4—
Double Play
In the fall of 1930 Lawrence found what he needed to make accelerators: two graduate students, very capable, hardworking, and dependent on his good opinion. One, David Sloan, we have already met; the other, M. Stanley Livingston, who came to the Physics Department with an M.A. from Dartmouth in the fall of 1929, sought a subject for a doctoral thesis. Both had financial support: Sloan his Coffin Foundation fellowship, Livingston a teaching assistantship. Livingston took up Edlefsen's problematic cyclotron; Sloan, Wideröe's proven, but useless, ion accelerator. Their working during 1930/31 demonstrated the practicability not only of both devices, but also of Lawrence's as yet tentative method of organizing work: simultaneous development of complementary instruments or parts by graduate students or postdocs, each of whom had responsibility and considerable independence on his own project. They were inspired to outstanding work by a spirit of cooperative competitiveness and by the bittersweet satisfaction of laboring at the edge of technology. "It is rather remarkable," Lawrence wrote one of his old professors, "how physics is attracting the best much as engineering used to do."[74]
The straight way proved the quicker. Sloan and Lawrence used mercury ions in a tube with eight electrodes driven by a 75-watt oscillator, with whose robust personality Lawrence had become acquainted during his summer at GE. They immediately got the expected eightfold amplification, 90-keV ions with the oscillator operated at 11 kV, some five times its rating. They extended the tube with thirteen more electrodes and got ions of 100 keV, then of 200 keV, exactly what they expected, with less than 10 kV on the oscillator (fig. 2.18). They reported this performance to the National Academy of Sciences in December 1930 and recommended their installation for its simplicity and economy. "An entirely practicable laboratory arrangement," viz., an evacuated tube 2.3 meters long containing electrodes run at 25 keV would give mercury ions of 125 kV.[75] To what purpose? Lawrence and Sloan
suggested studies of the properties of the high-speed rays, but did not stop to undertake them themselves. Instead, Sloan built a bigger tube, with thirty electrodes (plate 2.7). By the end of May 1931 he had passed the great artificial barrier and could boast of 0.01 µA of mercury ions with energies exceeding a million volts.[76]
Fig. 2.18
Sloan's linac. Mercury ions from the source at the bottom
accelerate through the canal ray tube and then through the 21
accelerating tubes to the collector at the top. Lawrence and
Sloan, NAS, Proc., 17 (1931), 68–9.
Although the long linear accelerator had only a short life at Berkeley during the 1930s, it contributed much more to the Laboratory than the inspiring achievement of a million volts. It gave experience in working with a powerful commercial oscillator that could put as much as 90 kV on an electrode; in the definitive version of the 30-electrode tube, Sloan got ions of 1.26 MeV, or an average gain of 42 kV per electrode. It also forced attention on the problem of the synchronization and focusing of the beam. Synchronization—keeping the beam in step with the rf field at the first short electrodes—was resolved by trial-and-error adjustments, which gave important information about the fates of ions that do not enter a gap when the peak voltage spans it. Focusing turned out to be easy: it was another case where nature favors the particle accelerator. Figure 2.19 shows the lines of force between neighboring cylindrical electrodes. Besides suffering acceleration along the tube, an ion is driven toward the axis of the accelerating system during transit of the first half of a gap and away from it during transit of the second. If it enters the gap at the optimum time, near the peak voltage, it will speed up and spend less time in the second than in the first half of the gap. The net result of the minuet is motion toward the axis: the openings of the electrodes act as lenses, which inhibit ions from leaving the accelerating system.[77]
The discovery and exploitation of automatic focusing opened the possibility for ever longer linear resonant accelerators. Sloan started on a new tube, with thirty-six electrodes, each able to hold 80 kV, to be driven by two oscillators working at a wavelength of 27 m. He expected to have mercury ions at 4.5 MeV. If successful, he would go to the next mystical threshold, 10 MeV. That would require an accelerating system forty feet long fed by eight power oscillators.[78] Such a system would no longer be the convenient laboratory accelerator advertised by Sloan and Lawrence, but a big project in radio engineering. Sloan did not proceed immediately to quicker quicksilver. By November 1931 he had begun to construct an x-ray tube of novel design, which was to
Fig. 2.19
Electrostatic focusing across the dee gap. AB is the median plane; the curves are the
lines of electric force at potential difference 2 V between the dees; the errant hydrogen
ion traverses the path indicated back toward AB. Lawrence and Livingston,
PR, 40 (1932), 29.
