Life and Work of Rolf Wideröe by © Pedro Waloschek, => Contents
It was in the Physics Association in Oslo that something happened which was of great importance to me. In the autumn of 1941 a lecture on particle accelerators was given at the Association and among the work described was that of Donald W.Kerst and R.Serber which had just been published in the American `Physical Review' [Ke41a][Ke41b]. The lecture was given by the physicist Roald Tangen from Trondheim who became a professor there in 1948 and then in Oslo in 1952.
See Box 4: Roald Tangen, Kerst and Wideröe
Kerst described in his article how he had built and put into operation a `ray-transformer' for electrons which he called `induction accelerator'. At the end of the acceleration the electrons had an energy which could ordinarily only be achieved by a high voltage of 2.3 million volts. The small piece of equipment had a circular tube with a radius of only 7.5 cm. Probably the most impressive result was contained in the summary: under optimum conditions, the electrons could produce X-rays which corresponded to those emitted by about one gram of radium. If one takes into consideration that one gram of radium had a value of about one million Kroner at that time, it is easy to understand why Kerst's little machine caused such a stir. Its use in hospitals, especially for radiation therapy immediately suggested itself.
Kerst had designed and built the 2.3 MeV ray-transformer at the University of Illinois where he worked. General Electric Company was very interested in his work. They had built the glass ring for Kerst's machine in their Valve Department, exactly to his specifications. When the article was published in Physical Review, Kerst was already on leave of absence at the G.E.Company's Research Laboratory. Here he built further machines of this type.
In a second publication which appeared in the same issue of the Physical Review, Kerst and Serber had formulated a theory of the ray-transformer which, in principle, can be regarded as a natural continuation of my ideas of 1928 [Wi28] as well as those of Ernest Walton [Wa29] which we had developed independently and at almost the same time. I shall say more about Walton's important contributions later on.
So it became clear that the ray-transformer did work after all - if things were done correctly. And this was like a thunderbolt for me!
I immediately went back to my calculations for the ray-transformer. For several months I worked on this in parallel with my work for NEBB, and in September 1942 I sent a fairly long paper to the magazine `Archiv für Elektrotechnik' in Berlin. In it I discussed Kerst's results as well as a few of my latest calculations and formulas [Wi42]. It was published in 1943. Later on I wrote a second article which contained a somewhat adventurous proposal for a 200 MeV betatron. It was submitted to the same magazine in July 1943 but for several reasons was never printed.
A very strange thing happened when my first article appeared. One day, it must have been in March or April 1943, several German Air Force officers came to NEBB wanting to speak with me. Norway had been under occupation since April 1940. I can't remember exactly whether there were two or three of them. I was standing next to my bicycle because I always cycled to NEBB. They asked whether we could go to the Grand Hotel together to talk about something. I countered that it was possible, but first I would have to fix my bike.
In the Grand Hotel they asked me to return to Berlin with them. They said that it could be a matter of some importance to my brother. My brother Viggo, as I already explained, was the director of `Wideröes Flyveselskap', the airline he had founded. It was closed down because of the War. But my brother had links to people who were trying to get refugees into England and this was, of course, strictly prohibited. They were found out. My brother was arrested, was tried in Oslo and, luckily, was not sentenced to death (as others were). Instead he was sentenced to ten years of severe imprisonment in Germany.
The German officers hinted that it may be possible to release my brother if I helped them. This decided things for me, and I agreed to go to Berlin. Two days later I was flown there for a short visit, and they told me about their plans to build betatrons. If I agreed to help them, they would in turn do everything they could to secure Viggo's release. At the time I knew that he was in Rendsburg jail and that he was not at all well.
They didn't tell me what the Air Force wanted with a betatron; I didn't find that out until later. In any case, I did not know at the time that anyone would want to use betatrons as weapons. I also would not have believed it within the realms of the possible. Of course they had one strong argument: They wanted to catch up with the Americans, regardless of any use the betatron may later be put to. The official line was that all this was being done to develop new and better X-ray apparatus for medicine and for non-destructive testing of materials. Betatrons were small and relatively manageable machines which could replace the high voltage set ups normally required to produce X-rays. They would for instance be useful in field hospitals.
