Superconducting RF accelerating cavities of different frequencies are presently operating on various accelerators world wide. The large majority of these cavities are made of niobium sheet, by lathe spinning and welding. In one case only, i.e. LEP2 at CERN, the majority of cavities is made of copper, internally coated with a thin layer of sputtered niobium. The main motivation for developing Nb-coated copper cavities must be found in the higher stability against quenching, consequent to the higher thermal conductivity of copper at liquid helium temperatures. Additional advantages are higher Qo values at low fields, insensitivity to trapped earth magnetic field and lower cost. Beside these advantages, the Nb-coated cavities unfortunately suffer from a faster Qo degradation with increasing the accelerating field. The performances of Nb-sheet and Nb-coated cavities are critically compared on the grounds of the large industrial production required by the LEP2 project.
APPLICATIONS OF SUPERCONDUCTING CAVITIES TO PARTICLE ACCELERATORS
Due to their higher accelerating field and lower power dissipation, superconducting cavities (SC) are increasingly used to accelerate particle beams for high energy physics experiments. Typically, fields in the range of 10 MV/m may be produced at mains consumption levels about two orders of magnitude lower than for normal conducting cavities (NC). The low power losses render SC's particularly suitable for continuous beam operation, which provides a higher signal to noise ratio in the particle detectors. Superconducting cavities also present a much lower beam impedance and thus allow both higher beam intensity and machine luminosity.
The practical relevance of these advantages depends on the accelerator type, as discussed in the following.
Continuous wave (CW) linacs and recyclotrons
To compete with conventional pulsed tinacs, high accelerating fields (> 15 MV/m) in CW mode would be required. Since such fields are not easily obtained, to avoid an expensive length increase of the machines the b~ams are recirculated via a racetrack magnetic system and reinjected into the linac several times (recyclotron).
Typical operating frequencies are in the range from 1 to 3 GHz. The pioneer of these machines, the Stanford recyclotron at HEPL [1], is still in operation as a free electron laser (FEL). The newcomer Darmstadt recyclotron [2] is exploited at 130 MeV for nuclear physics and as a FEL. CEBAF with a superconducting 4 GeV recyclotron as the heart of the laboratory will soon become operational [3]. Other examples of machines of this type are