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Optimization of superconducting cavities for CW applications

Final Report Summary - CW_SRF (Optimization of superconducting cavities for CW applications)

A particle accelerator is a machine that uses electromagnetic fields to accelerate charged particles such as electrons close to light speed in well-defined bunches. Large accelerators are used in particle physics e.g. the large hadron collider (LHC) at CERN or as synchrotron light sources for the study of condensed matter physics, e.g. the European X-ray free electron laser (XFEL) in Hamburg (Germany). Smaller particle accelerators have a wide variety of applications, including healthcare, security and industry. Currently more than 30000 are in operation worldwide. To transfer electromagnetic energy to kinetic energy of charged particles resonant radiofrequency cavities are widely used. Many applications such as the LHC or the European XFEL require using superconducting materials for technical and economic reasons.
The energy gain per unit length of a particle passing through a cavity is expressed in terms of the accelerating gradient. The larger its value the shorter a particle accelerator has to be. Since for large scale projects the accelerator length is up to several tens of kilometers a technology enabling larger accelerator gradients can yield multi million euro savings in initial and operating costs.
The maximum achievable accelerating gradient is proportional to the maximum surface fields the resonant cavity can maintain. While superconductors allow for lossless DC current flow of fields up to several TESLAs, at alternating fields superconductors exhibit intrinsic losses which increase by orders of magnitude when magnetic flux penetrates the material. When the applied field exceeds the lower critical field Hc1 it becomes energetically favorable for flux to be located inside the superconductor instead of being expelled by the Meissner effect leading to penetration in the absence of an energy barrier at the surface. Hc1 is generally about one order of magnitude below the maximum fields achieved in DC applications. Therefore the material of choice for superconducting cavities is niobium the material with the largest value of Hc1 (approximately 170mT). Theory predicts that defect free superconductors can potentially remain in the flux free Meissner state above this lower critical field up to the so called superheating field. Experiments on RF cavities, e.g. made of Nb3Sn, show that the practical achievable critical field under RF exposure can indeed exceed the lower critical field. However, the maximum fields remain far below the superheating field. Several approaches have been proposed and investigated by institutes worldwide to understand and overcome the current limitation of premature flux penetration. In this project, it has been shown that an additional superconducting nanometer thin surface coating can potentially increase the energy gain of charged particles passing through a cavity by about a factor of two (T. Junginger et al. SUST 2017 30 (12), 125012).
To measure the field of first flux penetration a dedicated muon spin rotation has been developed (T. Junginger et al. PRAB 21.3 (2018): 032002). Spin polarized muons with an average stopping distance of 130 µm were implanted one at a time into test samples. When the muon decays (half life=2.197 µs) it emits a fast decay positron, preferentially along the direction of its spin. By detecting the location of emitted positrons as a function of time with two detectors the spin precession of the muons and therefore the local magnetic field can be measured. The muon spin rotation technique is used a local magnetometer. Compared to standard magnetometry edge effects are well controllable and precise measurements of the field of first flux entry are possible. Our results, see Figure 1, show:
• For pure niobium the field of first flux entry is consistent with the lower critical field.
• Low temperature baking (120°C) increases the field of first flux entry slightly but significantly above the lower critical field.
• A layer of a material with a higher critical temperature on niobium can enhance the field of first flux entry by up to 40% from a field consistent with the lower critical field up to the superheating field.
• This enhancement does not depend on material or thickness.
• This suggests that the superconductor-superconductor boundary is providing effective shielding up to the superheating field of niobium, while the superconductor-vacuum boundary is not providing shielding above its lower critical field.
• Measurements above the critical temperature of niobium find a field of first flux entry consistent with the lower critical field of the layer.
Why does the superconductor-superconductor but not the superconductor-vacuum boundary provide shielding above the lower critical field? Physical (geometric) defects can act as vortex nucleation sites, by causing a local depression of the order parameter. We suggest that superconducting proximity effect can smooth out a non-ideal interface, which may have been previously allowing vortex nucleation. To strengthen this hypothesis we have performed numerical simulations based on modified Ginzburg Landau equations, which allow superconducting charge carriers in normal conductors. The results show that the proposed mechanism of Meissner state stabilization is indeed predicted by theory. These results are of great interest to develop superconducting cavities beyond state of the art and have been presented through invited talks at several international conferences and published in peer reviewed journals.