Advanced Cyclotron Systems

Transcription

In this video, we will touch on the way a cyclotron works. It is a machine used to accelerate charged particles along a spiral path to high voltages or energies. Cyclotrons usually are within a bunker or vault used for radiation shielding to protect people outside when the machine is operating. The walls are made of concrete approximately 2 m thick. This cyclotron was designed and manufactured in Belgium. It is a fixed energy machine that accelerates negatively charged hydrogen ions up to an energy of 18 MeV. It has a weight of approximately 20 tonnes. If we look inside the cyclotron, we can see some of its major components. The upper yoke of the magnet with its 4 south poles, the lower yoke with its 4 north poles, the ion source used to generate and introduce the ions, and the cyclotron dees that are used to apply the electric field to the ions for acceleration. Firstly, we introduce high purity hydrogen gas to the internally located ion source which is subjected to a strong electric arc. Negative hydrogen ions are created which are essentially hydrogen ions with a second electron added. Electrodes called dees are used to accelerate the negative hydrogen ions by the creation of an electric field using radiofrequency power. There are two high voltage dees each consisting of two plates, and four ground potential or dummy dees in this cyclotron. An electric field is created due to the potential difference between electrodes, allowing the ion to be accelerated between the dee and dummy dee. So the negative ion is pulled into the dee entrance between its two plates when it's at a positive potential. A negative potential is then applied pushing the ion away. This process is then repeated at the other high voltage dee. So by alternating the dee voltage at a fixed frequency, reaching four zones of acceleration in the cyclotron, and a magnetic field keeps the ion travelling in a circular path. As the ion moves in the presence of the electric field and the magnetic field, it experiences a force called the Lorentz force which some people may call centripetal force. The particle also experiences centrifugal force that, in the cyclotron, is equal but in the opposite direction. Therefore, we have a circular motion of ions in the cyclotron's uniform magnetic field. However as the ion gains kinetic energy with each turn, a spiral path is taken. The increase in radius is large initially, and then becomes smaller until the path overlaps. This is because the cyclotron's magnetic field is fixed in time, but the distribution in space can be changed. This magnetic field is called isochronous. This type of magnetic field ensures that the accelerated ions are in sync with the dee voltage and therefore keeps accelerating the ion as the kinetic energy increases. After approximately 300 turns, the ion is at the maximum radius of its trajectory. It passes through carbon foil or stripper, which removes the electrons thereby forming a positively charged proton. This change of charge, changes the pathway of the ion out of the circular orbit straight into a beam line or directly into a target. This process is happening rapidly, so a beam of protons is now achieved irradiating a target. The material from the irradiated target can now be transferred by remote control into a shielded work station or hot cell for processing and other uses.