

0, 52

0, 52

1, 00

1, 00

d\/d _ o,7 d j d ’ ~5J

0 , 78

0, 59

0, 90

0, 68

0, 68

d\/d o,9 d jd “ T o”

0 , 78

0, 60

0 , 74

0, 92

0, 92

= 1,0

z 0/ d = 0 , 3

II

c>

XII

XIII

Ud = 1,0 d0ld = 0, 2

0 , 86

0 , 67

0 , 89

0 , 85

0 , 85

XIV

l/d = 1,0 d0/d= 0 , 2

0 , 82

0, 58

0, 99

0, 59

0, 59

The data presented specify the kinetics of mass transfer in a vacuum system, the evolution of the sticking coefficients, and other parameters that are needed to design an electrophysical pump and to predict its operating characteristics. Examples of their use will be given in § 2.5 and in the following chapters. 2. 5. Algorithm for Planning and Optimizing the Performance of Electrophysical Pumps The arguments advanced in the preceding sections form the foundation of an algorithm for designing an optimal electrophysical pump. At the same time, this algorithm illustrates the fruitfulness and practical possibilities of the methods used in a detailed analysis of the molecular fluxes in complex vacuum systems. It should be stressed that an iterative approach is unavoidable in designing an optimal electrophysical pump.

GETTER A N D GETTER-ION VACUUM PUMPS

73

A key step in the designing process is the determination of the spatial distribution of the fluxes of molecules and the fluxes of active sites in the cavity of the pump being designed. This distribution can be analyzed by the method of angular coefficients already cited or other methods, viz., the Monte Carlo method, the integral kinetic method, and the method of equivalent surfaces. Here we shall use the method of angular coefficients. This method is preferable for the analysis of simple structures with a number of surfaces no greater than five (electrophysical pumps usually fall into this category): it is the most graphic method, and its use is based on tabulated characteristics of the geometric structure of the pump and does not require cumbersome calculations. When more complex structures, for example, geometrically branched systems based on electrophysical pumping devices, are calculated, the transition to other methods becomes necessary. The electrophysical pump being designed is regarded as a set of several surfaces with assigned properties. In the simplest case it consists of a cylindrical vessel with a partially shielded inlet cross section and a closed end surface and of a source of active sites. A uniform molecular flux with a density q0 passes from the chamber into the inlet cross section of the pump with an area Fop. We shall also assume that the flux of active sites emitted by the surface of the source FeVap is also uniform and has a density p0. Nonuniformity of the distribution of the fluxes of molecules and the fluxes of the active sites does not alter the essential points of the method being described. Only the calculation procedure is complicated. All the points belonging to the inlet cross section will be indicated by the subscript 1, and the points on the surface of the evaporator will be indicated by the subscript evap. The distribution of the fluxes of molecules and the fluxes of active sites in the cavity of a pump is essentially nonuniform. Therefore, using the local characteristics of the geometric structure, we can represent the flux density of molecules impinging on an elementary area dF(rt) around the point rt in the form (2.42) where q(rk) is the “intrinsic”* flux of molecules emitted by the area around the point r k9 and Q[dF(rf)9 dF (rk)] is the coefficient of total molecular exchange *In the present context the "intrinsic” flux includes all the kinds of fluxes of molecules emitted by a surface which are not associated with the fluxes of molecules impinging on it from other surfaces. In other words, it is the primary flux of molecules from the surface. The fluxes due to thermal, ion-stimulated, electron-stimulated, and photostimulated desorption, the fluxes of gaseous molecules injected into the vacuum from the surface under consideration, etc. are classified as “intrinsic.” In the case of the inlet

74

G. L. SAKSAGANSKII

between the areas dF(rk) and dF(rf), i. e., the relative number of molecules out of the total number emitted by the area dF(rk) which impinge on the area dF(rt) directly or after repeated reflection from other surfaces (the term resolving angular coefficients is also used for the coefficients 0 ). To simplify the expres­ sion for the coefficient of total molecular exchange between the areas dF(rk) and dF(r,), we shall henceforth write it in the form $>(rh rk). The distribution of the molecular fluxes over all the surfaces of a pump is determined by solving a system of integral equations of form (2.42). The exact calculations of the coefficients O is a cumbersome computational operation involving the determination of the sum of a slowly converging functional series. Some simple approximate methods, which will be considered in § 3.2, have been developed for calculating them in practical engineering work. For the time being, we shall only note that they are calculated from a system of algebraic equations containing the coefficients of direct molecular exchange?* . Thus, changes in the dimensions of a pump are reflected in the characteristics of its geometric structure only through the coefficients of direct molecular exchange i. e., the throughput of the pump, can be increased proportionally. This provides some basis to interpret the function A # as a universal parameter characterizing the perfection of the geometric structure of a pump, if it is regarded as the integral throughput coefficient of an electrophysical pump with preformed sorbing surfaces. Therefore, the optimization of a pump reduces to the multiparametric numerical analysis of the function A' and selection of the geometric proportions which correspond to its maximization. This limit, unlike the extremum in the

