Life and Work of Rolf Wider� by Pedro Waloschek, => Contents
11 Turin - the Beta-Synchrotron
At BBC, the construction of betatrons in which electrons reached 31 or even 45 MeV was a great success. However, there were good reasons for not using betatrons to achieve higher energies, as I knew well from past experience. At the end of the War (1944), the German Air Force had appointed BBC to make preliminary plans for a 200MeV betatron in accordance with ideas I had developed previously. I now believe it most unlikely that these proposals would ever have resulted in a machine capable of functioning.
Donald Kerst had already been successful in building and operating his second betatron (20 MeV) for General Electric in 1942 [Ke42]. W. F. Westendorp and E. E. Charlton then went on to build a 100 MeV betatron for the same company, and this was completed in 1945 [We45]. In the meantime, Kerst had returned to the University of Illinois where he built first, a model machine for 80MeV and eventually a gigantic betatron for 300 MeV. This was the largest machine of this type ever constructed and should be regarded as the final stage in the development of betatrons.
Size and construction costs prohibited competition at higher energies with synchrotrons which, by that time, had already been tried and tested. However, the betatrons proved their worth for practical uses below about 50MeV for a long time. Later on, linear accelerators were developed for use at somewhat lower energies and these prevailed, especially for therapeutic uses.
After I started work at BBC (Baden) in 1946, we only ever discussed betatrons of 31 to 45MeV. In the interim, I had also spent a lot of time thinking about other methods of acceleration, especially about that type of machine which McMillan called a `synchrotron'.
From 1953 onwards I was several times in Italy to talk with various physicists about the construction of synchrotrons. Professor Giorgio Salvini and the engineer Fernando Amman were planning a 1,000 MeV electron-synchrotron at the time. This was later built in the `Laboratori Nazionali di Frascati' south of Rome, where Bruno Touschek was also working at the time. This 1,000MeV synchrotron came into operation in 1959.
Also in 1953 I entered into negotiations with scientists at Turin University's Institute of Physics for a new, much smaller, accelerator. My contacts at the Institute were the head, Professor Gleb Wataghin who came from Russia and had also worked in Brazil for a long time, and Professor L.Gonella. I have fond memories of them both. Incidentally, the project was financed in equal parts by the `Consiglio Nazionale delle Ricerche' (CNR), FIAT in Turin and the University of Turin itself.
The purpose of this machine was to accelerate electrons to approximately 100MeV, mainly for experiments in nuclear physics for which the secondary production of neutrons was also rather important.
See Fig. 11.1: Accelerating tube of the Turin-synchrotron
and Fig. 11:.2 Diagramm of the Turin-synchrotron
It was clear to me by then that a betatron would not be the best solution for this task. Using the synchrotron principle we would be able to build a much smaller machine and achieve better results - at the target energy of 100MeV. However, a synchrotron requires an injector, that is, a pre-accelerator which provides the particles with a starting energy.
The physicists at the Turin Institute were willing to tread relatively unknown paths in order to produce a very compact, reliable and economical machine which may also in the future be used at other places of research. So we developed a rather original concept, although it did owe much to the investigations done previously in the U.K. by F.K.Goward and D.E.Barnes [Go46]. Dr.H.Nabholz worked with me on both the design for the project and the construction of the new machine.
The machine was to function as a betatron until the electrons reached 2 MeV. Then it would continue to increase the particles' energy like a synchrotron. For me, this was the longed for opportunity to use my ideas and knowledge of synchrotrons on a machine which I was going to build myself.
And, of course, the new project was based on our previous, positive experiences of constructing betatrons at BBC. Accordingly, the iron yoke was again made up of six sections arranged around a central body. Naturally, many other details were taken from our betatrons, but the important thing for me was the second stage of acceleration, with which we hoped to achieve 100 MeV.
I had already described in detail the principles and the theory for the operation of a synchrotron in my Norwegian patent of January 1946 [Wi46] (reproduced in Appendix 2) and for the first phase of the operation (as a betatron), we were going to realise a few ideas which I had patented in 1948.
This machine was going to accelerate electrons in both directions, as was the case with many of our earlier betatrons. We fixed the radius of the electrons' orbit at 29 cm and planned to use the Italian electricity network's frequency, i.e. 50 cycles per second _ which is what I had done with all my previous betatrons.
Part of the vacuum chamber was arranged as a curved drift-tube. In order to bring this about, a section of the inner surface of the chamber was coated with silver and connected to a high frequency voltage supply through a capacitor. As in my Aachen drift-tube, the electrons would be accelerated at both ends - but this time, the acceleration would occur once per revolution. However, this created many new problems and was not as easy as I had thought when I wrote my synchrotron patent in 1945 (see Fig.5 in Appendix 2). This was not a simple drift-tube like the one I had tested in Aachen.
We had problems with secondary electrons which appeared on the inner wall of the tube. We dealt with this, and a few other problems, by coating the drift-tube with a layer of graphite (which has a high electrical resistance). We cut grooves along the coating and came up with a few other tricks, all of which we described in a subsequent publication [Go64].
By 1956 it became clear that we would need more time than originally expected to build the machine, so BBC provisionally installed a 31MeV betatron in the Turin Institute. This was operated until the new beta-synchrotron was finally delivered.
When the 105 MeV machine was ready in 1959, the physicists of the Institute, and particularly Professor Gonella, were able to use it for many experiments. Gonella had also been active in installing and commissioning the machine. Together with my BBC colleague Nabholz, we subsequently wrote a report on the successful operation of the machine [Go64]. It contains many interesting details. More than anything it was important for us to demonstrate that such a machine had proven itself in practice, and furthermore, that it was relatively simple and cheap.
Even simpler and compacter linear accelerators were
developed later for this range of energy and these have pushed
aside both the betatrons and the small synchrotrons. Today these
linacs dominate the market. However, developments are still
possible and I assume that better and more compact machines will be
built in future.