Energy degrader enabling high-speed energy switching

A particle therapy system comprising:

• a particle accelerator to output a particle beam;
• a scanning magnet to move the particle beam;
• a control system;
• and
• an energy degrader comprising multiple plates, the multiple plates being controllable by the control system to change an energy of the particle beam by passing the particle beam through one or more of the multiple plates to an irradiation target, the multiple plates comprising:
• a first plate that is controllable by the control system to move across at least part a beam field in a first direction while the particle beam is incident on a surface of the first plate in the beam field and while the particle beam is moved by the scanning magnet across the surface of the first plate, where movement of the particle beam across the surface of the first plate is at least partly in the first direction, and
• a second plate that is controllable by the control system to move across the at least part of the beam field while the particle beam is moved across the surface of the first plate, the second plate being controllable to trail or to lead the first plate in the first direction such that the particle beam does not pass through the second plate during at least part of movement of the first plate and the second plate.