A Neutron Foundry
|View inside the partially opened Iinac chamber showing the drift tubes between which protons are successively accelerated.|
The principle of the Alvarez linac differs from that of the original Wideroe version (the same that inspired the invention of the cyclotron). Both use drift tubes to protect the particles from decelerating phases of the radiofrequency field. The Wideroe tubes, however, also function as electrodes, whereas the Alvarez tubes are passive, the accelerating field arising from electromagnetic radiation flooding the apparatus. The stems supporting the tubes bring electricity in the first case and only cooling water in the second.
|The 40-foot long radio-frequency cavity of the proton linac lifted out of its vacuum vessel.|
The Alvarez linac exploited sources of electromagnetic radiation at the 1.5 meter wavelengths developed for radar during the second world war. In acknowledgment of the contribution of the Laboratory to the MED, General Groves made it a gift of 750 surplus radar oscillators. They were to form the core of the equipment that Alvarez, Panofsky, and a team of specialists used to create the oscillating field within the linac that would direct and accelerate protons down its 40-foot length. The machine may be pictured as a cylindrical wave guide or resonant cavity containing a time-varying accelerating field everywhere the same. The field automatically bunches particles that enter the gaps between the drift tubes as the field is increasing there. Particles arriving slightly early (or late) receive a smaller (or larger) push than the mean, and come in better time to the next gap.
Distortions in the field introduced by the drift tubes make a sideways force in the gaps between them. As a result, some of the beam runs into the walls of the drift tubes. By placing a conducting grid across the opening of the tubes, Alvarez's group distorted the transverse field so that it always focused the beam. Another important accomplishment was the redesign of the circuits for the oscillator units Groves had provided, which did not work efficiently for their new purpose. Alvarez explained that wartime experience made the group's task unexpectedly easy: "It has been our experience all through the development of the linear accelerator that all problems involving the phasing of oscillators end cavities are an order of magnitude easier than people without direct experience in the field predict. Every time we tried something new in this line we were surprised at how easy it was to do what had first looked like a very tough job."
The linac, completed in 1948, gave a beam of unprecedented intensity and collimation: 0.4 milliamp, 85 percent of which could be directed on a target 3 mm in diameter. The emergent energy, 32 MeV, the highest available for protons before the 184-inch synchrocylotron accelerated them, was acquired partly from a conventional Van de Graaff accelerator used as an injector and mainly (28 MeY) from the linac. The construction, evacuation, and cooling of the copper cavity and its steel vacuum enclosure brought new experience in engineering. The copper liner, made of tong sheets specially milled, so closely resembled the fuselage of an airplane turned inside-out that its fabrication was subcontracted to the Douglas Aircraft Company. A system of integral water pipes prevented the 2.5 million watts of radiofrequency power developed within the cavity by the 200 MHz oscillator from melting the machine.
This impressive accelerator, with innards the size of a small plane and power requirements in megawatts, was but a "toy," as Lawrence said, compared with the "great particle reactor" he proposed to a subcommittee of the JCAE visiting Berkeley in October 1949. A huge apparatus of the Alvarez type would make free neutrons by bombarding heavy metals like thorium, and these neutrons, when absorbed by lithium, thorium, or depleted uranium, would create the useful explosive materials tritium, uranium-233, or plutonium, as the AEC might desire.
For a year and a-half, from January 1950 through the summer of 1951, the Laboratory refined the plan. According to the last design, the MTA (as the project was named) would give 500 milliamp of 350 MeV deuterons that would make a gram of free neutrons a day. To guard against arcing in the gaps, which had proved troublesome in pilot studies, the plan reduced the potential gradient in them and extended the cavity to 1500 feet, thereby assuming the maintenance of a vacuum of one billionth of an atmosphere in a volume of over four million cubic feet. The price would be more than $300 million (up $235 million over the first estimate) per machine, exclusive of target. The whole would be surrounded by a concrete wall 80 feet high and between 7 and 20 feet thick. Power alone would run $14 million a year, about two-thirds of the operating budget. Thus the hypothetical bottleneck of raw materials would be hypothetically broken. "If we can do that for a few hundred million dollars, why, let's get going and do it." The members of the JCAE to whom Lawrence addressed this remark agreed; they had no trouble with the money after receiving assurance, as one of them put it, that "neutrons theoretically are actually apparently there."
The primary target was to be thorium plates in stainless steel tubes cooled by liquid sodium. A magnetic field would sweep the tip of the beam across the tubes. Neutrons emerging from the thorium would fall on a secondary target of uranium, perhaps jacketed in aluminum as at the Hanford reactors and at least twelve feet thick; or into an absorbing lattice similar to the Hanford slugs imbedded in graphite, the slugs to consist of uranium tailings, lithium, or thorium, or any two, or all. The projected cost of the target: $55 to $70 million depending on the constitution of the lattice.
