4
E. Wilson and B. J. Holzer
Fig. 1.1 Van der Graaf
accelerator
Fig. 1.2 Wideröe’s sketch of
the ray transformer
1.2.2 The Ray Transformer
The earliest idea of how to overcome the limitations of electrostatic acceleration
involved using the time varying property of magnetic fields and came from the
inventive mind of Rolf Wideröe.
Beginning his studies at Karlsruhe Technical University in 1923, he wondered
if electrons in an evacuated ring would flow in the same way as the electrons in
copper if they replaced the secondary winding of a transformer . His notebooks of
that time contain sketches of a device he called a “ray transformer”; the first circular
accelerator and the precursor of the “betatron” [5].
These sketches show a beam tube, in the form of an annulus, R, placed in the
gap between the parallel poles or faces of a small electromagnet (on the left in Fig.
1.2). This magnet is in the form of a “C” and the field between the poles, B z , guides
particles in a circular orbit in the mid plane between the poles. A circular hole is cut
in each pole through which the yoke of the transformer passes linking the beam tube.
The primary winding of the transformer, labelled W 1 , is powered with alternative
voltage from the mains. The beam tube is placed where one would normally expect
the secondary winding. The beam within it carries the induced secondary current.