Big Complicated Machines, Geeky Physics

Big, Complicated Machines #14 – The Calutron


184” (184 inch) Cyclotron taken in 1942. Image: LBNL

Good morning everyone.

Today we’re going to talk about on odd offshoot of the cyclotron, actually it’s an odd offshoot of a cyclotron, it’s called a Calutron and it’s a device used to enrich uranium.

The calutron (I hate that name, just despise it) is another invention of E.O. Lawrence. Remember Ernest Eddy? I briefly talked about his invention of the cyclotron back in BCS #13, among other things. Anyway, Lawrence never intended to build any such device, he wanted to build bigger and better cyclotrons and had been doing just that throughout the 1930s. He and his associates built a string of them starting with the original 9 inch cobbled-together device, to a 27 inch 4.8 MeV device (a big improvement), a 37 inch 8 MeV device, and a 60 inch 16 MeV device. The experimenters that used the things just loved them and were discovering all sorts of new things, first of which is that the machines got inordinately larger (and more expensive, of course) as the power output increased. The sizes indicate the diameter of the acceleration chamber, not the size of the cyclotron itself. For example, the 60 inch device required the use of a 220 ton iron electromagnet, a picture of the device is after the jump.


The 60-inch cyclotron at the University of California Lawrence Radiation Laboratory, Berkeley, in August, 1939. Image: LBNL via Wiki

A magnificent thing isn’t it, but extremely large. They had to move the radiation laboratory into a nearby empty building (it was actually the Civil Engineering Testing Laboratory. The rad lab just stole it from them) starting with the construction of the 27 inch cyclotron because the device would have filled the lab almost entirely, and where would the theorists be able to have darts matches whilst having their huge thoughts? They soon filled up the CET building and had build a new one, the Crocker Laboratory, to house the 60 inch machine that you see above. Did I mention that all of this was rather expensive? It was.

After the success of the 60 inch machine and a Nobel Prize (for the original cyclotron), Lawrence started making plans and gathering loot to make a 184 inch cyclotron that would be hugely more powerful than the previous one. This machine would use a 4,000 ton magnet and have an output of over 100 MeV, a tremendous leap in power. It would need its own building (but of course) that was at least 160 feet in diameter and nearly 100 feet high in order to house the machine and the experimental facilities that would use it. Here is a picture of the building under construction:


184 inch Cyclotron construction site. Image:LBNL

You see that low upside-down horseshoe-looking thing in the center? That’s the magnet yoke, but not all of it. This diagram will help you to make sense of what you’re looking at:


184 inch synchrocyclotron Building diagram. Image: LBNL

So they are actually constructing the building around the magnet. Could you imagine trying to get that in through a door?

This was all happening in the late summer of 1941 and things were moving along nicely, and then a snag arose in early December: the damned Japanese bombed the living bejesus out of Pearl Harbor, and all of sudden we were at war.

I can just hear Lawrence saying, “Damn it! Now what?” when he heard that all projects were put on hiatus indefinitely.

That brings us back to the calutron, remember that?

There was a great deal of interest at the time in the separation and refining (enriching) of uranium isotopes, but there wasn’t a practical method to do so on an industrial scale, and that’s what the military wanted, and very badly. Lawrence decided on using the electromagnetic method because that’s what he was good at, and he just happened to have a huge magnet laying around that would work perfectly in his idea for uranium enrichment: a huge, powerful mass spectrometer. It was a great idea.

The way a mass spectrometer works is: a material is vaporized and then ionized with an electron beam. The ions are accelerated into a strong electromagnetic field which bends the ion beam. This causes the ion beam to separate into streams according to the mass of the ions, with the lighter ion streams bending a lot, and the heavier ones bending less. The streams fall onto detectors which provide signals of how much of each respective mass was in each stream. Here is a diagram that should help:


Schematics of a simple mass spectrometer with sector type mass analyzer. Image: Wiki

There was much design and experimentation work done to prove the concept, and they ran into a host of technical problems along the way. It sounds simple on the face of it, and on a laboratory scale it is: you’re only after a few billion atoms in that setting. Ramping the same effect up to industrial levels is something else altogether, as there is only 1.25 percent difference between the masses of U235 and U238 and the ion streams diverge to a very small degree, making collection of the different isotopes very difficult.

Lawrence finally proved that his idea worked, and the device was named ‘The Calutron’, a conglomeration of University of California (Cal-U) and the contractor of Los Alamos (??). It’s an entirely political construct and I hate it with a passion. It also sounds stupid. Why not ‘The Californicatron’? It’s just as stupid and doesn’t embarrass Los Alamos.

Anyway, that’s what a calutron is: a big mass spectrometer for separating uranium isotopes. In mid-1942 a design was settled on,  called ‘alpha’, and the military immediately started pouring money into Oak Ridge, Tennessee, to get a pilot plant constructed. Groves had a panic attack in early 1943 when he found a German cockroach in his shoe and was convinced that Nazi spies had stolen all their plans and had gotten the jump on them. He killed the pilot plant project and had construction start on the production unit, labeled ‘XAX’.


Alpha Track Calutron at the Y-12 Plant at Oak Ridge, Tennessee from the Manhattan Project. Image: LBNL via Wiki

There was a severe shortage of copper due to the war, so in order to make the windings for the huge electromagnets, General Leslie Groves borrowed 13,300 tons of silver from the Treasury. It was all actually returned, too, with the last few tons being returned in 1970. The military can get away with just about anything during wartime, as we have seen.

Meanwhile, in the spring of 1943, hundreds of trainee workers from Berkeley started arriving in Oak Ridge to build and test the magnets and to operate the thing once it was made. In late summer of 1943 XAX was ready for testing, and passed after surprising few teething problems. By November, two of the scheduled five alpha ‘racetracks’ (see the above photo and remember the nature of the people naming things) started up, and both failed almost immediately due to contaminated cooling oil. By January 1944 they got one working again, barely, and by April that had four ‘racetracks’ working, including the gummed up unit one.


An alpha calutron tank removed from the magnet for recovery of uranium-235. Image: LBNL via Wiki

There were still problems though. The alpha enriched U235 by ~12-15 percent and each ‘racetrack’ output around 200 grams a month, for a total of 1 kilo per month total. They tried running the output through a secondary refinement stage called ‘beta’, but the results were poor. Oppenheimer said the he needed ~10 kilos of 90 percent U235 to construct a working bomb, and the calutrons just weren’t producing enough. There was a gaseous diffusion process which was being built at the same time as the calutrons, but progress was very slow and the program was behind schedule. Groves leaned on the Manhattan Engineering District to help increase the output, and they constructed a thermal diffusion plant at Oak Ridge in 1944. In early 1945, the alpha calutrons and the 6 beta calutrons (once the bugs were worked out), along with whatever they could get from the gaseous diffusion and thermal diffusion plants and run through the beta calutrons, were producing weapons-grade U235. This material was shipped to Los Alamos and was used to make Little Boy.

Here are some more photos of the calutrons at Oak Ridge and the cyclotrons at Berkeley:



Image credits:

—LBNL = Lawrence Berkeley National Laboratory

—Wiki = Wikipedia article

Lawrence Berkeley National Laboratory

Project Gutenberg

Lawrence and His Laboratory: Episode 2: The Calutron

Ernest Lawrence’s Cyclotron: Invention for the Ages

Berkeley Lab History




Manhattan Project