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The ESS Accelerator for the Non-Expert

  1. Rapidly varying electromagnetic fields heat hydrogen gas in the ion source so that electrons evaporate from the hydrogen molecules. The principle is similar to boiling water in a microwave, however the frequency is considerably higher and the effect is stronger.  Hydrogen is the simplest element of nature and consists only of protons and electrons. Therefore, when the electrons have evaporated a plasma of protons remains. 
  2. The protons are guided from the ion source into the accelerator beam line, which consists of beam pipes and accelerating structures, both of which are under vacuum. Accelerating structures, which are distributed all along the linear accelerator and take up most part of it, kick the protons forward by electromagnetic fields. The process can be thought of as a surfer on a sea wave, except for the fact that the electromagnetic waves accelerate with the beam particles, in difference to sea waves that travel with constant velocity.
  3. There are magnets around the beam pipes in between the accelerating structures to focus and to steer the beam into the right direction. In the the accelerator closest to the ion source, where the particles travel at a low speed, it is necessary to simultaneously accelerate and focus the proton beam.
  4. After approximately 50 meters the protons have gained enough speed so they can be accelerated by superconducting cavities. These cavities are cooled by liquid helium to -271 °C. 
  5. The protons reach 96% of the speed of light before they hit the target.