play an important part in the technique and financing of the Laboratory.[79] Sloan finished the 36-electrode tube with the help of a graduate student, Wesley Coates, in 1932. It gave mercury ions with energies up to 2.85 MeV. Coates loosed these ions on various targets and discovered that soft x rays left the collision sites.[80] A similar investigation by another graduate student, Leo Linford, disclosed that each impacting mercury ion drove out around ten electrons, a result not uninteresting to designers of high-voltage vacuum tubes.[81]
While Sloan pushed Wideröe's technique to a million volts, Livingston struggled to reproduce the resonance experienced by Edlefsen. The earliest experiments recorded in Livingston's notebook took place on September 20, 1930. He had built a cylinder of metal 4 inches (10 cm) in diameter, about the size of Edlefsen's instrument, to serve as vacuum chamber; and he had access to a small magnet, capable of a maximum of 5,500 gauss, to control the spiralling ions. During October and November he found only feeble indications of resonance, and they came at values of the field H that depended upon the potential V of the oscillator. That violated the sound doctrine of equation 2.1. Livingston decided that his predecessor's success had been an illusion of faith.[82]
The obstacles Livingston had to overcome may best be indicated by following the path of ions that succeed in completing the spiral course. Obstacle 1. The source must give off an ion current iS strong enough that the survivors will constitute a detectable current iD at the collector (fig. 2.20). Livingston assumed that fewer than one in a thousand ions would run the course; to get an iD of a few microamps he would need an i S of a few milliamps. He did not know how to produce milliamps of protons. Lawrence wrote to the Forschungsinstitut of the Allgemeine Elektrizitätsgesellschaft, which he was told had perfected a proton source, but insufficient information returned and Livingston had to work with hydrogen molecule ions 
. These he generated in the center of the chamber by firing electrons from a radio-tube filament into the hydrogen gas that filled the chamber.[83]
Obstacle 2. The pressure P within the chamber must be small enough to make collisions between ions infrequent and large enough to accumulate an adequate i S . In practice a vacuum pump worked continuously, maintaining a pressure around 10–5 mm Hg,
Fig. 2.20
Livingston's cyclotron. Livingston, Production (his doctoral thesis), fig. 3.
at which the average distance between collisions equalled the length of the spiral course.[84]
Obstacle 3. The grid across the entrance to the dee, which Lawrence and Livingston wrongly thought necessary for shielding, must not soak up a sensible fraction of the beam; the final arrangement used parallel slits, which opposed less metal to the ions than a rectangular screen.[85]
Obstacle 4. An electrostatic force D must be established between a special plate and the dee at the periphery of the spiral to deflect the beam (if any) into the collector. Since the value of the deflecting force measures the energy of the ions, it is
important that the passage into the cup be so restricted that only ions in a narrow range of energies can be focused on it by D ; on the other hand, the passage must be wide enough to admit an iD capable of registering itself, and, ultimately, of inducing nuclear transformations.
