JLAB, Newport News, Virginia
The performance of SRF cavities made of bulk Nb at high fields (peak surface magnetic field greater than about 90 mT) is characterized by exponentially increasing rf losses (high-field Q-slope), in the absence of field emission, which are often mitigated by a low temperature (100-140 °C, 12-48h) baking. In this contribution, recent experimental results and phenomenological models to explain this effect will be briefly reviewed. New experimental results on the high-field Q-slope will be presented for cavities that had been heat treated at high temperature in the presence of a small partial pressure of nitrogen. Improvement of the cavity performances have been obtained, while surface analysis measurements on Nb samples treated with the cavities revealed significantly lower hydrogen concentration than for samples that followed standard cavity treatments.
IIT, Chicago, Illinois
JLAB, Newport News, Virginia
ANL, Argonne
We have shown previously that magnetic niobium oxides can influence the superconducting density of states at the surface of cavity-grade niobium coupons. We will present recent results obtained by Point Contact Tunneling spectroscopy (PCT) on coupons removed from hot and cold spots in a niobium cavity, as well as a comparative study of magnetic oxides on mild baked/unbaked electropolished coupons. We will also describe recent results obtained from coated cavities, ALD films properties and new materials using Atomic Layer Deposition (ALD).
JLAB, Newport News, Virginia
Several 9-cell cavities electropolished and tested at Jefferson Lab are found to be quench limited. Pass-band mode excitation measurements provide the first clue of candidate cells responsible for the quench limit. A second RF test with thermometers attached to the equator region of candidate cells (typically only 2 candidates) reveals a hot spot caused by excessive heating of the operational defect and hence determines its location. High resolution optical tools inspect the RF surface corresponding to the hot spot to image and document the defect. All defects in cavities quench limited ~ 20 MV/m are sub-mm sized irregularities near but outside of the equator EBW. In contrast, no observable irregularities are found in some other cavities that are quench limited ~ 30 MV/m. These two types of quench limited cavities have different response to a second EP processing. In this paper, we will give a summary of the test result and attempts to catalog the observed defects.
JLAB, Newport News, Virginia
This paper reports an on-going surface study of a superconducting radio frequency resonant cavity made of large-grain bulk niobium, which experienced anomalous RF energy loss in the medium field range. "Hot" and "cold" spots were identified via in-situ thermometry mapping of the BCP-etched single-cell cavity. The cavity was cut apart for surface investigation via high resolution electron microscopy (SEM), electron-back scattering diffraction microscopy (EBSD), optical microscopy, and three dimensional profilometry. Etching pits with clearly discernable crystal facets were observed in both "hotspot" and "coldspot" specimens. They were found in-grain, at bi-crystal boundaries, and on tri-crystal junctions. Two types of pits were observed with significantly different geometrical features, as observed with high resolution SEM. All "coldspots" examined had qualitatively low density of etching pits or very shallow pits at tri-crystal bound
BARC, Mumbai
JLAB, Newport News, Virginia
BARC-EBC, Mumbai
In recent years, large-grain/single-crystal niobium has become a viable alternative to the standard fine grain (ASTM grain size>6), high purity (RRR ) niobium for the fabrication of high-performance SRF cavities for particle accelerators In this contribution we present the results of a systematic study of the superconducting properties of samples obtained from four Niobium ingots (from CBMM, Brazil) of different purity. Measurements of bulk magnetization, surface pinning, critical temperature and thermal conductivity have been carried out on the samples subjected to different surface treatments such as buffered chemical polishing (BCP), 600C heat treatment, and low temperature baking (LTB). A correlation has been established between the LTB and the ratio of Hc3/Hc2. In addition, the phonon peak in the thermal conductivity data is suppressed by the presence of trapped magnetic vortices in the samples
JLAB, Newport News, Virginia
DESY, Hamburg
Performance of 3-Cell Seamless Niobium Cavities P. Kneisel, G.Ciovati, Jefferson Lab and X.Singer, W.Singer, I. Jelezov, DESY In the last several months we have surface treated and cryogenically tested three TESLA-type 3-cell cavities, which had been manufactured at DESY as seamless assemblies by hydroforming. The cavities were completed at JLab with beam tube/flange assemblies. All three cavities performed very well after they had been post-purified with titanium at 1250C for 3 hrs. The cavities, two of which consisted of an end cell and 2 center cells and one was a center cell assembly, achieved gradients of Eacc = 32 MV/m, 34 MV/m and 35 MV/m without quenches. The performance was limited by the appearance of the “Q-drop” in the absence of field emission. This contribution reports about the various measurements undertaken with these cavities.
JLAB, Newport News, Virginia
CEA, Gif-sur-Yvette
PKU/IHIP, Beijing
ORNL, Oak Ridge, Tennessee
BNL, Upton, Long Island, New York
Buffered electropolishing (BEP) is a Nb surface treatment technique developed at Jefferson Lab1. Experimental results obtained from flat Nb samples show2-4 that BEP can produce a surface finish much smoother than that produced by the conventional electropolishing (EP), while Nb removal rate can be as high as 4.67 μm/min. This new technique has been applied to the treatments of Nb SRF single cell cavity employing a vertical polishing system5 constructed at JLab as well as a horizontal polishing system at CEA Saclay. Preliminary results show that the accelerating gradient can reach 32 MV/m for a large grain cavity and 26.7 MV/m for a regular grain cavity. In this presentation, the latest progresses from the international collaboration between Peking University, CEA Saclay, and JLab on BEP will be summarized.