Life and Work of Rolf Wideröe by © Pedro Waloschek, => Contents
At this stage I would like to say a few words about Ernest Lawrence's work in America. Lawrence was of Norwegian extraction and his family name had originally been Larsen. He was a very interesting person, spirited, stubborn and full of enthusiasm. Furthermore, he had a definite thirst for adventure.
Lawrence once recalled in my presence that he had been at a conference in Berkeley (it must have been in 1928) where the presentations became rather tedious for a while. He therefore removed himself to the library and found my thesis in the magazine `Archiv für Elektrotechnik'. He looked at the pictures and formulae only, as he could understand little or no German. From these illustrations he gained an immediate understanding of my drift-tube principle. However, it was of great advantage to him that he didn't know the German language; he could not understand my reservations on the stability of the orbits in circular accelerators, as included in the essay.
Thereupon, Lawrence, who worked in the then `Radiation Laboratory' in Berkeley near San Francisco in the USA, together with his student David Sloan, built first a linear accelerator for Mercury ions with a total of fifteen tubes, and later one with even more [La31a], in exact accordance with the principle sketched in Illustration 3.5. He was thus able to accelerate ions to an energy of 1.3 MeV, i.e. as if he had 1.3 million volts at his disposal, although he in fact used only 48,000 volts of high frequency voltage. It was a tremendous achievement!
However, Lawrence was already suggesting that the drift-tube should be transformed into a D-shaped box (fig. 4.1) and that the particle paths should, with the help of a magnetic field, be `wound up' into a spiral. Thus he had invented the famous cyclotron (Fig. 4.2). He had discovered that, although the radii of the particle orbits increase as the energy grows, they require the same amount of time for each revolution, because their speed also goes up. Therefore, the frequency of the accelerating voltage could remain constant (although only as long as classical mechanics remained sufficiently accurate) and this greatly simplified the installation. He published these ideas with his student N.E.Edlefsen [La30] even though the first experimental tests were not at all successful. He was very confident really!
However, I must now mention that Rogowski's assistant in Aachen, Dr.Flegler, had the same idea some time around 1926. During a meeting held to discuss work in progress, Flegler asked whether it would be possible to wind the ion paths into a spiral. I replied that it would be very difficult to stabilise the circular orbits, which is exactly what I later wrote in my thesis. That is how Flegler's suggestion for a cyclotron was abandoned and I was the one who more or less killed the idea (see also Box 6).
In contrast, Lawrence, together with Stan Livingston (another of his then students), pursued this same idea and, in 1930, constructed the first functioning cyclotron for protons [La31b]. All they had was a four inch magnet from the laboratory's stock, and, with this small installation, they could accelerate hydrogen ions to a modest 80 keV. However, this did definitely confirm the principle - and Livingston was awarded a PhD on its basis [Li31].
Their second cyclotron had a magnet with a diameter of 10 inches and with this they were able to accelerate protons to 1 MeV as well as perform experiments. Thus (with M.G.White) they confirmed the nuclear disintegration, which had previously been observed by Cockroft and Walton in England. The third cyclotron had a diameter of 27 inches and in 1934 it accelerated heavy hydrogen nuclei (heavy hydrogen had just been discovered in 1931) to 5 MeV, which corresponded to 5 million volts - here too without having to resort to such a high voltage!
Afterwards Lawrence went on to build several more, very successful cyclotrons, and in 1939 was awarded the Nobel Prize. It was the start of large accelerator development for nuclear and particle physics at high energies. However, I didn't make this type of machine my particular business. This was partly because I was engaged on quite different activity at the time, but I did closely follow their emergence and progress.
See Box 2: Cyclotrons and Synchrocyclotrons
I came to the conclusion that this was not the best route towards achieving higher energies. The spiral orbits within these accelerators require a magnetic field which covers a large area and is best produced with an iron yoke. Not a major problem, as long as the energies were not too high. If, on the other hand, one wanted to go to higher energies, a limit was very soon attained, which was given by the magnet itself, by its weight and its cost. My ray-transformers encountered the same problem. The magnet required to accelerate to higher energies would have been much too large.
Yet I hoped to keep the particles within a relatively narrow ring tube, as was the case in the ray-transformer, and still manage to accelerate them - possibly without the bulky inner part, the accelerating induction field. This would have had some advantages over the gigantic D's in the higher energy cyclotrons and my thoughts were therefore levelled in that direction. This remained a dream however; I did not seriously occupy myself with this subject until later, when, for purely personal reasons, I found time for it - and this wasn't until 1945.
Apart from Lawrence's cyclotron, the Thirties saw another important step forwards. This was thanks to the work of many physicists, but perhaps in particular to that of Louis Alvarez. He too worked in the Radiation Laboratory in Berkeley, which is today known as the `Lawrence Berkeley Laboratory' (LBL). I should imagine that Alvarez developed his proposal on a line with Lawrence and Sloan's successful linear accelerator, because he became Lawrence's assistant in 1936.
With the advances of high frequency technology, Alvarez was able to build electrode systems in cylindrical boxes, in which resonant electromagnetic waves could then accelerate particles. Since then, two types of drift-tubes are distinguished, those of `Wideröe' and those of `Alvarez'. The latter have to be built into an `Alvarez-Tank' of very particular shape. In some modern linear accelerators both types of structure may even be applied.
See Box 3:
About Drift-Tubes and Waveguides,
Fig. 4.3: Different types of waveguide
and Fig. 4.4: The iris-loaded waveguide
Lawrence's main objective was to construct accelerators, particularly cyclotrons, and this he pursued like a man possessed. Yet the construction of larger and larger machines by his younger colleagues, assistants and students must have been motivated more by the disintegration of the atomic nucleus and other research into nuclear physics.
I suspect that this was also the case with Rogowski when he supported my ideas for a 6 million volts ray-transformer. He was a well educated, highly intellectual man with a most lively intelligence. We never spoke about these possible applications however, and neither did I refer to them in my thesis. It was probably premature to make mention of it at the time and would not have counted as serious physics. Rather, it would have been regarded as science fiction. I modestly wrote in my thesis, "It is possible that high energy ion beams may be of some importance to physics". Quite an understatement really, because ever since 1919, splitting the atom had been the leitmotif behind my interest in high voltage technology.
It is certainly pertinent to ask why I didn't continue to occupy myself with the interesting field of particle accelerators after I had finished working on my thesis in 1927 and 1928. Well, the cyclotron had not yet been invented and the first nuclear disintegrations with artificially accelerated particles did not take place until 1932. So it was quite simple really; I had finished my period of study and my first priority was to find a job. Therefore, I did not have time for more investigations in the field of particle accelerators.
I should add that when I was in Aachen I had no contact at all with other institutions (like Lord Rutherford's laboratory in Cambridge or the Radiation Laboratory in Berkeley) where the development of particle accelerators was just starting. So I did not see any particular reason to continue working in this area. Moreover I could not at that time think of any use for particle accelerators, other than splitting atoms - which I considered to be a far distant goal.
I wasn't particularly interested at that time in the option of
using high-energy electrons to produce harder (i.e. more powerful
or deeper penetrating) X-rays. Accordingly, I did not think of
X-rays for use in either the investigation of materials or in medicine.
I considered my work in Aachen as completed, and, for the
time being, concentrated on other tasks.