Life and Work of Rolf Wideröe by © Pedro Waloschek, => Contents
9 Oslo - the Theory of the Synchrotron
After the German troops left Norway in May 1945 and the Crown Prince returned, I was arrested in Oslo and taken to Ilebu prison. The Germans had previously used their buildings as a concentration camp which was known as `Grini', and many Norwegians still have sad memories of this place.
Sometime later I found out that one of my neighbours had reported me to the police because he knew about my expertise in regard to relays, and therefore believed that I had participated in the building of V2s in Peenemünde, and may even have invented them. Of course, that would have been a grave matter. We must not forget that London and Antwerp were still being attacked by V2s in April 1945 and there was no means of defence against them. During the winter of 1944/45 a total of 2,800 such rockets were launched over these two cities, each one carrying a ton of explosives - however, only a fraction reached their destination!
Luckily I had brought with me all the papers and documents related to the betatron construction in Hamburg. These enabled me to write an extensive report while in prison. And, when I completed it at the beginning of July 1945, I was released. Apparently I was helped by the famous accelerator expert Odd Dahl, whom I didn't know at the time. He had influential connections, but I imagine that a few others also had a hand in my release.
Although, as I mentioned earlier, NEBB (a member of the Swiss Brown Boveri Group) in Oslo were my employers for the entire duration of the War, and I had been `conscripted' to work in Germany, I do not believe that this fact contributed to shorten my stay in prison.
From prison I wrote a long letter to my wife, making plans for the future. NEBB had stopped paying my salary after my arrest, and I was worried about my family. I asked my wife to pay a visit to the director of NEBB in Oslo and to ask him for advice. He suggested that I should apply for a position at BBC in Baden, Switzerland.
See Box 11: The Experts' Report.
During my imprisonment I was also visited by the Norwegian physicist Gunnar Randers. He had spent some time in America and returned to Norway to devote himself to astro and nuclear physics. He was sent to talk to me, presumably because of the V2 rumours. It would be easy to check the date because it was the day of a solar eclipse and he had brought with him some blackened glass with which we could observe the sun. It was on July 9, 1945, about an hour after midday. I was given the opportunity to explain the facts about my activities in Germany and, at least in my opinion, he and I got on rather well.
A while later, a commission of experts was called to make a `professional assessment' of my activities and to clarify my position [Hy46]. I didn't notice much of these investigations myself but I am not very susceptible when it comes to this sort of thing. It is possible that some people made malicious statements about me, but either I did not understand them or I wasn't bothered.
I assume that the police authorities just wanted experts to answer questions they were not in a position to assess themselves. I think that's quite natural under the circumstances, but the mood in Norway was a little overheated at the time and things were not always thought through and considered calmly and justly. In any case, it does appear that there were serious doubts about my conduct during the War. I bear no resentment, but at the time I did appreciate that I would soon be leaving for Switzerland to continue my work.
Despite everything, the post-War suspicions did leave a certain after-taste for some people and I am glad now that everything appears to have been completely cleared up. And the flowers I received from the Royal Norwegian Ambassadors during the last years on the occasion of various honours have entirely convinced me that no one in Norway now thinks badly of me. I was always very proud of being Norwegian. I was frequently, and mistakenly, described as being German, the first time probably in an article by Professor Gustav Ising, which appeared in the Annuary of the Swedish Physics Association [Is33]. This must have caused the confusion.
My wife has a very clear memory of the second half of 1945 and especially of the winter of 1945/46. We had very little money, it was extremely cold, I had no passport and was practically unemployed in Oslo. I used this time to order and write down my thoughts on what was later called the `synchrotron'. I submitted these ideas and theories as a private patent in Norway on January31, 1946 through the agency `Tandbergs Patent Kontor' Oslo [Wi46]. I had heard some rumours about other people in the USA working on the same subject, so I did my best to finish as soon as possible. The text of this patent is rather complicated and contains many formulae which I can no longer understand today. But it also includes some very important ideas which I shall describe later.
A synchrotron is made of a ring-shaped vacuum chamber, on which a magnetic field is applied. This field increases with the energy of the particles and keeps them on the same orbit. In such a machine, part of the beam-pipe corresponds to a bent drift-tube, as illustrated in fig. 5 of my patent (see Appendix 2). At the ends of this tube the particles receive voltage kicks every time they pass through, that is, once at each revolution. During this process the particles are automatically kept together in small bunches, which practically `ride' on the accelerating wave. The frequency of the accelerating voltage and the speed at which the particles turn around must obviously match each other exactly. In a stable bunch the particles `oscillate' around their nominal position inside the bunch, as they are constantly being `pushed back' by the accelerating wave. These are the so-called `synchrotron oscillations'.