80

G. L. SAKSAGANSKII

previously discussed situation for electrophysical pumps with the continuous deposition of active sites, gives the absolute maximum of the integral charac­ teristics. The results of the present step include the determination of the layout and principal dimensions of the pump in a second approximation and the calculation of refined characteristics of its geometric structure on their basis, and a comparison of the latter with those adopted in step 2. In the case of considerable disparity, the iteration process must be repeated to find a third approximation. 6. Refined nominal values of the capture coefficient r n0p [see (2.47)] and the pumping speed Sn0p [see (2.48)] are calculated on the basis of the results of step 5. 7. The final parameters of the engineering design are selected, and the principal kinetic and operating characteristics of the pump are calculated. The values of the critical parameters cCTand G0cr for different gases may not be identical; they also vary as a result of the evolution of the thermophysical conditions and the transition to the pumping of multicomponent gas mixtures. Therefore, a multivariant calculation of electrophysical pumps followed by selec­ tion of the engineering solutions which meet the requirements of the most severe conditions is required. In § 3.2 the algorithm outlined here will be described in detail on the level necessary for the performance of technical engineering-design calculations.

Chapter 3

EVAPORATIVE GETTER AND GETTER-ION PUMPS WITH THERMAL DEPOSITION OF GETTER FILMS

3. 1. Design and Operating Characteristics of Evaporators Among the variety of physical designs that have been created, pumps with thermal deposition of getter films comprise the most numerous class of modem electrophysical pumping devices. Among the several dozen versions developed, only a comparatively small percentage are produced on an industrial scale. The required rate of evaporation of the getter depends on the gas load and the required degree of evacuation. It can be varied from the maximum possible value to practically zero by decreasing the power supplied and thus the temperature of the getter (Table 3.1 and Fig. 3.1). TABLE 3. 1 Averaged Characteristics of Industrial Evaporators of Various Types (with a titanium getter)

Type of evaporator

Solid-phase: resistive with direct heating resistive with indirect heating with electron-beam heating Liquid-phase electron-beam

Specific evaporation rate g/(sec*cm2)

Energy efficiency, 10-7 g/(sec'W)

Getter utilization factor

0-10-5

1-2

0. 3-0.7

0-2 X 10-5

0. 5-1.5

0. 6-0.7

0-2 X 10-5

0. 3-1.5

0.6-0.9

0-10-3

1-3

0. 8-0.9

Reduced-power conditions, however, are energetically unfavorable. The flux o f radiant energy from the surface makes up the bulk of the energy expended. The intensity of this flux per unit of evaporation rate decreases

81

82

G. L. SAKSAGANSKII

sharply as the temperature of the getter is increased. Therefore, the energy efficiency of evaporators, i.e., the rate of evaporation of the getter per unit of power introduced increases along the following series: solid-phase resistive evaporators (Tw « 1500 K) -» liquid-phase evaporators with electron-beam heating (7W« 2100 K) —* arc evaporators (Tw « 5 X 103) —> laser evaporators (7W« 104 K). The working temperatures indicated are for titanium. In pumps with a regulatable film-deposition rate, a periodic regime is preferable: the evaporator is switched on for a short time at the highest temperature. The duration of the evaporation period depends on the area and temperature of the surface being coated (the film-deposition surface), the evaporation rate, and the kind of gas being pumped. The duration of the pause is limited by saturation of the getter layer.

Fig. 3.1. Specific evaporation rate of metallic getters as a function of the temperature.

