Life and Work of Rolf Wideröe by © Pedro Waloschek,     => Contents

8   The Invention of the Storage Ring

At this point I would like to recall an important event of my Hamburg period. It happened during the autumn of 1943, on one of my vacation trips back to Norway. Ragnhild and I were staying in a hotel in a forest in Tuddal, near Telemarken, and Ragnhild unfortunately took rather ill with pneumonia.

I was lying on a grassy hill one day, observing the clouds in the sky, when I noticed two clouds moving towards each other, as if they were about to collide. This started me thinking about cars in frontal collisions and inspired me to make the following consideration: On frontal impact, most of the kinetic energy of both cars is transformed into destructive energy. On the other hand, if a car collides against one which is at rest, only part of the kinetic energy contributes to the destruction. Quite a considerable amount of it is used up to hurl the previously stationary car away and therefore is not available to destroy the two cars. This is a result of the laws of mechanics.

I had thus come upon a simple method for improving the exploitation of particle energies available in accelerators for nuclear reactions. As with the cars, when a target particle (at rest) is bombarded, a considerable portion of the kinetic energy is used to hurl it (or the reaction products) away. Only a relatively small portion of the accelerated particle's energy is used to actually split or destroy the colliding particles. However, when the collision is frontal, most of the available kinetic energy can be exploited. For nuclear particles, relativistic mechanics must be applied, and this would cause the effect to be even greater.

However, it is not so easy to achieve head-on collisions of very small particles against each other. A large number of particles are required and they have to be tightly bundled in order for any two to stand any chance of ever colliding with each other. At the time, I was thinking of atomic nuclei. Since Rutherford's experiments their approximate size was known and I could therefore estimate the probability of a collision. However, given the particle beams available at that time this was an utterly hopeless venture.

And this is where I had my second idea. If it were possible to store the particles in rings for longer periods, and if these `stored' particles were made to run in opposite directions, the result would be one opportunity for collision at each revolution. Because the accelerated particles would move very quickly they would make many thousand revolutions per second and one could expect to obtain a collision rate that would be sufficient for many interesting experiments. I gave the name `nuclear mill' to this storage ring, or rings, in which the collisions were to take place.

This exceedingly simple principle was not conceived of again until 1956, i.e. thirteen years later, in the USA [Ke56] [O'N56], when it was developed further and eventually put into practice. Also, in the USSR, at Novosibirsk, similar ideas appeared. However, the first storage ring was put into operation in 1961, and it was not in either the USA or the USSR, but in Italy. Many storage rings used in high energy physics were built in accordance with the principle of this first Italian machine.

After I returned to Hamburg I spoke with Touschek about my ideas and he said that they were obvious, the type of thing that most people would learn at school (he even said `primary school') and that such an idea could not be published or patented. That was fine, but I still wanted to be assured of the priority of this idea, and I thought the best way to do this would be to submit a patent. I telephoned my friend Ernst Sommerfeld in Berlin and we turned it into a very nice and quite useable patent which we submitted on September 8, 1943 (see facsimile in Appendix 1). This was given the status of a `secret patent'. It was not until 1953 that it was retrospectively recognized and published [Wi43a].

But we had taken Touschek's objections into consideration and did not state anything about the favourable balance of energy during a frontal collision in the patent, as this was considered a well known fact. Even so, Touschek was pretty offended.

However, the time was not yet ripe in 1943 for constructing storage rings. It was only years later that the accelerator experts came to be in a position to propose and build realistic storage rings for physics experiments. Before that, a whole series of technical problems had to be solved. It was even necessary to develop entirely new technologies. BBC therefore earned nothing from this patent.

The only particle accelerators which I was in a position at that time to propose for my `nuclear mill', were the betatrons so successfully built by Kerst. And those weren't really suitable for anything other than electrons. But I suspected that soon there would be ring-accelerators for other particles, quite apart from the cyclotrons already in existence. The latter, however, could not be used as storage rings because they did not have stable particle orbits with a constant radius.

