[Triumf-seminars] TRIUMF Accelerator Seminar today at 10:00

[TRIUMF Seminars]

Date/Time: Wed 2018-09-05 at 10:00

Location:  Auditorium          

Speaker:   Tobias Junginger (Lancaster University)

Title:     Ultimate accelerating gradient of superconducting cavities

Abstract: State of the art niobium superconducting radiofrequency cavities can reach accelerating gradients of up to 50 MV/m corresponding to a peak surface magnetic field above 200mT. Such a high field requires metastable operation above the lower critical field of about 170mT and is close to the ultimate limitation of the material set by the superheating field at about 220mT. To reach accelerating gradients beyond current state of the art technology alternative materials need to be investigated. Nb3Sn is currently the most promising candidate reaching RF critical fields above 120mT in pulsed and about 80mT in CW operation. This is large compared to the DC field of first vortex penetration of 28(12)mT as derived with the muon spin rotation technique. The lower critical field cannot be measured directly as it is the field at which it is energetically favorable for flux to be located inside the superconductor but not necessarily to enter if there is a surface barrier. Using the low energy muon spin rotation technique the London penetration depth has been measured from which the lower critical field of 28(2)mT consistent with the DC field of first vortex penetration is derived. These results suggests that Nb3Sn can indeed be operated in a metastable state above its lower critical field. However, a direct measurement of the critical RF field using an RF sample test cavity (Quadrupole Resonator) suggests that Nb3Sn cannot reach higher accelerating gradients than Nb if no precaution is taken to avoid premature flux penetration. Further muon spin rotation results show that a bilayer system of two superconductors can prevent premature flux penetration by creating an energy barrier at the interface between the two materials potentially enabling accelerating gradients beyond niobium technology.

Stimulants available 15 minutes before the talk.

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