So I agreed to go to Hamburg or, to be more precise, I was `subjected to compulsory work' with my more or less voluntary agreement (and obviously that of my employers NEBB). Initially I was to develop and build a relatively small betatron for 15 MeV and then perhaps a larger one somewhere near Mannheim. That was in the spring of 1943. I prepared the design of the 15 MeV machine in Oslo until the summer and also planned a few things for further development of this type of apparatus. In July 1943 I also applied for a first patent on betatron-construction which dealt with some details of the injection system.
Although I hadn't been directly involved in particle accelerators since my Aachen days, I had carefully followed the progress made in this field. I had thoroughly re-examined the literature on ray-transformers (later called betatrons) for my `Archiv für Elektrotechnik' report, and in doing so had come across a series of publications and patents. I still find all this developmental and pioneering work very interesting and would therefore like to mention some of it here, without getting too bogged down in technical details.
In 1937 I had already, quite accidentally, discovered an American patent which introduced a very similar idea to that behind my Karlsruhe ray-transformer. It belonged to Joseph Slepian, who worked for Westinghouse Company (see Fig. 6.1). He submitted the patent in America on April 1, 1922, and it was granted in 1927 [Sl22]. Slepian also made use of the induced electric field which appears around the core of a transformer to accelerate electrons within a small disk shaped vacuum chamber. Permanent magnets forced the electrons onto spiral orbits whose radii increased in size as the particle energy mounted. Finally the particles would hit either the walls of the tube or a suitable piece of material. The small size of the machine meant that it was only of use for relatively small energies (under 100,000 eV). It was designed to produce X-rays and was also given the name, `X-Ray Tube'. Slepian's machine could not follow my `2:1-condition' because his deflecting magnets provided him only with a fixed field, constant in time.
Slepian also submitted this patent in Germany, shortly after he had handed it in to the American patent authorities. As I learnt later [Ka47], a Dr. Smidt at the German patent office doubted the validity of his proposal for a long time. Smidt referred to the famous text book on electromagnetism by Abraham and Becker from which he thought he had learnt that it was impossible to deflect electrons within a magnetic field while simultaneously accelerating them. The German patent was therefore not granted until 1928.
But even before 1928, that is just before I completed my thesis in Aachen, ideas on the construction of ray-transformers had emerged in other places and some experiments were even started, albeit without much success. It seemed that it was not going to be that easy after all.
In 1927 at the Carnegie Institution in Washington DC, a group had already been performing experiments along this line [Br27]. The authors, Gregory Breit and Merle Antony Tuve, should have stood quite a good chance of success, but appear not to have pursued their plans any further. They did not know the 2:1 relation.
Around 1928, Ernest Walton, then at the Cavendish Laboratory in Cambridge, on the instigation of Lord Rutherford, did what was basically exactly the same as I was doing at about the same time, without, however, knowing anything about my work. First of all Walton built a machine which was very similar to my ray-transformer, although much more primitive, and it was not a success. He published his results in October 1929 and included the conclusions of some very interesting and important theoretical deductions on the stability of the electron-orbits [Wa29].
Since Walton's experiments with a ray-transformer were not successful, he built a linear accelerator which was also very similar to mine in Aachen, but again it was much more primitive. It was also fitted with a spark generator to produce the required high voltage. This machine had no chance of success.
And then Walton gave up these investigations. Together with John Cockroft, and with Rutherford's steady encouragement, they built their famous cascade generator, which came to be known as the `Cockroft-Walton' device. With this he achieved the first nuclear disintegrations with artificially accelerated particles, which I have already mentioned [Co32]. Cockroft and Walton were awarded the Nobel Prize for their achievements in 1951.
Ernest Walton subsequently returned to Ireland where he took up a professorial post. I sent him a letter once, enclosing copy of a lecture I had given at the `5th Nordic Meeting' held in 1983 in Geilo. He thanked me and wrote that he had been able to find much in this lecture that he had not known before. He had heard a little about my work by then.
