Trapped magnetic flux in superconducting niobium samples

Abstract

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.

The importance of the electron mean free path for superconducting radio-frequency cavities

“…With good shielding, typical ambient fields in SRF cryomodules are near 0.5 lT (5 mG); anywhere from 0% to 100% of this ambient field may be trapped during cooldown, depending on cooldown parameters, most notable of which is the spatial temperature gradient as the cavity crosses T c . 10,[32][33][34] For an operating gradient of 16 MV/m at 2 K, the results in Fig. 8 show that, unless more than $50% of such an ambient field can be expelled during cooldown, light doping is preferable over heavy doping.…”

“…6 Magnetic flux is usually trapped from ambient fields during the cool-down or during quench events by flux vortex pinning at impurities and defects. 9,10 Trapping flux increases the effective residual resistance R 0 of the superconductors, which can be detrimental to accelerator operation. Further, nitrogen-doping has been shown to lower the average quench field of SRF cavities, limiting the peak available accelerating gradient.…”

Efficient expulsion of magnetic flux in superconducting radiofrequency cavities for high Q applications

“…119, 213903 (2016) material in fraction of flux trapped during cooldown. 25,26 These studies were performed before it was recognized that it was important to control for thermal gradient, making it difficult to extract quantitative data, but qualitative trends were demonstrated. Material with larger grains appeared to have higher expulsion, which is consistent with the results in Figure 3, as well as with previous studies of high field pinning.…”

“…12,13 However, even in single crystal niobium samples, heat treatment appeared to improve expulsion, suggesting that crystal properties other than grain boundaries (e.g., dislocations) play an important role. 25 We can apply a simple model to attempt to predict the quality factor that would be achieved in the cavities from production groups 1 and 2, depending on the temperature gradient applied during cooldown (converting from DT using the approximate distance between thermometers of 20 cm). This model will require a number of assumptions that will not apply in every situation, but can provide a helpful case study.…”

Superconducting Radio-Frequency Technology R&D for Future Accelerator Applications

“…[89] Early indications from JLab suggest opportunities for reduction of both residual and BCS rf surface resistance with carefully managed surface chemistry during and immediately following ≥1200ºC bake of ingot Nb cavities [33,90].…”