The first accelerator (apart from the betatron) in which the particles turned around in a stable orbit was the `synchrotron', developed in several places after 1945. I worked on this subject myself. I shall later come to describe the problems related to these machines. It was not until ten years later, i.e. in 1956, that this new type of accelerator was proposed as a storage ring, by then a straight forward and natural idea.

I was not worried about the missing technology for my patent to be realized as yet. My main concern was the principle - for which I wished to secure priority for myself. So I put the vacuum problem (and others) to one side for the time being, although this had already caused me difficulties with Professor Gaede when I proposed my first ray-transformer in Karlsruhe. Now this was obviously a completely unresolved problem, because an even better vacuum was required to store particles for a longer time without colliding with molecules of the residual gas.

I was also aware of the fact that the orbits lacked stability. I had been dealing with this problem since my time in Aachen and knew how difficult it was. Kerst was the first to solve it in practice. There was also the problem of getting equally charged particles to run in opposite directions within the same tube. I came up with an adventurous proposal by which the particles would be guided by electrical fields. This never became realisable. It was simpler to use two rings with steering magnetic fields, and indeed this is how it was later done.

But none of this changed the fact that the best way to exploit the accelerated particles' energy was by frontal collision - today this is known as a `collider', of which there is a variety of types - and that storage rings provided the particles with more opportunities to collide (in fact many thousand per second), as is explained in my patent. Bruno Touschek could not shatter my optimism. Years afterwards he pioneered work in this direction himself - but more of that later.

In the meantime, construction of the 15 MeV betatron went according to schedule and it began operations in the summer of 1944. Intensity was very low at first, but eventually it could be increased sufficiently to be comparable with Kerst's second betatron of 1942 [Ke42]. This betatron accelerated electrons up to 20 MeV. Given his higher frequency (180 Hz, i.e. 3.6 times as much as we had) and his greater electron injection voltage (20 KV as opposed to our 7.5 KV), Kerst did in fact achieve approximately thirteen times our maximum intensity.

Later on, the X-rays produced did on occasion correspond to the radiation equivalent to an entire kilogram of radium, as it was reported by Kaiser [Ka47] - but usually it was only equivalent to about 30 gram, which is already pretty dangerous.

In the beginning we used a hot cathode to produce electrons but the filament could only provide us with the greatest intensity when it was in a favourable position. The result was that the intensity was constantly changing. Rudolf Kollath called it our `squirrel'. Later we used an oxide cathode and the source became more stable and the intensity more constant.

As I mentioned earlier, a 6 MeV betatron was built at around the same time in the Siemens-Reiniger factory near Erlangen, following Max Steenbeck's proposal. The X-ray specialist Konrad Gund was appointed to the job at the end of 1941. I went to visit him in November 1944. For a variety of reasons I did not believe that the machine would ever work. In particular, there were problems with the vacuum tube made of ceramic material, which is a very good insulator. Electrons leaving their nominal orbit penetrated the walls where they accumulated and eventually caused disruptive discharges in the wall which caused the vacuum to collapse. I was able to prevent this effect in my machines by using glass of weak conductivity (boron-silicate glass, C9) for the tube.

But we also discussed the machines' frequencies and I think I managed to persuade the Siemens team to use 50 Hz rather than the higher frequency of 550 Hz used by Gund. I heard later that this betatron was taken to Göttingen after the end of the War. Konrad Gund collaborated successfully with physicists and he went on to attain a doctorate [Gu46]. Gund, however, was psychologically unstable and he and his wife committed suicide in 1953.

One day, we were visited in Hamburg by Professor Gentner from Heidelberg and Professor Kulenkampp from Tübingen. They were full of praise for our results.

By the autumn of 1944 our betatron had progressed sufficiently for me to be able to hand over the work to Dr. Kollath and Gerhard Schumann. They did a good job and later published an in-depth report on it in `Zeitschrift für Naturforschung' [Ko47].

At about that time I was invited to a meeting at the Kaiser-Wilhelm Institute in Berlin at which were present several physicists. The meeting took place in a beautiful garden. I think Heisenberg may have organized it, but perhaps it was Gerlach. This conference was of a purely scientific nature. We all spoke freely and said exactly what we meant. As there weren't any Gestapo men present, nothing was kept secret.