Walton's work was continued by Leo Szilard and J. L. Tuck at the Clarendon Laboratory in Oxford. They built an iron-free betatron for higher frequencies and this work was also unsuccessful. It is possible to produce suitable magnetic fields for a betatron without using an iron yoke, but this was not made to work until many years later. Leo Szilard, whom I had met in Berlin, had emigrated to England after Hitler had come to power in Germany.
There was another article on ray-transformers in the `Archiv für Elektrotechnik' magazine published in 1936. It was written by W.W.Jassinski [Ja36] and contained a comprehensive mathematical investigation as well as some technical proposals which I did not find particularly useful at the time.
While I was correcting the proofs of my article for the `Archiv für Elektrotechnik' in 1943, the physicist Max Steenbeck from the Siemens Company in Berlin published an article in the magazine `Naturwissenschaften' [St43]. He stated that he had been successful in accelerating electrons to approximately 1.8 MeV with a betatron-tube in 1934 and 1935, and that he had also applied for several patents on this matter. I included this as a footnote in my article, on page 545. I also mentioned that Steenbeck roughly described a condition which the magnetic field had to satisfy in order to obtain stable electron orbits in a betatron. This condition was also included in two German patents [Ru33] and [St35] submitted in 1933 and 1935, and in an American patent [St36] which, incidentally, should have been known to Kerst. Steenbeck should therefore be considered as the inventor of this stability condition.
See Box 5: The Many Names of the Betatrons,
Box 6: About Max Steenbeck
and Box 7: The War of the Patents.
I met Max Steenbeck much later, during an International Conference on Betatrons in Jena in June 1964. It was a pleasant meeting and we had lots to discuss. While I was there, I also gave a lecture on the first ten years of multiple acceleration. The complete text was subsequently published in the periodical of the Friedrich-Schiller University in Jena [Wi64].
However, Steenbeck's stability condition should be regarded as an approximation of Walton's earlier, more general formulations. Steenbeck's condition is valid only in the immediate proximity of the particle's nominal orbit, whereas Walton's formulas also apply at greater distances. But Steenbeck's condition was easier to understand than Walton's somewhat more complicated and scarcely disseminated theory. Therefore, Steenbeck is generally regarded today as the author of the (simplified) stability condition. In his first patent (1933), Steenbeck formulated the condition rather vaguely: "...the magnetic field which serves to guide the particles, (is) characterised by the magnetic field dropping off from the centre to the sides...". More was not specified on the subject.
If particles are to stay on a constant circular orbit, it is important to ensure that they are guided by suitable forces. Particles which are not on the nominal course are then gently pushed back. Of course, the force which does the `pushing' also causes the particles to snake around their intended course, just as a sleigh would when in a rut. These backward and forward movements are called betatron oscillations. They can be radial as well as vertical (on a ring placed in a horizontal plane). Appropriate correcting forces are created in Walton and Steenbeck's magnetic fields within the betatron. They have to decrease proportionally to 1/rn going outwards, whereby the number n lies between 0 and 1. This is a more precise formulation of the intention in Steenbeck's patent.
In 1946, Donald Kerst provided a precise illustration of the history behind the betatron in a comprehensive and very interesting article for Nature magazine [Ke46]. He expounded on all published and unpublished work on the subject, in as far as he was aware of its existence. It seems certain to me however, that the basic idea for the construction of a betatron or ray-transformer was developed independently and in different places at the same time.
By the end of the summer of 1943, while still in Oslo, I had progressed far enough with my ideas and preliminary studies to be able to start constructing a betatron.
However, from July 25 to August 3, 1943, `Operation Gomorrah' had been executed: during the course of five air attacks Hamburg's centre and some outlying areas were almost completely destroyed by American and British bombs. An enormous number of people were killed, probably more than 50,000. After this, Hamburg was regarded as a `relatively safe' place, because it wasn't thought that anyone would find it worthwhile to subject the city to such intensive bombardment again.
And so I began my work in Hamburg, although I often
returned to Oslo which is where I wrote many of my reports. During
this period (the second half of 1943), and always with the assistance
of Ernst Sommerfeld, I submitted another four patents in
Germany which concerned the construction of betatrons as well as a
very special patent, which I shall describe later on.