A particle accelerator like a cyclotron, but which operates at variable frequency to account for the particles gaining energy, allowing for greater energies to be achieved.
2004: The synchrocyclotron was able to accelerate protons to sufficient energies that collisions with nuclei produce pions, the lightest particles that, we now know, are made from a single quark and an antiquark. — Frank Close, Particle Physics: A Very Short Introduction (Oxford 2004, p. 50)