We unanimously agreed that Schiebold's fantasies should be called off as they were so utterly unrealistic. On the other hand, it was decided that the betatron was a very interesting machine, especially with regard to the medicine and nuclear physics of the future. The hopeless `secret project' which aimed to shoot down aeroplanes with X-rays produced by betatrons was dropped in its entirety. The development of betatrons however, was to continue. Of course, in this case it was possible to maintain the official justification that the betatron project was of importance to medicine. It did not cost a lot of money and in any case, money did not play a tremendous role in Germany at that time.

I had several meetings with the directors and design engineers of Brown Boveri & Co. (BBC) on the construction of a 200 MeV betatron. Richard Seifert had awarded BBC a preliminary order for planning such a machine (from the German Aviation Ministry [Wi44]). Several possibilities were investigated and detailed drawings were produced, as was later reported by Hermann Kaiser [Ka47], but these plans were never realised. BBC's factories in Mannheim were in a fair state of destruction, and after Germany was occupied, all mention of these plans ceased completely. Kaiser, however, judged these efforts as the most progressive plans for future betatrons in Europe.

After receiving a final payment for my work from Hollnack I returned to Oslo in March 1945. This time I took the train. We had to stop in Denmark several times, because parts of the railway tracks had been sabotaged. I had another stop in Copenhagen to get my documents in order at the Norwegian consulate.

Our betatron was not the only thing that worked well in those days. The British Army was also doing well and rapidly approaching the city of Hamburg. The German Aviation Ministry therefore ordered the betatron to be moved to Kellinghusen, near Wrist, approximately 40 km North of Hamburg in central Holstein, as Dr.Werner Fehr from C.H.F.Müller reports [Fe81]. Here Seifert's family offered the use of a dairy in which Kollath and Schumann could install the betatron.

On May 3, 1945 British troops occupied the centre of Hamburg. They met no resistance. It appears that Hollnack immediately went over to their side with a full show of flags. Germany surrendered unconditionally on May 7, 1945 and we may assume that Berlin ceased to finance the work on the Hamburg betatron from that moment on. However, Kollath and Schumann got the betatron running in Kellinghusen without any major hitches, and even continued working and taking measurements until December 1945, as is documented in notes now kept in the ETH library [Ko45]. Bruno Touschek had also moved to Kellinghusen, at least for a while, as a few of his manuscripts regarding the theory of betatrons are marked with the name of that place [To45].

In 1947 Kollath and Schumann wrote the extensive report mentioned earlier on the performance of the betatron [Ko47], a work which was mostly done in Kellinghusen. In a footnote on the first page of this report are the words, "We would like to thank the gentlemen of the company C.H.F.Müller for their active support at all times and for their commitment to continuing this work". The footnote which immediately follows states, "We would also like to thank Mr. Richard Seifert, factory owner of Hamburg, for his willingness to offer us his assistance at all times."

From this we may deduce that both companies found a way for financing the work in Kellinghusen as they were themselves involved in the construction of X-ray apparatus (and are still today). Later however, the construction of betatrons was taken over by larger companies such as BBC and Philips-Eindhoven.

In December 1945, the British authorities decided to take the betatron, as part of the booty of war, from Kellinghusen to the Woolwich Arsenal near London. Apparently, Rudolf Kollath later on took charge of its operation in Woolwich where it was used for non-destructive X-ray inspection of steel plates and such like. The machine has since disappeared without a trace. Many, including myself, later attempted to find it, but with no success. It was most probably scrapped.

With hindsight, 1943 and 1944 were very positive years for me, despite all the problems. During this period I submitted ten very important patents on the construction of betatrons for BBC. When I returned to Norway in March 1945 I had already started thinking about issues relating to an even better accelerator which today is known as the `synchrotron'.

