Particle Accelerators: A Brief History M. Stanley Livingston Associate Director, National Accelerator Laboratory Harvard University Press : Cambridge, Massachusetts : 1969 Copyright © 1969 by the President and Fellows of Harvard College All rights reserved Distributed in Great Britain by Oxford University Press, London Library of Congress Catalog Card Number 69-1K03H Printed in the United States of America Preface Particle accelerators are among the most useful tools for re- search in nuclear physics and in high-energy particle physics. The rapid growth of these research fields has been due, in large measure, to the development of a sequence of electronu- clear machines for acceleration of ions and electrons. The high- intensity and well-controlled beams from these machines can be used to disintegrate nuclei, produce new unstable isotopes, and investigate the properties of the nuclear force. Modern high-energy accelerators can produce excited states of the ele- mentary particles of matter, forming new unstable particles with mass values much higher than those of the stable parti- cles. Fundamental questions can be asked of nature, and an- swered by experiments with these very high-energy particles. The field of high-energy particle physics is on the threshold of a significant breakthrough in our understanding of the parti- cles of nature and the origins of the nuclear force. Energies achieved with accelerators have increased at an al- most exponential rate during the past 35 years. The field has been characterized by a sequence of new concepts or inven- tions, each leading to a new machine capable of still higher energy, and each stimulating the development and construc- tion of a new generation of accelerators. At times, the new de- velopments came so fast that it was difficult to determine which laboratory or machine held the current energy record. A chronology of the major new steps in the development is in- cluded as an appendix, which lists the new concepts, first op- erations of new types of accelerators, and new energy records. Also included in the Appendix is a graph of the growth of en- ergies achieved by accelerators during the years 1930 to 1968, with extrapolations into the future. iii iv] Preface This monograph is largely derived from manuscripts of talks and lectures delivered at Harvard University. Four chap- ters were presented as Morris Loeb Lectures at the Harvard Physics Department from April 8 to 18, 1968. One chapter is a talk given at the Physics Department Colloquium on Decem- ber 11, 1967. And the final chapter is based on a National Ac- celerator Laboratory Report, NAL-12-0100, dated June 18, 1968. These chapters represent facets of accelerator history as ex- perienced and observed by the author. The monograph is not intended as a complete history of all accelerator development, and several significant types are only briefly mentioned, al- though they have played major roles. Rather, this is a limited compilation of those phases of accelerator development with which I have had close associations. It is to be hoped that ex- perts in other accelerator categories will also tell their stories of the origins and development of these machines. For a more detailed description of the technical develop- ment of accelerators the reader is referred to M. S. Livingston and J. P. Blewett, Particle Accelerators (McGraw-Hill, New York, 1962). A more complete report on the history of acceler- ator development can be found in M. S. Livingston, Ed., The Development of High-energy Accelerators (Classics of Science Series, G. Holton, Gen. Ed.; Dover, New York, 1966). M. Stanley Livingston Oak Brook, Illinois June 1, 1968 Contents 1 The Race for High Voltage 1 2 Ernest Lawrence and the Cyclotron—An Anecdotal Account 22 3 Synchronous Accelerators and How They Grew 39 4 The Story of Alternating-Gradient Focusing 60 5 Origins of the Cambridge Electron Accelerator 76 6 The 200-GeV Accelerator 88 Appendix: Chronology of Development of Particle Accelerators 109 References 113 Index 119 [1] The Race for High Voltage When J. D. Cockcroft and E. T. S. Walton first disintegrated lithium nuclei with accelerated protons of 500 kilovolts en- ergy, at the Cavendish Laboratory of Cambridge University in 1932, they opened a new era in science. This event may well be considered the origin of modern nuclear physics. It can also be taken as the starting point in accelerator history. A decade earlier in the same Laboratory, Ernest Rutherford had disintegrated nitrogen nuclei for the first time, using the naturally occurring alpha particles from radioactive elements which had energies of about 5 million electron volts (MeV). During this decade much had been learned to confirm the nu- clear character of atoms and to determine the magnitudes of nuclear binding energies. In a speech before the Royal Society in 1927 Rutherford 1 expressed his hope that charged particles would some day be accelerated to energies exceeding those of radioactive radiations, capable of disintegrating nuclei. Scien- tists in several other laboratories held similar views of the need for high-speed particles and of their value as probes for study- ing atomic nuclei. Despite the fact that the goal of several mil- lion volts seemed well out of reach of known techniques for producing high voltages, by 1929 work had started in several laboratories in several countries to develop the electrical ma- chines needed. This competition soon took on the aspects of a race for high voltage, and for the devices needed to accelerate particles to energies sufficient to disintegrate nuclei. The ini- tial goal was one million volts (1MV). HIGH-VOLTAGE MACHINES During the next few years many of the known techniques for producing high voltage were studied to see if they could be ex- 1 2] Particle Accelerators: A Brief History tended to even higher potentials. "Surge generators" had been developed for testing electrical equipment at very high volt- ages. These consisted of stacks of capacitors which were charged in parallel from a dc potential supply