How do you accelerate particles in a particle collider? One answer is superconducting radio-frequency (SCRF) cavities. To give particles energy as they move through an accelerator, physicists use cavities containing electric fields that oscillate. The changes in electric field help push the particles from one cavity to the next. These oscillations occur with the same frequency as radio waves, which is why this form of acceleration is called radio-frequency.
Superconducting refers to the way in which electric current is carried through these accelerating cavities. Electric current in a cavity may create friction—unless the cavity is created using special metals called superconductors. “Some metals have no resistance below a critical temperature,” says Fermilab scientist Camille Ginsburg. This means that these metals conduct electricity perfectly. Even in a superconductor, if electric current passing through a cavity encounters any bumps or impurities, the flow of electricity is interrupted and energy can be lost as heat. This is why cavities must be very clean and polished to a smooth finish. In proposed accelerators such as the ILC, the metal used is niobium, which becomes superconducting at temperatures below 9.2 Kelvin (-264°C). Keeping cool isn’t easy, however. To do this, each cavity is kept in a large thermos structure holding frigid liquid helium, typically at 2 Kelvin (-271°C).
There are a number of benefits to SCRF, explains Ginsburg. Among the most important are energy-efficient operation and a shorter accelerator than is achievable with conventional room temperature cavities.