Life and Work of Rolf Wideröe by © Pedro Waloschek, => Contents
3 Aachen - the First Operational Linac
Aachen was a rather unconventional place to work in. There were several assistants and PhD students who were investigating travelling waves, their penetration into transformer coils and suchlike. Dr. Flegler (he later became a professor in Beijing) was the head assistant.
In Aachen I met Ernst Sommerfeld. He was developing a small cathode-ray-oscilloscope under Rogowski's direction. Ernst was the son of the famous physicist Arnold Sommerfeld (see for instance [Ec93]). We became great friends and have frequently had the opportunity to get together again since and throughout our lives. He later specialised in the field of patenting, and before the War lived in Berlin where he worked as a patent agent for Telefunken. During the War he was called up and became an officer's driver for a while. He moved to Munich after the War, where he lived in his father's house and started his own company. Most of my patent applications (there were over 200 in all) were looked after and submitted by him.
Ernst often came to visit in Norway and we made several tours to the high mountains. During my period in Hamburg between the end of 1943 and March 1945 I visited him a few times and he also came to see me later on in Baden. Sadly, he died of a stroke in 1980. His father Arnold had been teaching in Aachen and had worked there for several years and I suspect that this was the reason why Ernst was working with Rogowski. Arnold Sommerfeld later went to the USA, and therefore I was able to get early information about Lawrence's work as well as the development of the cyclotron. However, I did not meet Ernst's father until many years later in Zurich where they had come to visit us.
In Aachen we had the opportunity to hear some very good lectures on electrical engineering by Rogowski, and on aerodynamics by Karman, who was later to go to California. We used to play tennis with Karman's assistants. The biggest departments in the Polytechnic were the metallurgy departments, this was primarily due to the Rhineland's industry and mines. Incidentally, I was the only Norwegian in Aachen during my time there.
I was soon busy building the ray-transformer. I believe that my workshop activities at the time were paid for by an institution for German Science called `Notgemeinschaft der deutschen Wissenschaft'. Fig. 3.1 shows my working place in the institute's cellar. The dimensions demonstrate how little space there was.
The city's power station supplied me with an iron yoke. It had been taken from a relatively small three-phase transformer and was about one metre tall. I had part of the yoke cut off in order to obtain a simple iron return path, that is, a two-phase transformer, and then I took out a piece to obtain two poles at the top. I used small iron plates to shape the induction and steering regions between the two pole areas. The drawings are shown in Fig. 3.2 and 3.3, and are excerpts of my dissertation.
The poles were shaped in such a way that the magnetic fields in the accelerating and deflecting regions followed the 2:1 ratio which I had already discovered in Karlsruhe, and which today is named after me. Of course, I had also made use of the simplification which is a result of this ratio: both the accelerating and deflecting fields were induced by the same coil. The correct ratio is provided by the shape of the magnet's poles. I had measured the fields between the poles quite accurately with test-coils and verified that they complied with the 2:1 ratio.
We had an excellent glass-blower in Aachen, for it was not the easiest of tasks to make the vacuum tight ring tube. The glass-ring was about 15 cm in diameter, and the tube had a cross section of 15 mm. It was fitted with a ground glass connection for the injection tube. The ring stood upright and the electrons were injected from above, as can be seen in Fig. 3.2. A vacuum pump was connected through another glass tube.
To produce and inject the electrons I used a source which was similar to those used in the cathode-ray-oscilloscopes of Rogowski and Flegler. It was quite a reasonable source of electrons; the electron beam was then focused by a long coil and there was a small entrance-slit, which I could open and close from outside.
During the early phase of my experiments, I shot the electrons into the evacuated glass ring tube with a weak starting field. Then I turned on the magnetic field by switching on the current and at the same time attempted to observe the accelerated electrons. The internal walls of the glass tube were covered with a fluorescent material which was supposed to give some fluorescent light when it was hit by electrons. In this way I hoped to observe some of the electrons after they reached their highest energy.
In theory, the electrons were supposed to reach an energy of up to 6.8 MeV, which, with a normal voltage generator, would have taken 6.8 million volts to achieve. At that point I had to lead the electrons away from their nominal path, that is, I had to `extract' them from their orbit, if I may put it this way. The coils of the magnet had a fuse. When the current reached its maximum, the fuse turned off the current and simultaneously turned on the current in another coil which was supposed to kick the electrons against the walls of the ring tube. It was all rather primitive and I described everything very precisely in my notebooks.
I fired the magnetic field many times by shutting the switch which is also shown in Fig. 3.3, but I could not see any accelerated electrons (there was no fluorescence on the inside of the wall). Of course, fluorescence is a rather poor method for detecting electrons, and I am sure that a good physicist would have thought of a much better way to do this.
