High Q0 Research: The Dynamics of Flux Trapping in Superconducting Niobium (original) (raw)
Related papers
Study of Trapped Magnetic Flux in Superconducting Niobium Samples
2011
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.
Applied Sciences
Reducing the size of ambient magnetic flux trapping during cooldown in superconducting radio-frequency niobium cavities is essential to reaching the lowest power dissipation as required for continuous wave application. Here, it is suggested that applying an alternating magnetic field superimposed to the external DC field can potentially reduce the size of trapped flux by supporting flux line movement. This hypothesis is tested for the first time systematically on a buffered chemically polished (BCP) niobium sample before and after high temperature annealing, a procedure which is known to reduce flux pinning. External low-frequency (Hz-range) magnetic fields were applied to the samples during their superconducting transition and the effect of varying their amplitude, frequency and offset was investigated. A few results can be highlighted: The influence of the frequency and magnitude of the AC fields on the flux trapping in the untreated Nb sample cannot be neglected. The trapped flux...
Magnetic flux studies in horizontally cooled elliptical superconducting cavities
Journal of Applied Physics, 2015
Previous studies on magnetic flux expulsion as a function of cooldown procedures for elliptical superconducting radio frequency (SRF) niobium cavities showed that when the cavity beam axis is placed parallel to the helium cooling flow and sufficiently large thermal gradients are achieved, all magnetic flux could be expelled and very low residual resistance could be achieved. In this paper, we investigate flux trapping for the case of resonators positioned perpendicularly to the helium cooling flow, which is more representative of how SRF cavities are cooled in accelerators and for different directions of the applied magnetic field surrounding the resonator. We show that different field components have a different impact on the surface resistance, and several parameters have to be considered to fully understand the flux dynamics. A newly discovered phenomenon of concentration of flux lines at the cavity top leading to temperature rise at the cavity equator is presented. V
Temperature dependence of the superheating field in niobium
This study experimentally investigates the temperature dependence of superheating field, Hsh, of niobium. Accurately determining this field is important both to test theory and to understand gradient limits in superconducting cavities for particle accelerators. This paper discusses theories that have been proposed in modeling the field and discriminates between them. The experimental procedure for measuring the temperature dependence of Hsh utilizes high power pulses to drive a niobium cavity resonator, ramping up surface magnetic fields extremely quickly. The moment any part of the cavity transitions between the superconducting and normal conducting state can be determined by measuring the quality factor of the cavity as a function of time. Oscillating superleak transducers are used to demonstrate that the transition to the normal conducting state is global in nature, showing that a fundamental limit is encountered. Finally, we see that 110- 120 C heat treatment of the cavity–a method commonly used to increase the quality factor at high accelerating gradients–may have the deleterious effect of reducing the superheating field of the material, which is the fundamental limiting factor in pursuing the maximal achievable accelerating gradient in superconducting niobium cavities.
Magnetic Flux Expulsion Studies in Niobium SRF Cavities
2016
With the recent discovery of nitrogen doping treatment for SRF cavities, ultra-high quality factors at medium accelerating fields are regularly achieved in vertical RF tests. To preserve these quality factors into the cryomodule, it is important to consider background magnetic fields, which can become trapped in the surface of the cavity during cooldown and cause Q0 degradation. Building on the recent discovery that spatial thermal gradients during cooldown can significantly improve expulsion of magnetic flux, a detailed study was performed of flux expulsion on two cavities with different furnace treatments that are cooled in magnetic fields amplitudes representative of what is expected in a realistic cryomodule. In this contribution, we summarize these cavity results, in order to improve understanding of the impact of flux expulsion on cavity performance. INTRODUCTION How strong is the impact of residual magnetic fields on the Q0 of a superconducting RF cavity? Trapped flux degrade...
Collapse of the critical state in superconducting niobium
Physical Review B, 2006
The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. arXiv:cond-mat/0607244v4 [cond-mat.supr-con] Giant abrupt changes in the magnetic flux distribution in niobium foils were studied by using magnetooptical visualization, thermal and magnetic measurements. Uniform flux jumps and sometimes almost total catastrophic collapse of the critical state are reported. Results are discussed in terms of thermomagnetic instability mechanism with different development scenarios.
Impact of geometry on flux trapping and the related surface resistance in a superconducting cavity
Physical Review Accelerators and Beams, 2020
In order to minimize the surface resistance in superconducting cavities, a deeper understanding of residual resistance due to trapped magnetic flux is necessary. For that purpose, a combined temperature and magnetic field mapping system is employed to map magnetic flux trapped in a superconducting cavity, and the related increase in surface resistance. By cooling down a 1.3 GHz TESLA single cell cavity several times with externally applied static magnetic fields with different orientations with respect to the cavity, a statement can be made about how the angle between the applied magnetic field and the cavity's surface affects flux trapping, and surface resistance. For example, a significantly higher increase in surface resistance is observed when the applied magnetic field is perpendicular to the cavity's surface compared to when it is parallel.