Department Proton Therapy
Our main accelerator is a k=130 isochronous separated-sector cyclotron. The particles are accelerated with electric alternating fields to the desired energy. Our cyclotron has two high-frequency systems for the acceleration and four magnets for bending the beam: Thus, the particles are brought to a path around the centre of the accelerator. Hence the name circular accelerator, albeit the particles follow a spiral through the machine. The ions from the pre-accelerators are injected near the centre and brought to their first turn. Each magnet deflects the ions by 90 degrees, bringing them repeatedly to the high-frequency systems (dees). Due to the acceleration in the dees the bending radius increases until the beam reaches the extraction point. Here the beam is extracted from the cyclotron and delivered to the irradiation room.
The cyclotron is an energy multiplicator with a fixed factor of 17. The very variable high-frequency system and the variable magnetic configuration permit kinetic energy from 1 MeV/u up to 72 MeV/u. The cyclotron provides a pulsed beam with a repetition rate between 10 MHz and 20 MHz (cyclotron frequency) and a pulse width of less than 5 ns. The high repetition rate permits both a quasi DC beam as well as the possibility of time structures.
There are two injectors available and they are chosen in dependance of the requirements. Both injectors are electrostatic accelerators.
The Van-de-Graaff-accelerator with a maximum high voltage of 6 MeV provides beams with extremely good sharpeness of energy. The ECR-(Electron-Cyclotron-Resonance) source developed for the high voltage pole produces both high and low charging status.
The Tandetron has a maximum high voltage of 2.05 MV which is used twice to full capacity: negative hydrogen ions are extracted from the Duoplasmatron ion source and accelerated towards the high voltage terminal. There the electrons are stripped and the positive hydrogen ion is accelerated a second time.
Due to its highly stable beam the Tandetron is typically used for the therapy.
Behind every injector there is an electric kicker that takes the beam rapidly away from the accelerator path. It provides pulse up to 50 ns with a maximum repetition rate of 2 MHz.
All along the beam line different collimators (round apertures, rectangular slits, pepperpots), diffusors and quadrupole magnets can trim, scatter and focus the beam to adjust its form, intensity and homogeneity.
The regular adjustment offers a pronton beam with a kinetic energy of 68 MeV, beam size from 1 mm (focussed) up to 35 mm (scattered) plus intensities higher than 100 nA.