Later on it became clear that it is possible to make both the test set-up and the measurements much simpler by exciting the magnetic field with alternating current, which was how I had planned it in my original sketches (instead of having to resort to awkward switching on and off). Well, I never got that far.
I had made no provisions for avoiding the effects of electrons which deposited on the internal walls of the ring. As I was soon to find out, `islands' of electrons formed in some places on the internal walls of the ring. They had an important role to play. These islands formed wherever the wall was hit by electrons running out of their nominal path. They produced an electric potential which reduced the energy of the injected electrons by about one third. I therefore had to adapt the field to this lower energy during injection. I had a faint hope that the charges on the walls would produce some stabilising forces, but this was not the case. However, I did finally manage to get the electrons to circulate in the ring approximately one and a half times.
Later on, the charge-islands were avoided by coating the inside wall of the ring with a slightly conductive graphite layer. If I compare all this to my experiences in Hamburg between 1943 and 1944 and in Baden after 1946 at BBC, I can say that it was not only the omission of a conductive wall coating (to draw off the electrons from the walls) which denied the machine it's success. The shape of the iron core (and thus the magnetic field which was created by it) and of the other magnetic iron parts was far too primitive and quite insufficient to meet a ray-transformer's (later known as a betatron) high requirements. To be more precise: The conditions required to stabilise the electrons' orbits were as yet unknown, and my Aachen machine was far short of satisfying such conditions. The injection too, was less than sufficient. I think it was fortunate for me that I did not continue with those ray-transformer experiments, but instead stopped immediately. My own insufficient experience and probably the conditions in Rogowski's laboratory were simply not adequate to the task.
When I realised that I was not having any success with the machine, I reported to Rogowski. He told me that he couldn't possibly grant me a doctor's degree for something that did not function. I was well aware of this, so I had to construct something that would work - and I already had a solution in mind.
As part of my reading in the Karlsruhe library I had come across a publication by Professor Gustav Ising in the Swedish magazine `Archiv för Mathematik, Astronomie och Fysik' [Is24]. In this article he proposed that electrons should be guided through a straight vacuum tube, inside a series of metal tubes (`electrodes') in which a so-called travelling wave was produced by high frequency alternating voltages. These voltages would be applied to the tubes through adequate delay lines. Fig. 3.4 shows Ising's original drawing. The particles would be accelerated as if they rode `on the front of the wave', in Ising's tube. I committed this article to memory and thought at the time that I may be able to make something useful of it one day, especially if my ring ray-transformer didn't work.
However, I already understood something about travelling waves and the many possible problems associated with them. The electrodes suggested by Ising, as sketched in his publication, would have reflected these waves, and I could see that it would not be possible therefore to produce any accelerating voltage. However, the basic idea was very interesting, and I developed from it the so-called `drift-tube'. This simple tube was connected to a high frequency voltage supply and (having the appropriate frequency and length) would accelerate electrically charged particles two times, namely once as the particle entered the tube, and a second time as it exited (see Fig. 3.5). While the particle is inside the tube, the voltage is reversed without affecting its motion.
Electrons are not particularly suitable for this type of accelerator. They rapidly reach such high speed that one would require either a very long tube or a very high frequency for the alternating voltage. At that time (1927), it was not possible to produce sufficiently high frequencies for such apparatus; at most one could perhaps count on a few megacycles, which is not enough.
Because of this I resolved to try the `drift-tube' principle with particles which were heavier and which would move at a much slower speed. I decided to use potassium and sodium ions, that is, potassium and sodium atoms which, because a few of their electrons are missing, have a positive charge. I am referring therefore, to so-called `anode-rays' which had already been known in physics for quite some time.
One of my tennis partners worked at the Institute of Metallurgy and he came to my aid, building the activator for the anode of the Kunsman-type which I used in order to produce the ion beam for my little accelerator. After that, the rest of the equipment was quite easy to construct. It was housed in an 88 cm long glass tube. A diagram of the installation taken from my thesis, is shown in Fig. 3.6. If I remember rightly the accelerator cost no more than four to five hundred Marks.
The ions went into the drift-tube at relatively low speeds. As they entered, they received a first voltage kick of up to 25,000 volts and as they exited a second one of approximately the same value. The voltage was reversed at just the right moment, when the ions were inside the tube. After this, the ions passed through a second tube which was not connected to the high frequency voltage, it was earthed. Then they moved between two electrically charged plates where they were deflected more or less, depending on their speed. Finally they reached a sensitive photographic plate of a type which in those days was already in use to make X-ray photographs. The accelerated particles `exposed' the emulsion's silver bromide grains (just as light would) and formed narrow stripes which I could measure after I developed the plates.
