High Tc superconductors for the future collider at CERN: Fundamental physics challenges materials science
CERN has recently started a Design Study for a possible next-generation high-energy hadron-hadron collider (Future Circular Collider – FCC-hh). The FCC-hh calls for an unprecedented center-of-mass collision energy of 100 TeV, colliding proton beams of 50 TeV steered in a 100-km circumference tunnel by 16 T superconducting magnets.
The number of new technologies and new approaches needed to build this machine is incredibly high, and many groups of scientists are already at work to try to solve the more complex problems
As an example, the proton beams emit synchrotron radiation at high power which, to optimize cryogenic efficiency, is absorbed by a beam-facing copper-coated screen held at 50 K. The surface impedance of this screen has a strong impact on the beam, and copper at 50 K does not allow sufficiently beam stability margins.
One possible approach is to use superconductors, however it is not obvious that an existing superconducting material can perform better than copper along a 100Km ring, at 50K, 16Tesla, in the presence of radiation.
In this lecture the general characteristics of the new CERN FCC accelerator will be first introduced. The need for a superconducting beam screen will be then discussed in detail. In the second part of the lecture, the surface impedance of superconductors in the foreseen operating conditions will be estimated using a simple phenomenological model. This will restrict the possible class of high temperature superconducting materials to be used in FCC. Finally, recent experimental results on YBCO tapes and electrodeposited TBCCO will be presented. The data show very good perspectives towards the realization of a high temperature superconducting screen, allowing high beam stability.