The transition to a periodic evaporation regime is usually advisable at pressures below 10-4 Pa. For example, a regime with a single evaporation period lasting several minutes over the course of 20-30 days is typical of electron storage rings and other ultrahigh-vacuum systems which operate without the injection of gas. The gas load in such systems consists only of the fluxes due to thermal and stimulated desorption. The evaporation rate of a getter is influenced not only by the temperature, but also by the composition and molecular concentration of the gases being evacuated. At 1400-1600 K titanium actively sorbs nitrogen, and a stable film of nitrides, which lowers the evaporation rate, forms on its surface. Similar processes also take place when an evaporator operates in a medium of oxygen and carbon monoxide. These processes restrict the permissible pressure of the active gases during the operation of evaporators to a value of 10“2 Pa. In inert gases evaporators can be switched on at far higher pressures.

GETTER A N D GETTER-ION VACUUM PUMPS

83

The main functional element of GPs and GIFs with thermal deposition of the getter films is the evaporator. Modern pumps employ both sublimation evaporators, in which the temperature of the getter being sputtered is below the melting point, and liquid-phase evaporators. The evaporator determines the maximum throughput of a pump and its service life. The most important parameters of evaporators include: the rate of evaporation of the getter and the range in which it can be regulated; the maximum permissible pressure at which the evaporator can be switched on; the energy efficiency; the getter utilization factor, which is equal to the ratio between the mass of the getter evaporated at the end of the service life of the evaporator and its initial mass (see Table 3.1). One of the most important vacuum characteristics of a getter is the degree to which it is saturated with dissolved gases (Tables 1.6 and 3.2). It appears that not all technologies make it possible to obtain titanium with compositions that are acceptable according to this criterion. Titanium iodide contains the smallest amount of impurities. TABLE 3.2 Mean Content of Gaseous Impurities (102 m3-Pa/kg) in Industrial Titanium Obtained by Different Methods Preparation method, type of titanium Calcium hydride: PTK IMP-1A Thermal magnesium: VT-1-1 VT-1-0 VT-1-00 Ribbon rolled from a powder Secondary refining Iodide

Hydrogen

Nitrogen Oxygen

46.6 —

0. 7 0. 8

2 .1

1.6 1.3 1.1 0. 4

0. 5 0. 4 0.4 5 X 10-2

1.2 1.0 0.8 0. 5

1.3

9 X 10“3 1.3 X 10-2 5 X 10-2 8 X10-2

Solid-phase resistive evaporators have become wisely used in modern industrial pumps. Pumps of the GIN and NIB types (Russian) employ a directly heated evaporator consisting of a bimetallic wire: a layer of titanium with a thickness 6 = 0.5do is deposited on a molybdenum core with a diameter d0 by the iodide method. The wires produced have diameters d0 = 0. 3, 0.5, 1.0, 1.5, and 2. 0 mm, and the deviation of the external diameter from the nominal value may reach 20%. The working current in such evaporators may be as high as

84

G. L. SAKSAGANSKII

200 A, and the highest operational rate of evaporation of the getter is achieved when the power supplied is about 18 W/cm2. A number of companies produce resistive wire evaporators made from titanium—nickel and titanium—molybdenum alloys. Such evaporators are more uniform and, therefore, less prone to overheat or melt than are the bimetallic evaporators which have a high-melting core and a titanium coating or a titanium wire wound on it; the titanium utilization factor reaches 0. 7. They can be switched on at pressures up to 10 Pa. The Perkin-Elmer evaporator consists of four spirals made from an 85% Ti + 15% Mo alloy, which are wound from wire with a diameter of 2. 1 mm and are placed on a flange with current leads. The length of the wire is 219 mm, its mass is 3.4-3.6 g, 1.5 g of which is the mass of the sputterable titanium, and the working current is 55 A at a voltage of 8 V. The evaporators of similar design produced by Balzers have a larger supply of sputterable getter (up to 3 g). Their length is 313-430 mm, the number of spirals on the flange is three, the maximum evaporation rate is 6 X 10“ 2 mg/sec, and the working current is 30-50 A at a voltage of 3-6 V. The wire evaporators from Leybold and other companies have similar characteristics. The use of wire evaporators made from a 94% Ti + 6% Pd alloy is promising. The catalytic activity of palladium, especially during the evacuation of hydrogen, makes it possible to lower the maximum permissible residual pressure of the gases being evacuated. The directly heated resistive evaporators have the simplest designs and power-supply schemes; however, they have low utilization factors and short service life. The indirectly heated resistive evaporators have somewhat better operating characteristics. A hollow cylindrical or spherical shell made from a getter material or a bimetal is heated by the radiant flux emitted by a wire heating elem ent in the cavity. A typical example of this type is the Ti-BALL™ evaporator produced by Varian. This evaporator has the form of a titanium sphere with a diameter of 32 mm, whose cavity contains a heating element. Wire holders fixed to a threaded bushing are soldered to the sphere, and current leads pass within the bushing. The bushing has detachable electrical contacts and a threaded mechanical coupling with a tubular stand, which is fastened to the flange with the current leads. When the evaporator ceases to function properly, the flange is dismantled, and the bushing with the evaporator is replaced. The initial mass of the titanium sphere is 50 g, 35 g of which make up the mass of the evaporable getter; the total mass of the bushing and the evaporator is 0.57 kg. The evaporation rate is determined by the power supplied and the heating regime; it can be regulated in the range from 3 X 10~3 to 0 . 15 m g/sec, which corresponds to a range of service life from 3500 to 70 h

