Travelling Wave Linear Accelerators

Communicated b y y. D. Cockcroft; MS. ABSTRACT. Acceleration of electrons by means of an electroma,onetiic wave travelling down a circular corrugated tube is considered. The relevint properties of corrugated tubes are derived and the factors influencing electron enxgies. Some numerical results are also quoted for a wavelength of 10 cm. The problems of high energy accelerators arc &scussed briefly and iIlustrated by calculations whici have been made for ci 500 M ~ V. accelerator. P accelerators using radio-frequency accelerating fields had been previously reported (Sloan.and co-workers 1931, 1934), and it was apparent that as a result of wartime advances in centjmetre-wave magnetrons and waveguide technique, linear accelerators compared: 'favourably with other forms of particle accelerators. Work began on electron accelerators, electrons being chosen both because of the technical simplidit3 associated with them and because there were important practical a6plications for comparatively low energies, 20 to 50 MeV., in the field of medical therap?. ?he type fi:st investigated was that in which-electrons are accelerated by elqctroq3gnetic waves travelling down a circular corrugated wave-guide ; the successful operation ofqan experinxntal model has been reported by Fry and co-worQers (1947). 1 '-'r+ '" '-" The object of the present paper is to set out the main factors influencing the design of a travelling wave accelerator. Although it is not proposed ro make detailed comparisons with other types of linear accelerator, mention will be made of some other methods which are available. The most important and obvious alternative is to accelerate the electrons by passing them through a scries of resonant cavities excited from a radio-frequency source. Successful experiments using this method have been reported from several sources It appears that these methods can yield energies comparable with the travelling-wave system, given the same amount of radio-frequency energy and the same overall length of accelerator (Walkinshaw 1947). There are, however, important practical differences not fully resolved, and neither type has as yet been proved superior to the other. Another type of combined travelling-wave and resonator accelerator using a rectangular corrugated guide has been investigated by Mullett (1946) but was not pursued, mainly on account of difficulties of manufacture of the necessary equipment. Travelling-wave systems using various configurations of bent waveguides have also been considered and rejected for reasons of difficulty with focusing, practical inconvenience or sometimes ineficient accelerating performance, for example the coiled-up waveguide (Hudspeth 1946). Other variations of these methods and a …