The Uranium Bomb^ the Calutron, and the Space-Charge Problem A participating scientist relates the story of a World War II realizing he had to deal with only two fractions, "^Li and '^Li, Smith suggested project dedicated to electromagnetically separating trying a geometry, illustrated in figure 1, uranium-235 from uranium-238. similar to that of the electron mag- netron. An arc ion source would be along the centerline parallel to the William E. Parkins magnetic field, and ions would be ac- celerated radially outward. They ranium! Uranium! Uranium!" A voice sbouted out into would describe circular paths, and the fields would be ad- U tbe night from the second floor of a dormitory in Oak justed so that the heavy fraction would collect near the Ridge, Tennessee. It was 6 August 1945. That day, Presi- 180° focus on a peripheral cylinder while the light fraction dent Harry S Truman bad announced to the world that the returned to be collected near the center. Smith reasoned US had dropped a new weapon, a uranium bomb, on the tbat the symmetry would eliminate space-charge fields in city of Hiroshima, Japan. For years, tbose of us on the the 0 direction (in the usual cylindrical coordinates). bomb project were cautioned not to say the word uranium, Space-charge fields would have r and z components, and but now it was okay. There were code words and code let- Smith had calculated how much ion current one would ex- ters for tbe things we worked with, and eacb of our new pect before the radial fields ruined the resolution in mass designs received a new name. The teletype messages tbat separation. went back and forth between tbe radiation laboratory in The experimental apparatus was readied in the sum- Berkeley, California, and the Y-12 plant in Oak Ridge were mer of 1941. Almost immediately, we observed clean reso- total gihherish. The purpose of our effort was to separate lution with higher currents than should have been possi- "P," or ^^''U from "Q," or ^^^U. Tbose were easy to remem- ble. Shortly thereafter, we realized that we had stumbled ber hecause P stood for precious and Q stood for qrap. onto a process wherein the ion beam automatically neu- Afew days later, another word burst on the scene with tralizes itself by ionizing residual gas in the vacuum cham- the issuance of the Smyth Report, the official government ber. The positive ion beam presents a potential well to elec- account of the history of the bomh project.' That word was trons. They are trapped while the ions they leave are "calutron." Now that the device had achieved its objective, immediately swept out along magnetic-field lines in the z Ernest Lawrence wished to give recognition to the Uni- direction. And tbe process is fast. Even when the acceler- versity of California by using the name calutron for the ap- ating voltage is swept at 60 Hz, the neutralization follows paratus developed to separate P from Q. He had made an the beam location perfectly. arrangement with report author Henry Smyth that the Furthermore, the process is self-limiting. Electrons name he included, but never divulged the deal until the accumulate while oscillating up and down along magnetic- war was won. field lines only until tbe potential well is filled. They move The calutron's separation method was based on elec- laterally with small cycloidal paths because of any resid- tromagnetic mass spectrometry. (See box 1 on page 46 for a ual space-charge fields perpendicular to the magnetic tutorial.) All critical material was transported in beams of field. Collisions of the electrons with gas molecules cause positive ions on wbich electric and magnetic fields could act. the electrons to start new cycloids—fortunately always The needed quantity of material demanded very intense closer to the center of the beam. {See box 2 on page 50 for beams witb a bigh density of electric charge. But the posi- more on heam neutralization.) tive beam itself should create so-called space-charge fields We at Cornell didn't know tbat at this very time, the whose repulsive forces would alter ion trajectories and pre- Uranium Committee, an arm of the Office of Scientific Re- vent the desired isotopic separation by mass from occurring. search and Development (OSRD) under Vannevar Bush, At least in 1940, any thinking physicist knew that. was negotiating with Smyth and Lawrence to start proj- ects at Princeton University and tbe University of Cali- Cornell University fornia. These would investigate whether space-charge ef- The calutron story starts around 1940 at Cornell Univer- fects might be overcome sufficiently to permit use of an sity. Fellow student A. Theodore Forrester and I were fin- electromagnetic method for quantity separation of ^^^U. ishing our graduate