Rhodotron Operating Principles

Rhodotron cavities are shaped as a coaxial line shortened at both ends and resonating at 107.5 MHz or 215 MHz. The beam crosses the cavity in the median plane through successive diameters. External window-frame magnets are used to bend back the electrons emerging from the cavity and to redirect them toward the cavity centre.

A high power RF system using a tetrode produces the electric field allowing an energy gain of 0.833 or 1 MeV per crossing. Ten or twelve crossings of the cavity (which means nine or eleven bending magnets) are required in order to obtain 10 MeV electron beams.

The cavity diameters of both the TT200 and TT300 is 2 meters. In these conditions, energy gain is 1 MeV per crossing. In these Rhodotron models, five and ten successive crossings are therefore needed to obtain respectively 5 and 10 MeV at the exit of the Rhodotron. A smaller cavity (diameter of 1 meter) was selected for the TT100 Rhodotron 10 MeV / 35 kW guaranteed power in order to have a very compact design. Energy gain of the TT100 is 833 keV per crossing. Therefore, 12 crossings are necessary to reach 10 MeV at the exit of this accelerator.

The electron gun is located at the outer wall of the accelerating cavity. In order to match the RF frequency (the electrons have to be injected into the cavity when the field is accelerating), the gun is pulsed at the RF frequency and the pulse width is equal to 60° of the RF period. Electrons are injected into the cavity at a voltage of about 35 - 40 kV. The use of the grid allows to modulate the emitted current.

The RF system consists of a voltage controlled oscillator followed by a chain of amplifiers.

The role of the deflection magnets is multiple: first, they are needed to send the electrons back into the cavity after each diameter crossing in order for them to undergo another accelerating cycle. The magnets also contribute to the focusing of the beam. It is possible to position a straight beam exit at each port, i.e. at energies of 1 to 10 MeV, in steps of 1 MeV.