[Triumf-seminars] TRIUMF Accelerator Seminar today at 14:30

[TRIUMF Seminars]

Date/Time: Wed 2023-11-01 at 14:30

Location:  ISAC II Conf. Room  

Speaker:   Ruth Gregory (TRIUMF)

Title:     Flux Expulsion in Coaxial Superconducting Radiofrequency Cavities

Abstract: This seminar explores the effects of different cool-down speeds and applied magnetic
fields on TRIUMF's coaxial superconducting radio frequency (SRF) cavities using COMSOL
simulations and experimental results. Flux trapping is a phenomenon in which some magnetic
flux remains trapped in an SRF cavity after it has been cooled down to superconducting
temperatures. Trapped flux is detrimental to cavity performance and should be reduced as
much as possible.
Previous studies have shown that elliptical cavities are very sensitive to applied external
magnetic fields, resulting in stringent requirements for the residual field and cavity cool-down
speed. Few such studies have been done on non-elliptical cavities such as half wave resonators
(HWRs) and quarter waver resonators (QWRs). Factors affecting flux expulsion include cavity
treatment, rf field distribution inside the cavity, external magnetic field direction, cool-down
speed, and cavity geometry. The effects of cool-down speed and temperature gradient on flux
expulsion was found to be insignificant for the QWR with a vertically applied magnetic field.
However, a horizontal magnetic field can be nearly completely expelled by a fast, high
temperature gradient cool-down.
Magnetic sensitivity describes how sensitive the surface resistance of a material is to an
external magnetic field and is an important characteristic of SRF accelerator design. Reducing
the magnetic sensitivity can improve cavity performance since in practice it is impossible to
eliminate all residual magnetic fields from external sources such as Earth's natural magnetic
field during a cool-down. Magnetic sensitivity can be affected by cavity frequencies and heat
treatments. A similar parameter to magnetic sensitivity used for this study is normalized Rmag.
The HWR's normalized Rmag is compared for different resonant frequencies after 400 and 120 degC
bakes, with the 120 degC bake resulting in lower normalized Rmag. The normalized Rmag was found
to generally increase with frequency for both the HWR and QWR. 
This Seminar is only in-person and there is no link.



______________________________

General TRIUMF seminar information available at http://www.triumf.ca/home/upcoming-events/about-seminars-lectures