Only the front end of the gigantic machine, the so- called Mark I, was ever built. Authorized late in March 1950 for $7 million (it had consumed $21 million by the end of 1951), it was to test the possibility of magnifying the proton linac to the size of the production accelerator (Mark II), make 50 milliamp of 30 MeV deuterons, and use them to create polonium-208, an agent of radiological warfare, from bismuth. Mark I contained the largest nothing in creation, an evacuated space 60 feet long and almost 60 feet in diameter. The inside of this cavity was bathed by the combined output of 18 oscillators each using an $8000 tube made by RCA; the total power supplied, 18 megawatts, would have met the needs of a community of 20,000. Each of the drift tubes had its own magnet focusing system and associated water cooling. The last and largest tube weighed 40 tons; the beam focusing magnets alone consumed 100 tons of copper windings. In order to be able to install and service these gigantic tubes, the team laid a standard gauge railroad track down the middle of the accelerator. That would no doubt have been the easiest of the machine's extraordinary features to extend to the quarter mile cavern provided in the final design of Mark II.
|Looking down the vacuum tank of Mark 1: railroad tracks were used to move the massive drift tubes within the vessel.|
Mark I came to life before the AEC "indefinitely postponed" Mark II in August 1952. It ran ten times, once continuously for thirteen hours with an average beam of 50 milliamp, before it was put to sleep in November 1953. Its builders judged it a success. It confirmed the possibility of holding a very high vacuum in a very large vessel, of maintaining the necessary potential gradient, and of introducing focusing magnets in the drift tubes. In achieving these ends, the MTA team had improved vacuum technology and diminished the lightning in the gaps. They built an ion source that emitted an ampere of deuterons or protons collimated to within four degrees. They trained themselves in remote reaches of high-frequency, high-power engineering. As the projectors summed up: "Mark I is believed to have provided by far the largest heavy particle beam of any machine ever built, and it provided information and developed technology which made it possible to build a substantially better high current accelerator at considerably reduced cost."
Other dividends came with Mark III, the production cyclotron Lawrence first brought to the attention of the AEC in the spring of 1950. Their rebuff as usual did not stop him. In November, having had tests made on a small model suitable for accelerating electrons, he returned to the Commission confident of making his gram of neutrons by the cyclotron as well as by the linac. The agency, concerned about the extraction of the beam, continued to prefer the straight path. Lawrence convinced himself that circular was better: "it is like the electromagnetic process in the Manhattan District," he told the JCAE. "Everyone felt that, thank God, this electromagnetic process will give us something; if we keep it going long enough, we will get a bomb." In September 1951 Lawrence formally proposed to make a prototype production cyclotron giving 15 milliamp of 300 MeV deuterons at a cost of $20 million exclusive of the target. He now explicitly rated it more promising than the Mark II linac, not only for its primary purpose, but also for "truly new possibilities that might well turn out to be of decisive importance which would give us a tremendous advantage militarily and peacewise." The Commission did not bite.
One reason for Lawrence's eagerness to proceed with the Mark III cyclotron was to test the principle of sector focusing, which had been suggested in 1938 by L. H. Thomas as a contribution to the debate about the practical limitation on accelerating particles imposed by relativity. Many physicists, including Lawrence, had recognized that if the magnetic field of a cyclotron grows radially it can speed up a particle in outer orbits to compensate for the slowing down arising from relativistically increasing mass. They also recognized that by so distorting the field they would lose the focusing into the plane of the median orbit that they customarily secured by a slight radial decrease in the field. Thomas thought of a way to, keep the spiraling orbits planar while allowing the average magnetic field to increase.
His trick divides the plane of the orbit into several pie-wedge sections and requires particles in alternate sections to be exposed to a relatively strong (and in intermediate sections to a relatively weak) magnetic field. The stronger sections bend the particles' paths more sharply toward the center than the weaker sections do. Orbiting particles cut obliquely across the magnetic field lines that bulge outward at the boundary from strong to weak field. This cutting induces a force that pushes a particle more strongly back toward the median plane the farther from the plane it is. Since the angle of the path with respect to the mean circular orbit changes sign at the other side of the wedge, the reverse bulge in magnetic field there also tends to drive errant particles back.
|The magnet yoke of the model for Mark III showing the three strong/weak field regions.|
A sector-focused cyclotron accordingly keeps accelerated particles approximately in phase even if their masses increase. It therefore can use a fixed frequency oscillator for the accelerating field. It runs continuously, and so can give a beam more intense than those of pulsed machines like the Alvarez linac and synchrocyclotron. Thomas' principle therefore seemed the obvious and perhaps the only way to Robert Thornton and his colleagues when Lawrence charged them with the design of a cyclotron for the MTA project. Their conception, the J-16 cyclotron, did not materialize. It would have produced deuterons of 220 MeV after acceleration between magnetic poles shaped to give three strong and three weak fields extending out to its full 312-inch diameter. They demonstrated its principle, however, on an electron accelerator made to one-tenth scale. It was the first machine to show the merit of sector focusing, which subsequently regulated the design of many research cyclotrons throughout the world, including the Laboratory's 88-inch machine.