dee

…two hollow semicircular electrodes, called dees, mounted back to back, separated by a narrow gap, in an evacuated chamber between the poles of a magnet. An electric field, alternating in polarity, is created in the gap by a radio-frequency oscillator.

… is applied by electrodes, called dees from their shape: each is a D-shaped half of a pillbox. The source of the voltage is an oscillator—similar to a radio transmitter—that operates at a frequency equal to the frequency of revolution of the particles in the magnetic field. The electric fields caused…