I would like to make another mention of my colleagues at C.H.F.Müller in Hamburg. As I recalled earlier on, there were Dr. Rudolf Kollath, Gerhard Schumann and Bruno Touschek. We were also actively supported by the engineers and designers in the company. I would like to make special mention of the head of the laboratory in which I got my working place, Ing. A. Kuntke. He lived outside of Fuhlsbüttel. His home was in a lovely wood and I visited him there a few times. I also remember Dr. Werner Fehr, who was very active at the time and whom I met several times later on in Remscheid. A few years ago he sent me a nice photograph of the Hamburg betatron. He wrote an interesting booklet on the history of C.H.F.Müller. I have already mentioned Ing. Friedrich Reiniger. He is still alive, as is Ing. Gert Krohn who was working on a linear accelerator for industrial purposes - if I remember rightly. We had many interesting discussions and the atmosphere at C.H.F.Müller was pleasant and cooperative.

I met Schumann once more in January 1945 before I left for Oslo and then heard no more of him for a long time. In his CERN-Report `The Touschek Legacy' [Am81] the famous physicist Edoardo Amaldi writes that Gerhard Schumann (born 1911 in Dresden) studied in Halle and Leipzig (where he worked with Smekal) and went to Heidelberg in 1950 to work with O. Haxel. He later studied fall-out problems by means of filtering methods and became an expert on exchange phenomena in the atmosphere.

During my period in Hamburg I also met other scientists with whom I had a good rapport. Political issues were very rarely mentioned during our conversations. However, I do believe that most Germans knew nothing of Hitler's atrocities against the Jews. We certainly never spoke about it.

Amongst the people I met was Dr. H. Suess. He lived a little way out of Hamburg and worked with O. Haxel and H. J. D. Jensen. Suess later became a professor at the University of California. At that time he was concerned with the abundance of the elements in the universe. Dr. Suess was absolutely opposed to Hitler and I was able to converse quite freely with him. He gave me the impression that scientists were doing everything in their power to prevent nuclear bombs being built in Germany. The only potential they perceived in splitting uranium was as a future source of energy. A small reactor was under construction in southern Germany, but one may presume that this was little more than a diversionary tactic.

I don't think that my work in Hamburg was used in any way for purposes of war propaganda, not even by way of a hint, especially not after the Berlin meeting. The miracle weapons were expected to come from Peenemünde. In my opinion, German morale was at a very low ebb during the second half of 1944. Of course the government did attempt to improve the general mood with a few propaganda tricks, and Goebbels was, after all, a talented copy writer. Nevertheless, I imagine that most people didn't think they stood a chance.

Before we take leave of my time in Hamburg I would like to say a few words about Bruno Touschek. He was a very young man at the time, a student. Touschek's mother was Jewish, and, obviously, that caused him many difficulties. As I mentioned before, Touschek worked with Dr. Egerer at `Opta' (previously `Löwe') in Berlin. Dr. Egerer was also the editor-in-chief of `Archiv für Elektrotechnik' at that time. It was probably Egerer who brought Touschek to us in Hamburg.

Touschek lived in Professor Lenz' house, where, as I said, I first met him. Lenz had some psychological problems and whenever there was an air-raid he would be so scared, that Touschek had to carry him into the cellar. Touschek was able to listen to Lenz and Jensen's lectures at the University while in Hamburg, but he was not officially registered as a student. As a `non-Aryan' he had already been forced to stop his physics studies in Vienna.

There was a place in Hamburg (the chamber of commerce [Am81]) where one could read foreign magazines and Touschek was a frequent visitor. This was noticed and the Gestapo arrested him in November or December 1944. He was jailed in Fuhlsbüttel, but was able to continue working for us there. We helped him as much as we could, but could not secure his release. I can remember that we brought his beloved books, some food and cigarettes to his cell, but I have no recollection of the schnapps he later spoke of. It was in prison that he wrote an important essay on `radiation dumping in betatrons' which he wrote in invisible ink in the pages of Heitler's book `The Quantum Theory of Radiation' [Am81].

As the British troops approached, Touschek was due to be transferred to Kiel in February or March 1945. He had a cold and was finding it difficult to carry his many books. One fell into a ditch and, while he was trying to pick it up (his condition was fairly poor), he was shot at from behind by one of the guards. He was only grazed behind the left ear, lost a lot of blood but was left for dead. When he heard passers-by speaking, he raised himself, was given treatment and then re-arrested and taken to Altona prison where, so he later said, things were `a bit more peaceful'.

