Conducting Foams for High Synchrotron Radiation Accelerators

Molecular gas desorption from the beam-pipe wall due to synchrotron radiation has been a major solved issue in the design of the LHC [1]. Nuclear scattering in the desorbed gas may limit the beam luminositity lifetime, and produce high energy protons causing thermal runaway and magnet quenching. The problem will be more serious for HL-LHC [2] and crucial for lepton Higgs factories [3]. In the LHC a copper-coated stainless-steel beam pipe liner kept at ≈ 20K by active Helium cooling handles the heat load of synchrotron radiation, photoelectrons, and image-charges [1]. A large number (∼ 102 m−1) of thin slots in the liner keep the desorbed gas densities below a critical level (∼1015 molecules/m3 for H2), by continuous cryopumping toward the co-axial stainless steel cold bore of the superconducting magnets, kept at 1.9K by superfluid He. Size, geometry and density of the pumping slots affect the beam coupling impedances [4]. The slot pattern should also be chosen so as to minimize the effect of trapped (cut-off) modes, and to prevent the coherent buildup of radiation leaking into the coaxial waveguide limited by the pipe and the cold bore [5]. Open-cell metal foams may help fulfilling such conflicting requirements of beam liner design.