The history of the invention of the synchrotron is very interesting. The idea must have been floating in the air at the time. In America, Edwin M.McMillan discovered the most important principle which was published in a very elegant article in the September issue of the `Physical Review' 1945. It was merely two pages long and became world-famous [Mc45].
At almost the same time in Moscow, Vladimir Veksler quite independently discovered the same principle [Ve45], and he described it in an extensive essay. Oliphant and his colleagues in England also appear to have found the principle (at least, part of it), again on a completely independent basis. And when I submitted the above mentioned patent I had no concrete knowledge of the others. I saw McMillan's publication only a few months later. Scientific contact and exchanges of information were much disrupted during the War.
My patent was based on the drift-tube which I had extended to a ring shaped machine which I called `l/2' and `l/4' resonant accelerators. But the patent also contained many other very important details which are now taken for granted when a synchrotron is built. For example, the stipulation that the accelerating frequency must be exactly fixed by the revolution frequency of the particles (a very important condition) is included therein.
Much later, during the construction of the 30GeV proton synchrotron (PS) at CERN in Geneva, one of the designers of this machine, Dr.Christoph Schmelzer (whom I knew well), proposed a different solution. He wanted to adjust the accelerating frequency to the revolution frequency of the protons using a computer. This, however, did not work. It was only after he had rigidly coupled the two frequencies that the machine was successful. Schmelzer called this a `phase-lock' and it became one of the most important construction criteria for further synchrotrons.
The magnets of a synchrotron only produce an adequate magnetic field in the relatively small ring shaped region of the particle-beam and not in the central part of the orbit, as had been necessary with both betatron and cyclotron. The significance of this is that a synchrotron machine for a given energy costs less _ or that with the same amount of money it is possible to build a machine for much higher energy.
Another interesting idea was the proposal for using a multiple of the revolution frequency in the acceleration. In doing so, the amplitudes of the synchrotron oscillations could be made smaller. This was very important for further reducing the size of the vacuum chamber and also of the bending magnets.
In his famous article McMillan announced that his institute, the Radiation Laboratory at the University of California, was already planning to construct a synchrotron for 300 MeV, and this actually went into operation in January 1949. However, back in 1946 in England, the two physicists F. K. Goward and D. E. Barnes [Go46] had already undertaken a precise verification of the synchrotron principle with a modified betatron. They constructed an acceleration drift-tube made of wire mesh around the beam-pipe (complying exactly with the conditions specified by McMillan as well as in my patent) with which they were able to continue accelerating the electrons after the betatron-action was finished. They achieved 8 MeV, which was twice the energy previously reached with the same machine as a betatron. The way towards further developments was already apparent at this very early stage!
For the sake of completeness, I should like to mention the principle of `strong focusing' which, although developed later, today belongs to the foundations of modern synchrotron construction.
It was at the beginning of August 1952 when, on my return voyage from Australia (where I had been lecturing on the betatron) I was travelling through the United States and got to Brookhaven. Here I met Odd Dahl and Frank Goward who had been sent over from Europe by CERN. We spent several days with Ernest Courant, Hartland Snyder, Stan Livingston and other interesting people who had developed the so called `strong focusing' method just a few weeks previously [Co52]. With the help of magnets of different form (`alternating gradients'), this allowed for further reductions in the beam's dimensions and therefore also in the size of the vacuum chamber. Thus beam focusing was strengthened, that is, the particles were better bundled. Following this principle it became possible to build larger accelerators at the same cost.
Whilst in Hamburg, I had proposed another method for improving the focusing of particle beams, the `lens-road', which I submitted as a patent in September 1943, just as we were starting to build our betatron. I had been thinking about this problem for quite a while. But in the end the `alternating gradients' were much simpler to realise and I would say that they were also more effective - they were just better.
Incidentally, the first inventor of this method was the Greek Nicholas Christofilos who had had this idea patented in March 1950 [Ch50]. However, it was not accepted and published until February 28, 1956. He worked for Westinghouse and I met him once during a conference in Russia.
Now I would like to go back to my life story. In Oslo I was given a passport just before Easter 1946 and I flew to Switzerland for a short visit where I met the very pleasant Professor Paul Scherrer. Years later, the large Swiss research centre in Villigen was named the `Paul Scherrer Institut' (PSI) after him. It is quite close to where I live now in Nussbaumen and not far from Baden (CH).
During this trip I also met one of the Boveris. I think it
was Walter Boveri, although it may have been Theodor
Boveri. We agreed that I would build a fairly large betatron for Brown
Boveri (BBC) in Baden. In those days we did not believe that this
could be done in Norway. The infra-structure was not suitable;
for example, there were no adequate glass-blowers and no
vacuum technology to speak of.