Sensitivity to trapped flux in high-purity large-grain niobium based on cavity measurements

9 Nov 2018, 09:37
17m
30/7-018 - Kjell Johnsen Auditorium (CERN)

30/7-018 - Kjell Johnsen Auditorium

CERN

190
Show room on map

Speaker

Rongli Geng (Jefferson Lab)

Description

Surface resistance arising from trapped flux is experimentally measured, by which the sensitivity to trapped flux is derived. Measurements are carried out with single-cell L-band SRF cavities made of high-purity large-grain niobium materials, immersed in a uniform externally applied magnetic field generated by a solenoid whose axis overlaps the cavity axis. The surface resistance is found by using the standard technique for Q0 measurement and the customary G/Q0 analysis.Q0 values at a fixed low surface field are used. The trapped flux is found by measuring flux densities at a selected location using a single-axis fluxgate magnetometer attached to the cavity outer surface: Ba*[1-(Bsc-Bnc)/(Bsc^(0)-Bnc)], where Ba is the applied external field, Bnc and Bsc is the local flux density measured just above and below Tc, respectively, during a field-cooling of the cavity whose Q0 is then measured, Bsc^(0) is measured in a separate zero-field-cooling by keeping the cavity in the same location and turning on the identical applied field Ba at a temperature well below Tc . Several magnetometers are placed at various locations. It is found that the sensitivity to trapped flux in high-purity large-grain niobium to be 1.9 nOhm/microTesla on average. This is to be compared to 3-9 nOhm/microTesla in high purity fine-grain niobium and 10-50 nOhm/microTesla in nitrogen-doped high-purity niobium reported by other groups. We will discuss the measurement results as well as the measurement techniques.

Presentation materials