GHz Niobium Research Articles

Investigation of Electropolishing for High-Gradient 1.3 GHz and 3.9 GHz Niobium Cavities.

Electropolishing (EP) has become a standard procedure for treating the inner surfaces of superconducting radio-frequency (SRF) cavities composed of pure niobium. In this study, a new EP facility was employed for the surface treatment of both 1.3 GHz and 3.9 GHz single-cell cavities at the Wuxi Platform. The stable "cold EP" mode was successfully implemented on this newly designed EP facility. By integrating the cold EP process with a two-step baking approach, a maximum accelerating gradient exceeding 40 MV/m was achieved in 1.3 GHz single-cell cavities. Additionally, an update to this EP facility involved the design of a special cathode system for small-aperture structures, facilitating the cold EP process for 3.9 GHz single-cell cavities. Ultimately, a maximum accelerating gradient exceeding 25 MV/m was attained in the 3.9 GHz single-cell cavities after undergoing the cold EP treatment. The design and commissioning of the EP device, as well as the electropolishing and vertical test results of the single-cell cavities, will be detailed herein. These methods and experiences are also transferable to multi-cell cavities and elliptical cavities of other frequencies.

Reducing two-level systems dissipations in 3D superconducting niobium resonators by atomic layer deposition and high temperature heat treatment

Superconducting qubits have arisen as a leading technology platform for quantum computing, which is on the verge of revolutionizing the world's calculation capacities. Nonetheless, the fabrication of computationally reliable qubit circuits requires increasing the quantum coherence lifetimes, which are predominantly limited by the dissipations of two-level system defects present in the thin superconducting film and the adjacent dielectric regions. In this paper, we demonstrate the reduction of two-level system losses in three-dimensional superconducting radio frequency niobium resonators by atomic layer deposition of a 10 nm aluminum oxide Al2O3 thin films, followed by a high vacuum heat treatment at 650 °C for few hours. By probing the effect of several heat treatments on Al2O3-coated niobium samples by x-ray photoelectron spectroscopy plus scanning and conventional high resolution transmission electron microscopy coupled with electron energy loss spectroscopy and energy dispersive spectroscopy, we witness a dissolution of niobium native oxides and the modification of the Al2O3-Nb interface, which correlates with the enhancement of the quality factor at low fields of two 1.3 GHz niobium cavities coated with 10 nm of Al2O3.

Effect of Cathode Rotation and Acid Flow in Vertical Electropolishing of 1.3 GHz Niobium Nine-Cell Cavity

Effect of Cathode Rotation and Acid Flow in Vertical Electropolishing of 1.3 GHz Niobium Nine-Cell Cavity

High field Q slope and the effect of low-temperature baking at 3 GHz

A strong degradation of the unloaded quality factor with field, called high field $Q$ slope, is commonly observed above ${\mathrm{B}}_{p}\ensuremath{\cong}100\text{ }\text{ }\mathrm{mT}$ in 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 above ${\mathrm{B}}_{p}\ensuremath{\cong}100\text{ }\text{ }\mathrm{mT}$. The measurements show that a high field $Q$ slope phenomenon limits the field reach at this frequency, that the high field $Q$ slope onset field depends weakly on the frequency, and that the high field $Q$ slope can be removed by the typical empirical solution of electropolishing followed by heating to 120\ifmmode^\circ\else\textdegree\fi{}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.

Evidence for thermal boundary resistance effects on superconducting radiofrequency cavity performances