GETTER A N D GETTER-ION VACUUM PUMPS

85

(Figs. 3.2 and 3.3). The maximum working pressure depends on the rate of evaporation of the getter |x and is equal to 0.3 Pa (p, = 0 . 15 mg/sec), 3 Pa (p. = = 0.06 mg/sec), and 7 Pa (p. = 0. 03 mg/sec) for a standard gaseous medium. In an argon medium the evaporator can be operated at pressures up to 10 Pa.

Fig. 3. 2. Evaporation rate of titanium as a function of the operating time of the Ti-BALL™ evaporator at various powers. Fig. 3.3. Evaporation rate of titanium in the Ti-BALL™ evaporator as a function of the power supplied for two heating regimes, viz., a cyclic regime with variation of the power from 200 to 750 W (curve 1) and a continuous regime (curve 2).

The power supply, control unit, and timer assign the working cycle and provide for regulation of the rate and total time of evaporation. The latter can be set in the range from 3 to 12 min, and the cycle duration can be set in the range from 15 min to 24 h. A power of 200 W is supplied to the evaporator during each pause. Varian also produces a similar evaporator of smaller size with a usable supply of titanium equal to 15 g; it is called the MINI-Ti-BALL™ evaporator. In this model the evaporator proper has a hemispherical shape and a diameter of 21 mm. The main characteristics of this evaporator are as follows: a total length o f 229 mm, a maximum power consumption of 380 W, a maximum working current of 50 A, a mass of the evaporator as a whole equal to 0.91 kg, including the mass of the replaceable part, which is equal to 0. 45 kg. The evaporator is interchangeable with the directly heated resistive evaporators from Varian and can be used with the diode and triode SIP’s produced by this company. The rate of evaporation of the getter in solid-phase evaporators is usually controlled by varying the power supplied or the working current in accordance with plots of the type presented in Figs. 3.3 and 3.4. However, the dimensions of the evaporator proper and the properties of the getter vary to a fairly appreciable extent during operation: the diameter of directly heated wire evaporators decreases strongly and unevenly, the original shape and structure of the surface of indirectly heated resistive evaporators are destroyed, and the

86

G. L. SAKSAGANSKII

thermophysical characteristics change as a result of the deposition of the getter films and saturation of the getter with the gases being pumped. This results in violation of the optimal thermal regimes and calls for correction of the power supplied, especially toward the end of the service life of an evaporator.

Fig. 3.4. Rate of evaporation of titanium in a model TSC-20 directly heated resistive evaporator as a function of the working current and the power supplied to the heating element. The supply of titanium in the evaporator is 20 g, and the evaporation rate under recommended conditions is 0. 01 mg/sec. Fig. 3.5. Temperature dependence of the current density due to thermionic emission / (1) and of the specific rate of evaporation of titanium p0 (2) for a titaniummolybdenum wire.

