Linear Electron Accelerators

Talk by Mr. S.G. Hulbert (member) at Adult Education Centre, Worthing on Friday 5th November, 1982.

Mr. S.G. Hulbert gave a talk on the Linear Electron Accelerator or linac. He defined the Linac as a machine designed to propel electrons to very high energies, often very much greater than could be achieved with any realisable voltage gradient. The need for high energy electrons arises in research into the structure of matter and in the production of very hard X-rays for medical and industrial use.

The basic idea is to pass a very high frequency radio wave along an evacuated tube (Wave-guide), under which conditions an axial component of the electric vector is produced and this electric field is made to accelerate the electrons. This is possible if the electrons can be made to hold their positions relative to the wave and so the phase velocity of the wave and the electron velocity have to be matched. This enables the electron to surf-ride the wave. In an unobstructed wave-guide the phase velocity of the rf wave is greater than the velocity of light. Reduction of phase velocity is brought about by inserting a series of irises along the tube.

The source of high frequency energy considered was the klystron and a short description of this device was given. It was mentioned that typical frequencies were in the range 3-10GHz and powers of around 30MW were used. For such high powers it is necessary to operate the equipment intermittently so pulse techniques are employed.

The power from the Klystron is fed into the linac. It was shown that by judicious tailoring of the wave considerable bunching of the electrons could be achieved. Once high speed had been reached the relativistic increase in mass led to the bunched electrons being in a stable position relative to the wave.

Some of the practical problems in construction and operation were dealt with. At the very high frequencies involved the skin depth is of the order of one micron and so a very high surface finish is required. Certain critical dimensions have to be accurate to within 5 microns. High electrical conductivity is required to avoid wastage of power. This also implies perfect electrical contact between irises and wave guide tube. Some ways of achieving this were mentioned.

There followed a series of questions and answers on many aspects of the subject. Mr. Beeken initiated the final phase by asking why in the figures given by Mr. Hulbert, electron mass increased with voltage. The usual formula for this effect, quoted in reply, served only to provoke further lively exchanges, led by Mr. Lionel Hill, in which the reality of this and other relativistic effects was questioned.

A vote of thanks was proposed by Mr. L. G„ Hill, who commented upon “the hidden talent among the Association Memembership.”