The majority of the literature on superconducting cavities for particle accelerators concentrates on the interaction of a radiofrequency (RF) electromagnetic field with a superconductor cooled in liquid helium, generally either at a fixed temperature of 4.2 K or 1.8 K, basing the analysis of experimental results on the assumption that the superconductor is at the same temperature as the infinite reservoir of liquid helium. Only a limited number of papers have extended their analysis to the more complex overall system composed of an RF field, a superconductor and liquid helium. Only a few papers have analyzed, for example, the problem of the Kapitza resistance, i.e. the thermal boundary resistance between the superconductor and the superfluid helium. Among them, the general conclusion is that the Kapitza resistance, one of the most controversial and less understood topics in physics, is generally negligible, or not relevant for the performance enhancement of cavities. In our work presented here, studying the performance of 6 GHz niobium (Nb) test cavities, we have discovered and studied a new effect consisting of an abrupt change in the surface resistance versus temperature at the superfluid helium lambda transition Tλ. This abrupt change (or ‘jump’) clearly appears when the RF measurement of a cavity is performed at constant power rather than at a constant field. We have correlated this jump to a change in the thermal exchange regime across the lambda transition, and, through a simple thermal model and further reasonable assumptions, we have calculated the thermal boundary resistance between niobium and liquid helium in the temperature range between 4.2 K and 1.8 K. We find that the absolute values of the thermal resistance both above and below the lambda point are fully compatible with the data reported in the literature for heat transfer to pool boiling helium I (HeI) above Tλ and for the Kapitza interface resistance (below Tλ) between a polished metal surface and superfluid HeII. Finally, based on the well-documented evidence that the surface status of metal to liquid helium influences the heat exchange towards the fluid, and specifically the Kapitza resistance below Tλ, we have tested an anodization process external to the cavity, comparing the performances of the cavity before and after external anodization. The tests were done without breaking the vacuum inside the cavity or modifying the inner superconducting layer in any way, and were repeated on different samples. The results show that when the cavity is externally anodized, both the Q-factor and the maximum accelerating field increase. Again, when the oxide layer is removed, the Q-factor shifts towards a lower level and the maximum accelerating field is also reduced.

Development of Vertical Buffered Electropolishing for Its Post-Treatment Technology on 1.5 GHz Niobium SRF Cavities

We report the latest research development of vertical buffered electropolishing on its post-treatment procedure as well as the effects of several major post-treatment techniques for buffered electropolishing (BEP) processed 1.5 GHz niobium (Nb) superconducting radio frequency (SRF) cavities. With the established post-treatment procedure, an accelerating gradient of 28.4 MV/m is obtained on a single cell cavity of the cebaf shape. This is the best result in the history of BEP development. The cavity is limited by quench with a high quality factor over 1.2 × 1010 at the quench point. Analyses from optical inspection and temperature-mapping show that the quench should be originated from the pits that were already present on the cavity before this BEP treatment. All of these factors indicate that this procedure will have a great potential to produce better results if cavities without intrinsic performance limiting imperfections are used.

Analysis of the surface resistance of the superconducting accelerating cavities of the S-DALINAC

The quality factors of the 20 cell, 3 GHz niobium cavities of the S-DALINAC are smaller by almost a factor of three compared to their design value. Combined with the limited cryogenic power of the helium refrigerator in use, this restricts the maximum electron energy of the S-DALINAC in continuous wave operation. Extensive studies to elaborate the origin of this degradation are presented. A 120 ∘ C baking combined with appropriate diagnostic tools for an attempt to improve the Q values has been applied to nine of the 11 cavities. The impact of this treatment on the BCS and residual resistance was evaluated and both were improved. Additionally, quality factor measurements of all modes of the TM 010 passband allowed to derive a spatial distribution of the mean surface resistance. The RF losses in the end cells are systematically higher compared to the inner cells. The studies presented in this article allowed to develop a model describing the Q degradation of the resonators and thereby identifying possible prospects to cure this limitation.

Study of superconducting Nb coated 1.5 GHz accelerating cavities

Single cell 1.5 GHz niobium coated accelerating cavities have been developed. Nb film was deposited by cylindrical magnetron sputtering. The film deposited onto the cavities was analyzed by SNMS for depth profile composition and by XRD for microstructure. Critical temperature (Tc) and residual resistance ratio (RRR) of the films were investigated on Al 2O 3 samples, giving (9.25 ± 0.05) K for Tc and 14.5 for RRR. First results of RF measurements on coated cavities showed superficial resistance values in the μΩ range. Possible superficial resistance sources are discussed.

Observation and cure of severe Q-degradation of superconducting niobium cavities

Abstract The observed Q -degradation of a superconducting 3 GHz niobium cavity of the Darmstadt electron linear accelerator S-DALINAC made from high purity material with RRR = 280 is described. It is shown, that the effect is due to hydrogen contamination of the material and can be explained by precipitation of the hydride phase Nb 4 H 3 during cooldown of the cavity. The conditions influencing the amount of precipitation are reported. Heating of the cavity in UHV is one possible way of cure and has been applied successfully to S-DALINAC cavities, as it is shown in this paper.