Several methods are used to continuously monitor the thermal regime of an evaporator. The best results are provided by direct measurements of the rate of evaporation (or deposition) of the getter atoms. The ionization or quartz meters used for this purpose, however, are comparatively cumbersome and have low sensitivities. Indirect methods based on thermionic emission are more convenient. One of the methods is based on the similarity of the plots of the dependence of the evaporation rate and the thermionic current of a heated body on the temperature (Fig. 3.5). The evaporation rate can be stabilized over the course of a long period of time by controlling the thermionic current, and the getter utilization factor can thereby be increased. To measure the thermionic current, a grid-like collector is placed around the evaporator, and a positive potential of 1-1.5 kV is supplied to it in order to carry off all of the thermionic current from the evaporator. The employment of this method in industrial wire evaporators ensures the stable operation of pumps with a getter utilization factor as high

GETTER A N D GETTER-ION VACUUM PUMPS

87

as 0. 6; the fluctuations in the evaporation rate do not exceed 20% of the nominal value. Better reproducibility is provided by the method based on the phenomenon of amplification of a thermionic current in activated bimetallic cathodes; the activating role is played by the film being deposited. Such cathodes satisfy the relations (3.1) (3.2) Here / w_Ti an& /w are the current densities of the thermionic emission from a tungsten surface covered with a layer of adsorbed titanium atoms and from the surface of a clean tungsten filament; A, B, and C are constants; T is the temperature of tungsten corresponding to the equilibrium concentration of adsorbed titanium atoms at the flux density of the atoms being deposited qTi. A quasi-linear correlation between the gain of the thermionic current k and the flux density of the titanium atoms being deposited qTi can be achieved by selecting a temperature T at which the equilibrium coverage of the substrate 6 would satisfy the requirement BqTi < 1. Thus, the simplest meter is a diode with a tungsten cathode that is accessible to the atoms evaporated. However, the accuracy of such a meter is low due to its instability with respect to the temperature. A differential measurement system has considerably better characteristics. In this case, along with the principal measuring diode, there is a second similar diode, which, however, is completely shielded from the titanium vapor; the cathodes in both diodes are identical and form a series electrical circuit. The difference between the thermionic currents of the measuring and shielded diodes or the ratio between these currents can serve as a measure of the flux density of the atoms being deposited. The plots in Fig. 3.6 illustrate the results obtained on an experimental model of a meter at different values of the thermionic current density of the shielded diode / w. The dashed line shows the course along which this current density must vary to ensure constancy of the factor by which qTi is multiplied; the curve was constructed for the arbitrarily selected relationship k — 1 = 1.7. It is seen that the linearity of the plot of k = /(^ Ti) is maintained over a broad range of values of qTi. The pressure does not influence the metrological characteristics of the meter in the range of values from 1(T5 to 1CT1 Pa investigated; an assessment of its sluggishness is given by the time delay of 0.5-5 sec, which decreases as qTi is increased. The cathodes have a comparatively low temperature (1700-2100 K), which ensures their long life.

88

G. L. SAKSAGANSKII

Fig. 3.6. Gain of the thermionic current at different values of its density as a function of the flux density of the titanium atoms impinging on the cathode.

The best energy efficien cy characteristics and the maximum getter utilization factor are observed for electron-beam evaporators. The first pumps based on them (the Russian model STON titanium cryosorption pump and the AVTO-20M ultrahigh-vacuum pumping unit based on it) employed a liquid-phase electron-beam evaporator, in which the required service life is ensured by periodically supplying titanium wire to the melting zone. The evaporator has the form of an electron gun with a directly heated tungsten cathode and a titanium anode, which serves as the target and is placed in a transverse magnetic field. The accelerating voltage is equal to several kilovolts. A magnetic field formed with the aid of an electromagnet turns the electron beam through an angle of almost 270°. This makes it possible to keep the cathode and the electrode system of the electron gun completely outside of the zone where the titanium is deposited and to thereby increase their service life and reliability. A titanium wire with a diameter of 1.5 mm is wound onto the drum of a feeding mechanism; the amount stored is sufficient for the operation of the evaporator over the course of several thousand hours. The evaporator is mounted on a flange in a horizontally oriented tube at the axis of the pump on the lid of the heated housing. To protect the chamber being evacuated from being coated by the getter, the inlet cross section of the pump is shielded by a disk placed directly over the evaporator. The titanium condenses on the walls of a double-walled cylindrical insert, which is coaxial to the housing; liquid nitrogen is supplied to the cavity within the insert from a container. When the evaporation rate of titanium is 0. 1 mg/sec, the pumping speed is equal to 30 (H2) and 10 (N2, 0 2) m3/sec, respectively. The ultimate pressure achieved by the pump after high-temperature outgassing is less than 10~9 Pa; the power consumption is 1.7 kW, and the flow rate of liquid nitrogen when the evaporator is switched on is about 4 dm3/h . The AVTO-20M pumping unit based on the STON sorption pump includes an oil-vapor diffusion pump with a nitrogen trap