On December 1, 1930 Livingston recorded his first success: with a newly designed detector, he had killed the dependence of H on V . "At last we seem to be getting the correct effect."[86] He then tried what values of oscillator frequency f , dee potential V , and pressure P gave the sharpest and strongest rise in iD as he brought the electromagnet through the value H = 2pfme / c calculated for resonance. With f = 2.5 ·106 and V = 300, he got a rise around H = 3300, where theory placed it. If the rise did record the presence of resonantly accelerated 
ions, they each had an energy W of 6 keV:
which, for and R = 4.8 cm, the radial distance to the entrance to the collector, is 9.6·10–9 erg or 6,000 eV. These ions had accelerated in twenty steps over a spiral path of ten complete turns.[87] That was most encouraging if true. Over the Christmas holidays Livingston borrowed a magnet capable of almost 13 kilogauss (kG), over twice the limit of 5.5 kG with which he had been working. On January 2, 1931, he got good resonance at 12.4 kG, V = 1,800 volts, for a calculated energy W of 70 keV attained in forty-six steps. With the magnet at its maximum, 12.7 kG, W = 80 kV, Livingston detected resonance at this setting with a little less than 1,000 volts on the dee, indicating eighty-two crossings of the dee gap, or forty-one entire turns. That was enough for a thesis, if not for disintegration.[88]
Although Livingston marshalled his evidence very well, it did not amount to a demonstration. The resonances came sharply at the predicted places, to be sure; but i D showed other bumps too, which often exceeded the resonance peak. Figure 2.21 gives a typical best case; but as appears from figures 2.22 and 2.23, the curve of iD against H depended sensitively on P and D , and on V as well. In the drawings D and B are resonance peaks, the former for ions that travel through a complete circle in one period of the rf field, the latter for ions that go through a half turn in a period and a half. Livingston explained A and C as consequences of background ionization, photoeffects, partially accelerated ions, scattering, and so on, and supported his special pleading by arguments that need not be repeated. What was needed was a clean measurement of W ; but to obtain the value of the deflecting potential D (= W ) that made iD a maximum required a collimation so tight
Fig. 2.21
Livingston's thesis results, 1: the ordinate is iD , the abscissa the magnetic field
H . Livingston, Production , fig. 5.
Fig. 2.22
Livingston's thesis results, 2: iD against H at several pressures in the
"vacuum" chamber. Livingston, Production , fig. 8.
Fig. 2.23
Livingston's thesis results, 3: iD against H for various values of the deflecting
potential D. Livingston, Production , fig. 11.
that the beam entering the collector became too weak to measure.[89]
The point about inferring rather than measuring the energy of the particles bothered Lawrence. "We can make them spin around alright," he wrote Swann in January, "but we have not been able to determine how many times and therefore what speeds we have been able to produce."[90] Although by April 1931, when it was time to declare results, the difficulty had not been overcome, Lawrence and Livingston had no doubt that they had demonstrated the way to high energy for light ions. Their declarations differ subtly, however, in accordance with their places, personalities, and ambitions. Livingston, concluding his thesis: "There appear to be no fundamental difficulties in the way of obtaining particles with energies of the order of magnitude of one million volt-electrons." Lawrence, concluding a talk to the American Physical Society: "There are no difficulties in producing one million volt ions in this manner."[91]
A test was already being implemented. In a move typical of his later practice, Lawrence had started on the next bigger machine as soon as experiments with the one in hand gave the first indications of success. On January 9, a week after Livingston's ions had appeared to reach 80 keV, Lawrence wrote around for advice about building a magnet with the specifications he and Edlefsen had given: R = 10 cm, H max = 15 kG. He received useful information from Kenneth Bainbridge, then at the Bartol Research Foundation, who had just built a similar magnet to practice mass-spectroscopy in Aston's manner. According to Bainbridge, materials would cost around $700.[92] Lawrence turned to the University for money, an advance, he called it, on (indeed almost over) his next year's research allowance. The University's Board of Research, still headed by Armin Leuschner, recommended the advance, which the new president of the University, Robert
Gordon Sproul, approved, setting a pattern for the early development of the Laboratory. Both had recently had the opportunity to take Lawrence's measure. The preceding semester, with the advice and prompting of G.N. Lewis, they had defeated an attempt by Northwestern University to lure him away. Lawrence had emerged from the negotiations with a full professorship, at a high salary and a low age, and with easy access to the chief financial and administrative officer of the University.[93]
