Life and Work of Rolf Wideröe by © Pedro Waloschek, => Contents
14 Some Retrospectives and Dreams
As I speak about my life, I find that I return frequently to a few very special events which I now consider to be the most important stages of my work. While I was actually involved with these things I wasn't really aware of their relevance or future importance, since everything I did generally gave me much pleasure and I always concentrated completely on whatever I was doing. Thus I built relays with just as much enthusiasm as I later constructed betatrons. And I was always particularly interested and motivated if there were new ideas involved.
However, what always comes to my mind first is the Aachen drift-tube. Proving that it was possible to accelerate electrically charged particles with alternating potentials and without having to use the restricted possibilities of the (at that time, usual) d.c. voltage, appears to me as my most fundamental piece of work. This was the major result which I presented in my dissertation in 1927 and it does appear to have had the most far-reaching consequences. Added to this was the happy circumstance that this work was widely disseminated and well known everywhere. It is definitely one of the most quoted publications on particle accelerators.
The `bent drift-tube' appeared first in Lawrence's cyclotron and later in the accelerating cavities of the synchrotron. The latter now seems much more important to me because the synchrotron formed the basis of storage rings. My discovery of the stabilized particle orbits in synchrotrons might also have been quite important. However, the further development of the drift-tube which took place at almost the same time as the cyclotron, starting with Alvarez' resonators, via the cavities with standing waves and finally resulting in the iris-loaded wave guides with travelling waves of modern linear accelerators, is also very interesting. All this began in 1927 with the first drift-tube in Aachen.
The 1943 patent containing my invention of storage rings [Wi43a] was probably very important but it was kept secret for ten years. As I could not see any practical use for it myself (there were still too many technical problems which needed solving), I did not speak much about it. I was not to explain my proposals again until the 1956 accelerator conference in Geneva [Wi56], after Kerst and O'Neill had rediscovered the principle. However, others took the development further while I was fully occupied building betatrons. I am therefore very pleased that I had had the right idea thirteen years before my colleagues, but I can't blame them if they've sometimes forgotten me, since they would have often spent years working on these projects. Many very beautiful storage rings were built while I was busy with other problems.
I think it is pretty clear from my story that I was deeply committed to my work with relays. I guess my contributions to this field were quite good and I believe that my relays were also very useful. Although it might not be of great interest to particle physicists and doctors, this work was creative and I am rather proud of it.
I endow my work in the field of radiation therapy with a certain amount of status. It is on this subject that I had the opportunity, for the first time in my life, to be active as a scientist at a highly regarded institution, the ETH. This was a completely new experience for me and I was able to let my imagination run free without having to take into consideration the interests of an industrial company. It must be said however that BBC, who were still employing me, were not at all opposed to my lecturing, because I was in some way contributing to the sale of betatrons. I hardly published any technical articles or applied for patents during this period, but concentrated instead on writing for scientific periodicals and giving lectures.
See also Box 15: Widerröe's Life at a Glance
My increasing interest in radiation therapy was a logical continuation of the war against the tumour cells with our new weapon: the megavolt beam. After all, the patients needed urgent help and I took part in this with a great deal of enthusiasm.
However, while I was busy with all these other things I never lost sight of particle accelerators. I kept up to date by reading periodicals and speaking with my many friends.
That is how I came to follow the exciting development of cyclotrons while still in Berlin, through the news which Ernst Sommerfeld used to bring to me from his father. Of course the situation was rather more difficult during the War, but from about the end of the 1940s onwards a completely new scientific spirit took over. Communication between scientists became desirable. Unrestricted travel, mutual visits and international conferences meant that people knew just about everything that was happening in their field. One even knew most of the participants on a personal level, which was essential for the impressive progress in the field of particle physics and the structure of the smallest particles of matter.
Nowadays it is easy to keep quite well informed on many areas of research, as long as one has enough spare time for reading - and a few good friends. So, even after my retirement, I could not refrain from studying the basic problems of particle acceleration. Only through experiments at even higher energies will we be able to obtain new knowledge which should finally lead us to a comprehensive theory of the structure of all kinds of matter.
Well, after the successful eras of cyclotrons, synchrotrons and now storage rings, we have gone back to basics: Experts agree that probably there will be no bigger rings in future and that linear accelerators will be built instead. I have already mentioned the reasons for this: Electron and positron rings are limited by their synchrotron radiation, and proton rings are disadvantaged by their need for stronger magnets and by the cost of gigantic rings. After all, plans can only be made for those accelerators which can realistically be built with the means available, and obviously, these means are limited.
