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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.
Much of this is incorrect. Superconductors have zero DC resistivity when below their load line because the Cooper pairs carry the current. The unpaired electrons do no participate. In AC superconductors at high frequency, the unpaired electrons react to the AC fields. The fraction of unpaired electrons declines exponentially as one gets away from the critical temperature, lowering the residual resistance of the cavity, but there is an irreducible residual resistance at all temperatures. Cavities are operated below 2.17K, the lambda point of superfluid helium, because the heat transfer is orders of magnitude greater than in normal liquid helium.
Thank you for your comment! To keep this explanation brief and simple, you are quite right that I’ve neglected to explain Cooper pairing and how superconducting metals work (which hopefully will find its way into a future LCpedia entry). However, I hope this explanation, although very simplistic, is still consistent with the basics of superconducting radio-frequency.Thanks again for your more detailed explanation.
I’m curious about SCRF, the four-letter abbreviation. I do see that usage sometimes, but more often in the 27 years during which I’ve been paying attention to accelerator technology, I’ve seen superconducting radiofrequency technology called not SCRF, but just SRF. Maybe others will comment on this admittedly fine, but maybe not entirely trivial, point. Thanks.
Dear Steven,
funny you should point this out; we had recently raised the same question in a different context and had decided to use ‘SCRF’ rather than ‘SRF’. As far as we understand, the terms that these acronyms stand for are identical and it’s a matter of taste, or habit, to use one or the other. We should have probably pointed this out in the original article though, so thank you very much for raising the point. If however there are subtle differences between SRF and SCRF that we were not aware of we are happy to learn!
Thanks. Allow me please to be a bit parochial, though with grounding in historical fact. In 1985, CEBAF (at the lab later named for Thomas Jefferson) was approved as a room-temperature, conventional-technology linac and pulse stretcher ring. But Hermann Grunder, the director, and others had the vision — and took the big risk — to get it switched to what we always called SRF, which was then proven only at laboratory scale, not at large scale. That was 27 years ago. Ever since, we and others have called it superconducting radiofrequency (SRF) technology. (And by the way, I never saw omitted a noun like “technology” or “science” from the end of the phrase.) It’s true that over the years — and especially in recent years when others have begun to profit from the original success of Hermann’s risk — the phrase SCRF has cropped up from time to time. And it’s also true that at some level it doesn’t matter, as long as readers and listeners understand what’s being discussed. But the fact remains that when Jefferson Lab adopted Cornell’s technology, it was universally called SRF, both then and later. I’m just not sure that a quite late change in the common parlance is wise — though again, it may not matter much.