Magnetic flux trapped during the cooldown of superconducting radio-frequency cavities through the transition temperature due to incomplete Meissner state is known to be a significant source of ...radio-frequency losses. The sensitivity of flux trapping depends on the distribution and the type of defects and impurities which pin vortices, as well as the cooldown dynamics when the cavity transitions from a normal to superconducting state. Here we present the results of measurements of the flux trapping sensitivity on 1.3 GHz elliptical cavities made from large-grain niobium with different purity for different cooldown dynamics and surface treatments. The results show that lower purity material results in a higher fraction of trapped flux and that the trapped flux sensitivity parameterSis significantly affected by surface treatments but without much change in the mean free pathl. We discuss our results within an overview of published data on the dependencies ofS(l,f)onland frequencyfusing theoretical models of rf losses of elastic vortex lines driven by weak rf currents in the cases of sparse strong pinning defects and collective pinning by many weak defects. Our analysis shows how multiscale pinning mechanisms in cavities can result in a maximum inS(l)similar to that observed by the FNAL and Cornell groups and how pinning characteristics can be extracted from the experimental data. Here the main contribution toScome from weak pinning regions at the cavity surface, where dissipative oscillations along trapped vortices perpendicular to the surface propagate into the bulk well beyond the layer of rf screening current. However, the analysis ofSas a function of only the mean free path is incomplete since cavity treatments change not onlylbut pinning characteristics as well. The effect of cavity treatments on pinning is primarily responsible for the change ofSwithout much effect onlobserved in this work. It also manifests itself in different magnitudes and peak positions inS(l), and scatter of theS-data coming from the measurements on different cavities which have undergone different treatments affecting bothland pinning. Optimizations of flux pinning to reduce flux sensitivity at low rf fields is discussed.
We present detailed experimental and theoretical investigations of hotspots produced by trapped vortex bundles and their effect on the radio-frequency (rf) surface resistance Rs of superconductors at ...low temperatures. Our measurements of Rs, combined with the temperature mapping and laser scanning of a 2.36-mm-thick Nb plate incorporated into a 3.3-GHz Nb resonator cavity cooled by the superfluid He at 2 K, revealed spatial scales and temperature distributions of hotspots and showed that they can be moved or split by thermal gradients produced by the scanning laser beam. These results, along with the observed hysteretic field dependence of Rs which can be tuned by the scanning laser beam, show that the hotspots in our Nb sample are due to trapped vortex bundles which contain ~ 10 super(6) vortices spread over regions ~ 0.1-1 cm. We calculated the frequency dependence of the rf power dissipated by oscillating vortex segments trapped between nanoscale pinning centers, taking into account all bending modes and the nonlocal line tension of the vortex driven by rf Meissner currents. We also calculated the temperature distributions caused by trapped vortex hotspots, and suggested a method of reconstructing the spatial distribution of vortex dissipation sources from the observed temperature maps. Vortex hotspots can dominate the residual surface resistance at low temperatures and give rise to a significant dependence of Rs on the rf field amplitude Hp, which can have important implications for the rf resonating cavities used in particle accelerators and forthin-film structures used in quantum computing and photon detectors.
The performance of superconducting radio-frequency Nb cavities at high radio-frequency (rf) fields in the absence of field emission can be limited by either a sharp decrease of the quality factor Q 0 ...( B p ) above peak surface magnetic fields B p ∼100 mT or by a quench. We have measured Q 0 ( B p ) at 2 K of several 1.3 GHz single-cell Nb cavities with different grain sizes, and with different ambient magnetic fields and cooldown rates below the critical temperature. Temperature mapping and a novel magnetic field mapping systems were used to find the location of “hot-spots” and regions of trapped magnetic flux. The use of a variable input coupler allowed further exploring the dissipative state. The results showed a remarkable thermal stability in some cavities with up to 200 W of rf power dissipation at 2 K, whereas other cavities quenched at much lower rf power. We observed a narrow distributions of the onset fields of hot-spots which were not affected by thermal cycling or by conditions which favor the formation of Nb hydrides. Furthermore, a poor correlation was found between the location of hot-spots and trapped vortices. We suggest that the totality of our experimental data can be explained by a sharp increase of the residual surface resistance above 120–140 mT due to the field-induced breakdown of a proximity-coupled metallic suboxide layer at the surface.
As a result of collaboration between Jefferson Lab and niobium manufacturer Companhia Brasileira de Metalurgia e Mineração (CBMM), ingot niobium was explored as a possible material for ...superconducting radiofrequency (SRF) cavity fabrication. The first single cell cavity from large-grain high purity niobium was fabricated and successfully tested at Jefferson Lab in 2004. This work triggered research activities in other SRF laboratories around the world. Large-grain (LG) niobium became not only an interesting alternative material for cavity builders, but also material scientists and surface scientists were eager to participate in the development of this technology. Many single cell cavities made from material of different suppliers have been tested successfully and several multi-cell cavities have shown performances comparable to the best cavities made from standard fine-grain niobium. Several 9-cell cavities fabricated by Research Instruments and tested at DESY exceeded the best performing fine grain cavities with a record accelerating gradient of Eacc=45.6MV/m. The quality factor of those cavities was also higher than that of fine-grain (FG) cavities processed with the same methods. Such performance levels push the state-of-the art of SRF technology and are of great interest for future accelerators. This contribution reviews the development of ingot niobium technology and highlights some of the differences compared to standard FG material and opportunities for further developments.
Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study ...of a large-grain 1.5 GHz single-cell cavity made of “medium purity” Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800–1400°C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2×1010 at 2.0 K and peak surface magnetic field (Bp ) of 90 mT were achieved reproducibly. A Q0 value of (5±1)×1010 at 2.0 K and Bp=90mT was obtained after heat treatment at 1400°C . This is the highest value ever reported at this temperature, frequency, and field. Samples heat treated with the cavity at 1400°C were analyzed by secondary ion mass spectrometry, x-ray photoelectron spectroscopy, energy dispersive x ray, point-contact tunneling, and x-ray diffraction, and revealed a complex surface composition which includes titanium oxide, increased carbon, and nitrogen content but reduced hydrogen concentration compared to a non-heat-treated sample.
A strong degradation of the unloaded quality factor with field, called high fieldQslope, is commonly observed aboveBp≅100mTin elliptical superconducting niobium cavities at 1.3 and 1.5 GHz. In the ...present experiments several 3 GHz niobium cavities were measured up to and aboveBp≅100mT. The measurements show that a high fieldQslope phenomenon limits the field reach at this frequency, that the high fieldQslope onset field depends weakly on the frequency, and that the high fieldQslope can be removed by the typical empirical solution of electropolishing followed by heating to 120°C for 48 hrs. In addition, one of the cavities reached a quench field of 174 mT and its field dependence of the quality factor was compared against global heating predicted by a thermal feedback model.
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