work under the direction of Lloyd P. Lawrence, who was in the process of building a giant 184- Smith, who had obtained a contract to separate a quantity inch magnet at UC Berkeley, volunteered the use of it and of lithium-6 for use in an experimental study of a new can- the existing 37-inch cyclotron magnet. He would try the cer therapy. Knowing about the space-charge problem and classic Dempster mass-spectrometer arrangement'^ and see how well he could do. Smyth proposed a time-of-flight method he called the isotron, invented by Robert R. Wil- Bill Parkins retired from Rockwell International, where he was son. It used no magnetic fields, but employed a broad two- director of research and technology for the energy systems dimensional ion source to increase currents. Both of those group. He now lives in Woodlarid Hills, California. projects were started in late 1941. © 2005 American Inslitule ot Physics. S-0031-9228-0505-020-9 May 2005 Physics Today 45 Word reached Lawrence of our work Figure 1. The radial magnetic separator at at Cornell. He contacted Smith and in- Cornell University was used to separate vited the three of us to join his proj- lithium isotopes. This cross section ect at Berkeley. Pearl Harhor had shows the relationship of its ion tra- recently been attacked, and the jectories with those of the con- country was at war. We felt a ventional Dempster mass spec- duty to go, although Smith trometer (blue area). Also would have to return within shown are a circular arc (red a few months. In mid- area) struck trom the cathode February we boarded a (A), short alternating sec- train in Ithaca and left tions of metal tubing and the ice and snow for screen at ground potential sunny California. From a (B), and matching sections railroad station on the of metal tubing and open way, we mailed a manu- mesh grid (C) at the accel- script to Physical Review erating potential. The loca- with the request that, be- tion of the collector pocket cause it should now be re- for the lighter isotope is indi- garded as secret, its publi- cated (D), as is the focus cation he postponed until the where the heavier isotope col- war was over. We wished to lected (E). For ease of viewing, the elements A, B, and C have get credit for having discovered been somewhat enlarged. and explained the automatic self- neutralization of intense ion beams where there are no applied electric fields. Our manuscript was received on 18 February 1942 and published on 1 Decem- until the additional width is equal to the ber 1947 after declassification.^ separation distance of the two isotopes being sep- arated, then tbe useful beam current limit has been The Berkeley radiation laboratory reached. The current / of the desired isotope in milliamps per centimeter of height of beam in the magnetic-field di- When we arrived at Berkeley, experiments were already rection is^ under way in a small vacuum-chamber tank placed be- tween the poie pieces of the 37-inch magnet. Also, tbere was arc-ion-source development using a smaller magnet /=8.55x VH, from the cosmic-ray program. Getting sufficient ion cur- rent from the source was the greatest problem. By mid- where rj is tbe fractional abundance of the desired isotope, March the ion currents were up and, for the first time, ex- Aj and A^ are the atomic weights of the two isotopes being ceeded those possible without some space-charge separated. Vis tbe accelerating voltage, and H is the mag- neutralization. netic field in gauss. The current above which resolution in mass separa- For separating '^'^'^U from '^^''U, the maximum / of ^^"^U tion is lost may readily he calculated from tbe divergence is 4 X 10"^ niA/cm for a voltage of 35 kV and a magnetic of the ions at the heam's boundary, which is caused by the field of 3500 gauss. With that current, it would take more space-charge field there. In the Dempster spectrometer, tban 5 years to accumulate 1 kg of'^^•''U with 1000 separa- the beam is accelerated at the ion source from a narrow tor units having beams 60 cm in height and operating at slit that is long in the direction of tbe magnetic field. Tbe full capacity without interruption. That was considered beam takes tbe shape of a douhle-bladed wedge bent into unachievable, so our challenge was to see how much beam a semicircle, and comes close to focusing at tbe 180" point. current might be increased above the space-cbarge limit. If the space-charge field widens the focus even further, A significant change took place in the beginning of Box 1. Electromagnetic Mass Spectrometry espite their varied geometries and field combinations, field Hare perpendicular and the ion moves in a straight line Dmass spectrometers incorporate two steps, each of orthogonal to both fields.