The proton accelerator COMET
Where do the protons come from? For radiation therapy with protons, PSI uses its own proton accelerator, the compact superconducting cyclotron COMET (COmpact MEdical Therapy cyclotron). The accelerator contains an ion source in which hydrogen atoms are continuously broken down into negatively charged electrons and positively charged protons in fractions of a second. An electronic field sucks the protons into the cyclrotron, which is also known as a ring accelerator. There, the protons rotate 630 times on a circular path before they are guided through a vacuum tube where they are bundled into a 5 to 7 mm wide beam with the help of magnets and guided at two-thirds the speed of light about 50 metres to one of the therapy facilities, the gantries. A kicker magnet at the output of the cyclotron is used to switch the beam on and off quickly. The switching time is 200 µsec from the trigger signal of the therapy control system to the disappearance of the beam signal at the treatment site. Additional switch-off elements (safety function) are the reduction of the cyclotron high frequency, the switching off of the ion source as well as several mechanical beam stoppers.