For CERN’s new LHC proton collider, superconducting magnets will not be the only super- conducting technology in the 27 km ring.
When the collider was commissioned in 1989, the energy of CERN’s LEP electronpositron collider was 50 GeV per beam. After a dedicated period of running around the Z particle resonance, LEP’s energy has been increased to 100 GeV per beam.
Behind the success story is the conversion of the machine from conventional radiofrequency to superconducting cavities to feed accelerating power to the circulating beams. Early on, research and development work for LEP showed that cavities made of niobium-coated copper were more effective than those of the more expensive solid niobium. The LHC is set to use this technology from the outset.
The LHC’s radiofrequency must be a multiple of 200 MHz, the operating frequency of the upstream SPS synchrotron, to allow rapid transfer of many SPS proton bunches, but not so high as to make for operational incompatibility. Thus it is set at twice that of the SPS.
The radiofrequency scenarios for LEP’s electrons and positrons, and the LHC’s protons, are very different, even though they both use a 27 km ring. Electrons, being very light particles, lose a lot of energy per turn by synchrotron radiation, which has to be replaced continually by the “accelerating” cavities. Most of LEP’s radiofrequency power is transferred to the beam and then dissipated by synchrotron radiation.
Proton beams lose little energy in this way. The main role of the LHC cavities is to keep the many proton bunches tightly bunched to ensure high luminosity at the collision points and to deliver power to the beam during energy ramping. Matching these radiofrequency conditions using conventional copper cavities would lead to unacceptable displacement of the beam crossing points.
Superconducting cavities with small losses and large stored energy are the best solution. This leads to a design using single-cell accelerating cavities with large beam tubes, similar to those considered for the new generation of electronpositron colliders.
The LHC will use eight cavities per beam, each capable of delivering 2 MV (an accelerating field of 5 MV/m) at 400 MHz. The cavities will operate at 4.5 K (the LHC magnets will use superfluid helium at 1.8 K). For the LHC they will be grouped in fours in cryomodules, with two cryomodules per beam, and installed in a long, straight section of the machine where the interbeam distance will be increased from the normal 195 to 420 mm. The cavities are being made by spinning and electron-beam welding, with the surface niobium being added by magnetron sputtering.
For LHC cavities, an ingenious mechanical tuner has been designed and successfully tested, to cope with the larger detuning range of the LHC cavities and their increased stiffness (compared with LEP cavities).
The experience gained with LEP couplers, which were once a very critical element of the LEP2 project, has led to the design of state-of-the-art LHC couplers, which link the cavity to the RF power system.