Life and Work of Rolf Wideröe by © Pedro Waloschek, => Contents
Specially shaped boxes, usually cylindrical, were soon developed, based on the tanks devised by Alvarez. Known as `resonators', these could be made to oscillate at high frequency. Because both ends are held on earth potential, a standing electromagnetic wave is produced inside, which accelerates electrically charged particles as they fly past at the right moment.
Today, many accelerating cavities are built this way and are then used in both linear and circular machines (such as synchrotrons and storage rings). Acceleration corresponds to several hundred thousand volts per metre of resonator structure.
Nowadays, resonators, which have their internal surface cooled to 4K are in use and, since they are made of particular materials like Niobium, they become superconducting. A higher accelerating voltage (gradient) is achieved (several million volts per metre during continuous operation) and heat losses are much reduced.
However, a second type of accelerating tube was developed in parallel, in which the high frequency power is introduced at one end and withdrawn again at the other. This causes a `travelling wave' to form internally which is also capable of accelerating electrically charged particles flying through at the right time. This complies with Ising's original idea [Is24], but can only be made to work by providing the internal surface of the tube with a very particular shape - as Wideröe had already realized in 1927. A usual type of such tubes is called `iris-loaded waveguide' (see Fig.4.4).
It was possible to achieve acceleration gradients of 17 MeV per metre in normally conducting linacs of several kilometres length, like the Stanford Linear Accelerator. Almost twice as much is realized in small machines, such as those used today for medical purposes.
High frequency technology, which was developed to
serve radar and television as well, is the prerequisite for operating
all these accelerating devices. High performance transmitters with
a frequency between 300 and several thousand MHz are
utilized, thereby employing very large transmitter tubes with power
outputs reaching the megawatt region. The electromagnetic waves are
sent into the cavities through specially designed `wave guides'
(accurately shaped metallic tubes), instead of cables.