2 research outputs found
Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities
A model for the onset of the reduction in SRF cavity quality factor, the
so-called Q-drop, at high accelerating electric fields is presented. Breakdown
of the surface barrier against magnetic flux penetration at the cavity equator
is considered to be the critical event that determines the onset of Q-drop. The
worst case of triangular grooves with low field of first flux penetration Hp,
as analyzed previously by Buzdin and Daumens, [1998 Physica C 294: 257], was
adapted. This approach incorporates both the geometry of the groove and local
contamination via the Ginzburg-Landau parameter kappa, so the proposed model
allows new comparisons of one effect in relation to the other. The model
predicts equivalent reduction of Hp when either roughness or contamination were
varied alone, so smooth but dirty surfaces limit cavity performance about as
much as rough but clean surfaces do. When in combination, contamination
exacerbates the negative effects of roughness and vice-versa. To test the model
with actual data, coupons were prepared by buffered chemical polishing and
electropolishing, and stylus profilometry was used to obtain distributions of
angles. From these data, curves for surface resistance generated by simple flux
flow as a function of magnetic field were generated by integrating over the
distribution of angles for reasonable values of kappa. This showed that
combined effects of roughness and contamination indeed reduce the Q-drop onset
field by ~30%, and that that contamination contributes to Q-drop as much as
roughness. The latter point may be overlooked by SRF cavity research, since
access to the cavity interior by spectroscopy tools is very difficult, whereas
optical images have become commonplace. The model was extended to fit cavity
test data, which indicated that reduction of the superconducting gap by
contaminants may also play a role in Q-drop.Comment: 15 pages with 7 figure
AUGMENTATION OF CAVITY OPTICAL INSPECTION BY REPLICAS WITHOUT PERFORMANCE DEGRADATION
Abstract Although cavity optical inspection systems provide a huge amount of qualitative information about surface features, the amount of quantitative topographic information is limited. Here, we report the use of silicone-based RTV for replicas and moldings that provide increased details of topographic data associated with the optical cavity images. Profilometry scans of the molds yield micrometer-scale details associated with equator weld structures and weld pits. This confirms at least two different types of pits, one which is bowl-shaped, and one which has a small peak at the bottom. The contour information extracted from profilometry can be used to evaluate mechanisms by which pits and other features limit RF performance. We present calculations based on a conformal transformation of the profiles above. We also show that application of the replica followed by rinsing does not adversely affect the cavity performance