Ideas are not subject to any such considerations. The limitations are set only by the intellect of human beings themselves. The theoretical possibilities with regard to accelerating particles by electromagnetic means (i.e. within the scope of the Maxwell equations which have been known since the 19th century), are nowhere near being exhausted, and technology surprises us almost daily with innovations which in turn allow us to broach new trains of thought. Although many of the ideas in this field which appeared over the last decades were not successful, it is possible, in principle, that there are yet more fundamental breakthroughs to be made. They could allow us to advance to energies unimaginable today. We have to remember that the things we build today appeared utterly utopian 50 years ago.
I would like to mention such an alternative as a vision of the future, not because I am fully convinced that it is good or correct, but because I consider it important that we maintain our confidence in further developments, however adventurous they may appear.
This story begins in 1956 at the International Conference on Accelerators in Geneva where Veksler presented a report on a very peculiar idea which rather impressed me. A fast bunch of particles was to be made to `meet' or `overtake' a slower bunch of other particles and thus `sweep it along' in its path. He indicated a number of possibilities. As some of Veksler's statements did not seem quite right to me, I thought the matter over and wrote down my results in April 1986. Veksler had christened his methods `coherent acceleration'. This name is apt since the particle bunches have to act on each other as entireties, i.e. they must be `coherent', and the individual particles must not interact. During ordinary acceleration we look at individual particles. We do not consider effects which affect the entire bunch until later, when the orbits are being corrected - not during the acceleration process itself.
I had come to think that it would be best if a bunch of protons could be `hit' from behind by a bunch of positrons (104 positrons for each proton), and in my considerations I just took as an example the data of the particle bunches which could be available at the HERA rings in Hamburg, i.e. 800 GeV protons and 30 GeV positrons. Measured in the rest frame, the positrons will have an energy of 17.1MeV. The results are rather startling. It becomes possible to accelerate the proton bunches so that each proton has an energy of several hundred TeV, and under the best conditions even over one thousand TeV. It is therefore possible to achieve extremely high energies by coherent scattering of particle bunches. In comparison, the protons in the LHC storage ring proposed at CERN would reach no more than 8 TeV.
For my 1986 calculations (and an addendum I made with J.F.Crawford) I had to assume certain bunch sizes and I also mentioned many of the difficulties that may be expected (We never published these considerations - I just sent a few copies to my friends). The major factor for the realisation of this method is the size of the particle bunches. I used the actual dimensions of the HERA bunches, which are several centimetres long, a few millimetres wide and only a couple of tenth of a millimetres high. However, the coherent scattering principle would work much better if it were possible to make the bunches much smaller. At the time I made my calculations, this was still thought to be unrealistic.
And this is really where the point of my story lies. In 1992 I was informed of the new plans for linear accelerators to be built in the future, since storage rings have now reached their limits. Much higher collision energies should be achieved in future, but as a first goal, electrons and positrons would be shot against each other using two linacs, each one providing particles with an energy of just a few hundred GeV, a value not at all accessible to storage rings for this type of particles.
See Box 16: Wideröe's Memeberships
The things that interested me most however, were the dimensions of the particle bunches in these linacs. I was very surprised when I heard that the aim was to have bunches which were about a factor one thousand smaller than those available with today's machines. This is the only way to achieve reasonable collision rates - a conclusion I had already reached in 1943, when I invented the principle of storage rings with colliding beams - just to overcome this difficulty. If in the past we have considered a few tenths of a millimetre as possible transverse beam dimensions, we are now talking about tenths of micrometres. For some projects, people are even speaking of hundredths of micrometres, which is the same as ten nanometres. The particle bunches which are going to interact coherently will have to be localised in space and steered with even greater precision. When this precision is achieved, it will perhaps be possible to think of other mechanisms, apart from `coherent bunch collisions', with which to accelerate particles to extremely high energies.
However complicated and utopian all this may seem to us now, it would undoubtedly be of great interest for physics research, if protons with 1,000 TeV were available. Today, this kind of energy can only be found in cosmic radiation, that is, in particles arriving from intergalactic space - and then, only very rarely.
It would be easy to come to the conclusion that the builders of accelerators who follow such fantastic ideas were completely mad, if we had not all been party to the developments of the last decades: A few years ago no technically versed person would have believed that the precision which is now used in the production of millions of CD-disks' would ever be possible. This example shows that we should never lose courage and that we must continue to aim for goals which lie far beyond us, even if they are still absolutely held to be at times unattainable.
With this I shall end this story of my life, but not before I
have thanked the readers for having made it this far and for their
interest and patience.