and then dis- charged through cross-connected spark gaps to develop surges of high voltage of a few microseconds duration. The highest- voltage surge generator was built in the General Electric Co.2 plant at Pittsfield, Mass., in about 1932; it was capable of pro- ducing voltage surges of over 6 MV, and was used for break- down tests of electrical insulators and other equipment. In 1930 A. Brasch and F. Lange 3 in Germany applied 2.4- MV voltage pulses from such a surge generator to a crude vac- uum chamber made of alternate rings of metal and fiber tightly compressed between end plates. The peak surge cur- rent was of the order of 1000 amperes; the discharge tube prac- tically exploded on each surge and had to be cleaned and reas- sembled frequently. A metal-foil window at the grounded end allowed a beam of high-energy electrons and gas ions to emerge into the air, where it produced an intense blue glow extending outward as far as 1 meter. Presumably this dis- charge was attended by some nuclear disintegrations, but they were not identified. An even more extreme approach was an attempt to utilize the high potentials developed in the atmosphere during elec- trical storms. In 1932 C. Urban and others stretched an in- sulated cable across a valley between two peaks in the Alps. From this cable a spherical terminal was suspended. During thunderstorms high potentials would develop between this terminal and the valley floor; sparks several hundred feet long were observed. Plans had been made to install a discharge tube for the acceleration of particles but were abandoned when Dr. Urban was killed by lightning. Another engineering test installation was developed by R. W. Sorensen4 at the California Institute of Technology in the early 1920's, for the Southern California Edison Company, 3] The Race for High Voltage which was using 220-kilovolt lines at 50-cycle frequency for long-distance transmission of electric power. In this technique three 250-kV transformers were arranged in series and mounted on insulating platforms; an "exciter" winding at the high-potential end of the secondary of one transformer was used to supply the primary winding of the next transformer. With this arrangement the rms potential of the high-voltage terminal of the third transformer was 750 kV above ground and the peak voltage exceeded 1 MV. The installation was used for years for the study of high-voltage breakdown of elec- trical devices. In about 1928 this system was taken over by C. C. Lauritsen 5 and his associates of the California Institute of Technology, to be used as a voltage source for acceleration of particles. They first developed X-ray tubes operating at poten- tials up to 750 kV, using a single large porcelain insulator for the vacuum chamber. By 1934 they developed a positive-ion accelerating tube with potentials up to 1 MV and started a program of nuclear research. Eventually, the cascade trans- former was replaced by a belt-charged electrostatic generator, which was found to be more suitable for nuclear experiments. The Tesla coil is a resonance transformer which can gener- ate oscillatory pulses of high potential. In its usual form the primary circuit has large capacitance and low inductance, and is excited by the discharge of a series spark gap; the secondary is wound of many turns with high inductance and low distrib- uted capacitance, with the same resonant frequency as the pri- mary. In the early 1930's a group at the Department of Ter restial Magnetism of the Carnegie Institution of Washington, consisting of M. A. Tuve, G. Breit, O. Dahl, and L. R. Haf stad,6 attempted to develop the Tesla coil as a voltage source for positive-ion acceleration, with the secondary coil immersed in oil for insulation. They reported peak potentials of up to 3 MV. They also developed discharge tubes in which the ap- plied potential was divided between multiple tubular elec- trodes; this became an accepted technique in the future. How- 4] Particle Accelerators: A Brief History ever, the oscillatory character of the potential obtained from the Tesla coil made it unsuitable for particle acceleration. The Carnegie group abandoned it in 1932 in favor of the belt- charged electrostatic generator developed by Van de Graaff. Several modifications of the resonance transformer were de- veloped by others. In 1933 D. H. Sloan 7 at the University of California built a radiofrequency resonance device operating at 6 megacycles per second or 6 megahertz (MHz). It was used as an electron accelerator to generate X-rays of up to 1.25 MV; an installation of this type built by M. S. Livingston and M. Chaffee gave many years of service at the University of Califor- nia Hospital in San Francisco. Another type, resonant at 60 Hz, was developed by Ε. E. Charlton 8 and associates at the Schenectady laboratory of the General Electric Company in 1934, which was also used as an X-ray generator at potentials up to 1 MV. All the devices described above have the limitation of pro- ducing either pulsed or alternating potentials, and this is their basic fault as ion accelerators. Several of them have been rea- sonably successful as electron accelerators for the production of X-rays, where the stability requirements are not so severe and the X-ray tube acts as a rectifier; but they have all failed as sources of positive ions. The successful techniques that have survived in the compe- tition are those which develop steady direct voltages and which can be regulated to maintain constant voltage with good precision. A system that has succeeded in this respect is the common direct-current power supply, consisting of an alternating-current transformer, a rectifier, and a filter circuit to smooth out the voltage ripple. Although the transformer- rectifier circuit had been available for many years, the voltages to which it had been developed (less than 100 kV) were far too low to be useful in nuclear physics. However, a variation known as the voltage-doubling or voltage-multiplying circuit can produce considerably higher potentials; it had been de-