GETTER A N D GETTER-ION VACUUM PUMPS

89

for pumping out inert gases; the pumping speed for argon is 90 dm3/sec. The dependence of the pumping speed of the pumping unit on the pressure of the gases being pumped is characterized by a broad plateau; the drop in the pumping speed begins only at pressures equal to 1.3 X 10“5 Pa (H2) to 1.3 X 10-4 Pa (N2, 0 2, air). The models of electron-beam evaporators from Perkin—Elmer employ rod-shaped titanium anodes. Their end surfaces are heated by a focused electron beam em itted by a stationary glowing cathode. As the end surfaces are evaporated, the rods are displaced with the aid of a screw mechanism, so that the relative positions of the anode and cathode remain unchanged. This ensures e ffe c tiv e focusing of the electron beam and maintenance of the required evaporation rate with relatively low power consumption. At the maximum evaporation rates of titanium equal to 0 . 14 mg/sec (for the EBS model) and 0.3 m g /se c (fo r the BTS m odel), the evaporators consume 0 . 5 and 2 kW , respectively; the usable supply of titanium is equal to 40 and 500 g, respectively. The BTS evaporator is intended for pumps with pumping speeds up to 102 m3/sec. Its maximum diameter is 203 mm, and its total length is 800 mm, including about 400 mm in the vacuum. Solid-phase evaporators with heating by a scattered electron beam will be described in § 3.4. 3.2. Engineering of Evaporation Pumps The procedure described here gives the details of the algorithm for designing electrophysical pumps presented in § 2. 5 as applied to evaporation pumps. We shall assume that the type of evaporator, the layout, and a certain range of geometric proportions (dimensions) have already been selected for the pump being designed with consideration of the operating conditions and structural considerations and on the basis of preliminary quantitative evaluations in the preceding steps of the designing process. The purpose of the calculation is to refine these relationships and to determine the principal characteristics of the pump, viz., the pumping speed and its kinetic effects, the ultimate pressure that can be achieved, the optimal temporal regime and service life of the evaporator, etc. During the calculation it is necessary to determine the spatial distribution of the fluxes of gaseous molecules, the fluxes of getter atoms, the coefficients of direct and total molecular exchange, and a number of other characteristics of the geometric structure of the pump being designed. Most of the characteristics sought have continuous distributions over the surfaces of the pump; therefore, a

90

G. L. SAKSAGANSKII

mathematically flawless approach to such calculations must be based on a fairly cumbersome solution of a system of integral or integrodifferential equations. The cost of the results attained under a rigorous approach is excessively high from the practical point of view. To avoid the computational difficulties, we shall replace the continuous distribution of fluxes over the surfaces of the pump by a discrete distribution. We shall then separate these surfaces into zones, and within each zone we shall assume that the distributions of the fluxes of molecules and the fluxes of getter atoms are uniform. This permits drastic simplification of the problem and the transition from integrodifferential equations to algebraic equations. The surfaces are divided into zones in such a manner that the requirement for uniform distributions would, in fact, be fulfilled in a certain approximation. This can be done very simply for the layouts presented in Tables 2.3 and 2.4: each zone should correspond to a region within which \ in(r) « const and \ M(f) « const. It can be done for other layouts on the basis of heuristic arguments, and the division determined can be refined in subsequent stages of the calculation. The number of zones should be sufficiently high at the sites of sharp alteration of the geometric structure, i. e., sites of constriction, expansion, and turns; the accuracy of the calculations increases as the number of zones is increased. The tendency to increase the accuracy by increasing the number of zones should, however, be kept within reason. The number of equations in the system to be solved increases as the square of the number of zones, while the error increases only in inverse proportion to it in a first approximation. To maintain consistency with the symbols adopted in § 2.5 in all the equa­ tions and expressions written below, we shall indicate that a site on a surface belongs to the i-th zone by means of the symbol rf. Distribution of Flux Densities. The molecular flux densities sought are determined by solving a system of equations of the form (3.3) where qin(rt) is the flux density incident to the i-th zone, q(rk) is the intrinsic molecular flux emitted by the it-th zone, n is the total number of zones, and rk) is the local coefficient of total molecular exchange between zones rt and r k with consideration of the possible repeated reflections of the molecules on other (/-th) surfaces. The local coefficients were calculated from a system of equations of the form

GETTER A N D GETTER-ION VACUUM PUMPS

91

(3.4) where