IV.

Betatron

When electrons are accelerated, they undergo a large increase in mass at a relatively low energy. At 1 MeV energy, an electron has three times as much mass as an electron at rest. Synchrocyclotrons cannot be adapted to make allowance for such large increases in mass. Therefore, another type of cyclic accelerator, the betatron, is employed to accelerate electrons. The betatron consists of a doughnut-shaped evacuated chamber placed between the poles of an electromagnet. The electrons are kept in a circular path by a magnetic field called a guide field. An alternating current is applied to the electromagnet, and the electromotive force induced by the changing magnetic flux through the circular orbit accelerates the electrons. During operation, both the guide field and the magnetic flux are varied to keep the radius of the orbit of the electrons constant.