How do the hyperfrequency cavities used in accelerators work?
Electric field distribution in a resonating cavity
RF cavities are resonators (kind of like the sound boxes of musical instruments) that enable to store and amplify the electric field used to accelerate a beam of charged particles in an accelerator.
In order to get accelerating fields of about 45 millions volts per metre, a radiofrequency wave must be injected in the cavity. Currents from 10 to 1000 billions of amperes per square metre flow on the internal surface of the cavity and heat it. We could not get such strong continuous fields with a normal conductor: the surface of the cavity would start melting !
This is not what happens with a superconductor. Even if in this case, it does not conduct perfectly because it uses alternating waves, radiofrequencies, the resistance of a superconductor is still 100 000 times smaller than that of copper, hence the main interest of this technology for accelerating cavities. This is not the only advantage: the use of superconducting cavities also influences the design of the accelerator and the quality of the beams obtained. Here is an example: their shape, more open, makes the lining of the beam easier; when the latter has to reach several dozens kilometres, it becomes a substantial argument !
M. Wilson, Superconducting magnets for accelerators: a review, IEEE Trans. Appl. Superconductivity 7 (1997) 727: http://cdsweb.cern.ch/record/316262?ln=fr
Ph. Lebrun, Superconductivity and Cryogenics for Future High-Energy Accelerators, CERN/AT 2007-4 : http://cdsweb.cern.ch/record/1026936?ln=fr
Ph. Lebrun, Cryogénie et supraconductivité pour le grand collisionneur de hadrons (LHC) du CERN, CERN-LHC Project report 802 (2004) : http://cdsweb.cern.ch/record/811057?ln=fr
CERN Accelerator School on Superconductivity and Cryogenics for Accelerators and Detectors
8 - 17 May 2002, Erice, Italy : http://cdsweb.cern.ch/record/503603
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SACM website: http://irfu.cea.fr/Phocea/Vie_des_labos/Ast/ast_service.php?id_unit=2
