The linear accelerator
Another type of accelerator is shown in Fig. 47.9. It is called a linear accelerator and consists of a series of co-axial tubes connected alternately as shown in Fig. 47.10 and given an alternating potential difference from a very high frequency a.c. source.
A narrow beam of particles is injected into the tubes from a suitable ion source and the frequency of the a.c. voltage is adjusted so that it reverses direction while the
particles are traversing a tube. They are thus given an impulse and accelerated across the gap between tubes. Inside a tube they coast along with uniform velocity, since there is no electric field inside a hollow charged tube. By this method particles can be speeded up to very high energies: Its main advantage is that insulation problems are minimized. The particles are accelerated by a series of small potential differences instead of one or two large ones as in the Van de Graaff and Cockcroft and Walton methods respectively.