Before being injected into the main accelerator, the particles are prepared by a series of systems that successively increase their energy. The first system is the linear particle accelerator LINAC 4 generating 160-MeV negative hydrogen ions (H− ions), which feeds the Proton Synchrotron Booster (PSB). There, both electrons are stripped of the hydrogen ions leaving only the nucleus containing one proton. Protons are then accelerated to 2 GeV and injected into the Proton Synchrotron (PS), where they are accelerated to 26 GeV. Finally, the Super Proton Synchrotron (SPS) is used to increase their energy further to 450 GeV before they are at last injected (over a period of several minutes) into the main ring. Here, the proton bunches are accumulated, accelerated (over a period of ) to their peak energy, and finally circulated for 5 to while collisions occur at the four intersection points.