Touschek was freed by the British troops in June 1945. As I mentioned earlier, he then went to Kellinghusen where he wrote several interesting (theoretical) reports on the betatron [To45]. In several of them he developed ideas which I had suggested in our discussions, and had even submitted for patenting. His particular skill for theory and mathematical formulations was of great help. It was a very pleasant collaboration.

Touschek didn't publish this work and never mentioned it in his curriculum vitae. However, these reports must have come in useful when he went to Göttingen in early 1946. At about that time Konrad Gund's 6 MeV betatron built at Siemens Company in Erlangen was going to be installed at Göttingen. By the summer of 1946 Touschek had completed his thesis (it was on the theory of betatrons) under the supervision of Professors R.Becker and H.Kopfermann.

Afterwards Touschek went to Glasgow where he obtained a PhD in November 1949. From December 1952 Touschek worked at Rome University. As a theoretical physicist, he made important contributions and wrote many very interesting publications during the course of his life.

But Touschek was also the first to break the ice in the field of storage rings. In Rome at the beginning of 1960, he proposed the construction of an electron-antielectron storage ring [To60]. This was completed within less than a year at the Laboratori Nazionali di Frascati in the beautiful hills to the South of Rome. It was the first storage ring ever to function, so it was the first time my patented ideas of 1943 were actually used in practice.

See Fig. 8.1:   A photograph of the AdA storage ring.

Electrons and their antiparticles (the positrons), have exactly the same mass, but electrical charges of opposite sign. They can therefore run in the same ring (and magnetic field) on identical orbits but in opposite directions and will then meet in certain places. According to Touschek (and to my patent), these encounters could eventually result in frontal collisions. Touschek's rather theoretical ideas were put into practice in Rome by brilliant experimental physicists.

In two other projects, similarly small storage rings of different types, were also built. One was in the USA, prompted by Gerry O'Neill [O'N56] (see Fig. 8.2) and another in Akademgorodok near Novosibirsk (then USSR). Construction of these two had started before, but they did not become operational until after the Frascati storage ring. In each of these two cases, two electron rings were placed tangentially next to each other. An interesting experiment was conducted on the American rings to check the validity of quantum electrodynamics.

After my 1943 patent, I was never really involved in storage ring construction, instead I concentrated on betatrons, a realistic task by then. But I did meet Touschek several more times, the last time was in 1975. He died of liver failure in 1978. He had been rather too partial to a drop of alcohol, and that was probably his undoing.

Touschek's machine in Frascati was primitive, but also interesting. It was given the name Anello d'Accumulazione (AdA, Fig. 8.1) which in Italian corresponds precisely to the term `storage ring'. As I mentioned earlier, a single ring was used to store both electrons and positrons in opposite directions and to make them collide. Basically it was two storage rings within a single tube, exactly as I had proposed in my patent of 1943.

However, a storage ring is merely a synchrotron with particularly good stability. I shall return to this subject later on. In AdA the particles could be stored at approximately 200 MeV. The machine as a whole had an external diameter of only 1.6 m, and the electron orbit was about 4 m in circumference. AdA went into operation on February 27, 1961. Touschek would spend hours watching a few stored electrons through a small telescope. A single electron gives off so much light during its orbit that it becomes clearly `visible' (this is part of the so-called `synchrotron-radiation'). AdA was later taken to Orsay, south of Paris, where positrons were also injected into it and made to collide with electrons [Am81]. The performance of AdA is best described in the PhD thesis of the Orsay physicist Jacques Haissinski [Ha65].

See Box 10:   Vacuum in Storage Rings

The development of storage rings led to gigantic machines which were often close to the limits of the available technology and financial resources. They were used to make very important discoveries, especially in relation to the quark structure of matter. The `Large Hadron Collider' (LHC) should make collisions between protons of 8 TeV (1 TeV = 1,000 GeV) feasible at CERN, in the 27 km long tunnel of the electron-positron collider LEP. Two proton storage rings will be fitted into the same tunnel and the beams will be guided towards each other at various points. This machine should help to solve some of the important remaining problems regarding the structure of matter.

Let's get back to my life story. By 1945 I had problems of quite a different nature to contend with, especially after I returned to Norway.