Following a few calibrating measurements, the ions' final energy for each accelerating voltage was precisely determined. The readings taken with the potassium and sodium ions showed that everything was functioning as planned; the ions really were accelerated twice by the same high frequency alternating voltage and finally achieved a speed for which one would otherwise have required 50,000 volts! For the first time it was thus proven that it is possible to accelerate electrically charged particles several times using high frequency alternating potentials. It was therefore possible to accelerate particles as if one had available very high voltages without, however, having to take recourse to a correspondingly high voltage device.
There was also no reason to doubt that my procedure could be repeated as often as desired using a sequence of such drift-tubes in order to accelerate the particles to even higher energies. In principle, it was possible to `extend' them indefinitely to achieve ever higher energies. In fact there is today such a linear accelerator at Stanford University in California, which, over the years, has been extended until now it is approximately 5 km long. It accelerates particles as if 50 thousand million volts were available. My little machine was a primitive precursor of this type of accelerator which today is called `linac' for short. However, I must now emphasize one important detail. The drift-tube was the first accelerating system which had earth potential on both sides, i.e. at both the particles' entry and exit, and was still able to accelerate the particles exactly as if a strong static electric field was present. This fact is not trivial. In all naivete one may well expect that, when the voltage on the drift-tube is reversed, the particles flying within would be decelerated - which is clearly not the case.
After I had proven that such structures, earthed at both ends, and in which acceleration could take place several times, were effectively possible, many other such systems were invented. However, I will refer to some of these at greater length later on.
There are exact reproductions of my little Aachen installation in various museums, namely the German Museum (Munich), the German Röntgen-Museum in Remscheid (Lennep), the Norwegian Radiumspital in Oslo, the Norwegian Technical Museum in Oslo, the Swiss Technorama in Winterthur and the Smithsonian Institution, Washington DC, USA. It must be said, however, that the reproductions are more beautiful than the original I built in Aachen. These models (which, with the addition of a few components are even capable of functioning) were built in 1982 in the Radiumspital in Oslo. Their construction was suggested by a friend who worked there, the physicist Olav Netteland. Regrettably, before work could begin he suffered a serious stroke. We therefore tried at first to have the models built at BBC in Baden, but this proved to be too expensive. In the end they were made by an apprentice at the Radiumspital in Oslo, exactly to my specifications. Another similar model is now being built at the research centre DESY in Hamburg, also in the apprentices workshop.
The important invention however, was the drift-tube, driven by high frequency voltage. It supplied the foundation for the development of particle physics with high energy accelerators, particularly with reference to the ideas which arose for the cyclotron and for the synchrotron. The principle of the `synchrotron', using a bent drift-tube, for example, was patented by myself in Norway in January 31, 1946 [Wi46]; a facsimile of this patent is reproduced in Appendix 2. Moreover, my original simple drift-tube was the starting point for the development of all later variations of `accelerating cavities' used in circular as well as linear machines. Of course I made a big mistake when I did not have the drift-tube immediately patented in Aachen.
Rogowski took hardly any notice of my work. I don't think that he ever as much as looked at my linac. It was expected that my thesis would be published in a periodical and I had no problem getting it into `Archiv für Elektrotechnik' [Wi28]. The publication is almost identical to my thesis; only the Lenard curves are missing. Rogowski and Professor L.Finzi (physics) were my examiners. I had no problems there either and I finally obtained my title of `Doktor-Ingenieur' on November 28, 1927.
It is not that easy to write such a doctoral thesis. I was given no instructions and wrote everything myself. In my thesis I also mentioned a few methods and principles for achieving higher voltages with potential-fields, for example Marx generators (a set of parallel and series capacitors) and similar installations. Unfortunately, there were a few printing mistakes in the thesis, but these were corrected in the English translation which was not written until about 1965. This was when I was a consultant at DESY and, as I clearly remember, many people helped me with the translation, including G.E.Fischer, F.W.Brasse, H.Kumpfert and H.Hartmann. This translation appeared in the book `The Development of High-Energy Accelerators', which reprinted important publications on this subject [Li66]. I did have a few problems with Stan Livingston who was editing the book. He wanted to publish only the section on the functioning linac, so I had to battle with him and said, "either you take the whole thing or nothing at all". In the end he accepted it in its entirety, including the piece on the ray-transformer.
See Fig. 3.7: Wideröe in front of a model of his linac