Another pattern began then too: the estimated budget fell far below the true cost. Lawrence acquired the additional $500 he needed from another familiar source, the National Research Council, from which he had had a grant-in-aid for his photoelectric studies.[94] His second cyclotron still served the express purpose of bringing physicists to high energy at low cost: no unusual funding was required. Manufacture of the magnet was entrusted to Federal Telegraph, which brought it to the Physics Department on July 3, 1931, just after Lawrence had returned from the symposium on the production of high-energy particles at the American Physical Society meeting in Pasadena, where he heard Tuve describe the acceleration of protons and ions to a million volts by a Tesla coil. Tuve had depreciated that great barrier as a "moderate" voltage. Lawrence went back to Berkeley hoping to set protons spinning in less than two weeks.[95]
He did. He wrote on July 17: "The proton experiment has succeeded much beyond our fondest hopes!!! We are producing 900,000 volt protons in tremendous quantities—as much as 10–8 ampere. The method works beautifully." The published announcement followed familiar lines. "With quite ordinary laboratory facilities proton beams having great enough energies [to effect disintegration] can readily be produced." All it takes is a magnet with pole pieces nine inches in diameter, a maximum field
of 15 kG, and a few accessories, like the 500-watt short-wave power oscillator Lawrence borrowed from Federal. One gets ions of over 500 keV, an amplification of at least 100, and full conviction that a million volts are around the corner and ten million less than a dream away.[96] In August 1931 Lawrence was awooing in New Haven. He received a telegram from the secretary of the Department: "Dr Livingston has asked me to advise you that he has obtained 1,100,000 volt protons. He also suggested that I add 'Whoopee'!"[97]
There remained the problem of direct measurement. By November Lawrence and Livingston had obtained a current iD strong enough to allow the necessary collimation by applying to their instrument the very important discovery made by Sloan and Lawrence that grids worsen the focusing of the circulating beams. Lawrence alerted two main doubters of cyclotronics, Lauritsen and Tuve, to his latest accomplishment. "Recently we have put our experiments on a very sound basis by proving quantitatively by electrostatic deflection that the particles we are measuring are the expected high speed protons or H2 + ions."[98] To get high final voltages, Livingston had to put a relatively high potential on the dee. He could not go beyond an amplification of seventy-five steps. According to Lawrence, Livingston thought that he had struck the relativistic limit, where increase of particle mass with velocity destroys the synchronization expressed in equation 2.1. In fact, imperfect regularity in the controlling magnetic field was the culprit. Lawrence suggested placing small soft-iron shims shaped as in figure 2.24 where they would do the most good. "A little work with this led to the most gratifying results," he wrote Tuve soon after the new year. On January 9, 1932, Lawrence and Livingston attained protons at 1.22 MeV with only 4,000 volts on the dee, an amplification of over 300.[99]
Fig. 2.24
Lawrence's earliest designs for cyclotron shims. Lawrence, "Notebook,"
6 Jan 1932 (39/2).
It was time to write up. One knows the format. The principle of magnetic resonance acceleration had proved itself with "quite modest laboratory equipment," which functioned best with nature's cheap gridless focusing and a few shims that looked like tears. With a magnet costing $1,200 and with a 20-kW power oscillator, protons could be spun around 300 times to an energy of 1.22 MeV. To be sure the final current was small, only 0.001 µa. But by improving the source, multipying the shims, putting in two dees with 50 kV between them, and using a magnet giving 14 kG between pole pieces only 114 cm in diameter, it would be "entirely feasible" to go to twenty-five million volts.[100] The technical achievement was mainly Livingston's; the inspiration, push, and, above all, the vision of future greatness, were Lawrence's. His professorship gave him the place to stand, and Livingston gave him the lever, to move the world of physics. Compare the case of Thibaud, who, also in 1932, had made a little cyclotron, which he ran in the gap of a 10-kG magnet with 20-cm (8-inch) poles; he took as his ion source a discharge tube that fed the vacuum chamber through a narrow canal. Thibaud therefore could maintain his ion source at a pressure considerably higher than the pressure in the chamber and he obtained a proton current a thousand times larger than Livingston's. It is not clear that he caused his ions to resonate; it is certain that he had no one like Lawrence to push him along; and no further progress was made in cyclotroneering in France for several years.[101]
When this account was completed, in February 1932, Lawrence had long since sought a magnet to raise his energy by a factor of ten. It is almost superfluous to add that he had begun raising money for it the preceding July, as soon as protons resonantly accelerated to 500 keV appeared in the collector of the second cyclotron. It was "practically certain," he said then, that he could get to 10 MeV, or even to 20 MeV, if he could put his hands on a suitable magnet and oscillator. He reckoned he needed $10,000, or possibly $15,000, for the purpose. That took him out of the range of University research funds and NRC grants-in-aid. Another angel had to be found, he wrote the man he hoped would act the part, "if we are to proceed immediately towards the goal of 20,000,000 volts."[102]