Cooling Dynamics Through Transition Temperature of Niobium SRF Cavities Captured by Temperature Mapping (original) (raw)
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Trapped magneticflux is knownto be one cause of residual losses in bulk niobium SRF cavities. In the Meissner state an ambient magnetic field should be expelled from the material. Disturbances such as lattice defects or impurities have the ability to inhibit the expulsion of an external field during the superconducting transition so that the field is trapped. We measured the fraction of trapped magnetic flux in niobium samples with different treatment histories, such as BCP andtempering. Thedifferencesbetweensingle crystal and polycrystalline material as well as the influence of spatial temperature gradients and different cooling rates were investigated. In addition, the progression of the release of a trapped field during warm up was studied.
Trapped magnetic flux in superconducting niobium samples
Physical Review Special Topics - Accelerators and Beams, 2012
Trapped magnetic flux is known to be one cause of residual losses in bulk niobium superconducting radio frequency cavities. In the Meissner state an ambient magnetic field should be expelled from the material. Disturbances such as lattice defects or impurities have the ability to inhibit the expulsion of an external field during the superconducting transition so that the field is trapped. We have investigated the effect the treatment history of bulk niobium has on the trapped flux and which treatment leads to minimal flux trapping. For that purpose, we measured the fraction of trapped magnetic flux in niobium samples representing cavities with different typical treatment histories. The differences between single crystal and polycrystalline material as well as the influence of spatial temperature gradients and different cooling rates were investigated. In addition, the progression of the release of a trapped field during warm-up was studied. We found that heat treatment reduces trapped flux considerably and that single crystal samples trap less flux than polycrystalline niobium. As a consequence, the single crystal sample with 1200 C baking trapped the smallest amount of field which is about 42%. Moreover, the release of the trapped field during warm-up was observed to progress over a broad temperature range for the baked single crystal samples.
High Q0 Research: The Dynamics of Flux Trapping in Superconducting Niobium
2013
The quality factor Q0 that can be obtained in a superconducting cavity is known to depend on various factors like niobium material properties, treatment history and magnetic shielding. We believe that cooling conditions have an additional impact, as they appear to influence the amount of trapped flux and hence the residual resistance [1 – 3]. We constructed a test stand using a niobium rod shorted out by a titanium rod to mimic a cavity in its helium tank to study flux trapping. Here we can precisely control the temperature and measure the dynamics of flux trapping at the superconducting phase transition. We learned that magnetic flux can be generated when a temperature gradient exists along the rod and when the niobium transitions into the superconducting state it subsequently remains trapped. Furthermore, it was shown that the cooling rate during isothermal cooldown through the transition temperature can influence the amount of externally applied flux which remains trapped. The ac...
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We report a strong effect of the cooling dynamics through Tc on the amount of trapped external magnetic flux in superconducting niobium cavities. The effect is similar for fine grain and single crystal niobium and all surface treatments including electropolishing with and without 120 °C baking and nitrogen doping. Direct magnetic field measurements on the cavity walls show that the effect stems from changes in the flux trapping efficiency: slow cooling leads to almost complete flux trapping and higher residual resistance, while fast cooling leads to the much more efficient flux expulsion and lower residual resistance.
Impact of Trapped Flux and Thermal Gradients on the SRF Cavity Quality Factor
2012
The obtained Q0 value of a superconducting niobium cavity is known to depend on various factors like the RRR of the Niobium material, crystallinity, chemical treatment history, the high-pressure rinsing process, or effectiveness of the magnetic shielding. We have observed that spatial thermal gradients over the cavity length during cool-down appear to contribute to a degradation of Q0. Measurements were performed in the Horizontal Bi-Cavity Test Facility (HoBiCaT) at HZB on TESLA type cavities as well as on discand rod-shaped niobium samples equipped with thermal, electrical and magnetic diagnostics. Possible explanations for the effect are discussed.
AIP Conference Proceedings, 2006
The accelerating gradients in superconducting RF cavities can be limited by excessive temperature rise on the inner surface. In some circumstances, such as high RF frequency or anomalous losses, improved heat transfer at the niobium-helium interface can increase the achievable gradient. Different surface morphology techniques have been applied to reduce the limitation at the thermal interface. These techniques include varying surface roughness and forming cooling channels (embedded fins) in the surface. Heat transfer measurements on several niobium samples of different surface states, but same bulk purity at both Helium I and II (super-fluid) temperature regimes are planned. Initial measurements to validate the experimental setup on one sample are presented. Comparison of the test measurements is made with the existing literature data. The surface characterization uses a high-resolution 3D optical nano-scope. Finally, the influence of these interface surface techniques on the performance of the cavities will be discussed.