KEK, Ibaraki, Japan
Takayuki Kubo, arXiv:1307.5943 [physics.acc-ph](Submitted on 23 Jul 2013)
Fermilab, Batavia, USA
ISSP, Chernogolovka, Russia
Magnetic flux penetration may produce additional losses in superconducting radio frequency cavities. All the existing models for flux penetration are based on the formation of Abrikosov vortices. Using high resolution Bitter decoration technique we have investigated magnetic flux distribution patterns in cavity cutouts at the perpendicular magnetic fields of 10-80 mT. At low fields <20 mT the magnetic field penetrates in the form of flux bundles and not Abrikosov vortices, the situation characteristic of type-I superconductors. With the increase of the magnetic field up to 30 mT "bundles" first merge into a connected structure and then break up into individual Abrikosov vortices at ~60 mT and a well-known intermediate mixed state is observed. Such magnetic field driven transition from type I to type II superconductivity has never been observed before in any existing superconductor. For the case of flat samples we have observed a coexistence of both "bundles" and Abrikosov vortices in one experiment. Our results show that high-purity cavity grade niobium is a "border-line" material and behaves as a type-I superconductor at lower fields and type-II at higher fields.
Michigan State University, East Lansing, Michigan, USA
FRIB, East Lansing, Michigan, USA
Previous cavity tests identified a strong dependence of achievable accelerating gradients on the amount of material removed from the surface. Samples extracted from the iris and the equator of a half cell fabricated by Jefferson Lab using large grain Nb were examined to identify underlying mechanisms. Electron backscattered diffraction (EBSD) was used to measure the crystal orientations on the cross sections of the samples. Results demonstrated the presence of a surface damage layer, which contained higher dislocation content than the bulk due to the deep drawing process. The depth of the damage layer depends on crystal orientations, and damage to the iris is more severe than at the equator. From the EBSD data, the damage depth was estimated to be about 100 microns. The samples were then heat treated at 800°C and 1000°C, and the same areas were examined again for the effects of heat treatment on the healing of the damage layer. While the damage layer accounts for some of the performance gain from chemical surface removal, the depth of the damage layer in polycrystalline cavities remains an open question.
Michigan State University, East Lansing, Michigan, USA
FRIB, East Lansing, Michigan, USA
SRF Cavities can be formed by deep drawing slices from Nb ingots with large grains. Crystal orientation dependent slip system activities affect the shape change of ingot slices during deep drawing, and form a dislocation substructure that affects subsequent recrystallization and ultimately, cavity performance. Two groups of single crystal tensile specimens with different orientations were extracted from a large grain ingot slice. The first group was deformed monotonically to 40% engineering strain. Analysis revealed that slip was preferred on {112} planes. The second group was heat treated at 800°C for two hours, and then deformed incrementally to 40% engineering strain using an in situ tensile stage. Crystal orientations and surface images were recorded at each increment of deformation. Results indicate that the heat treated group had lower yield strengths, and the details of slip activity differed in the annealed samples. Active slip systems were investigated and compared to the first group. Direct observations of dislocations were performed in selected specimens using electron channeling contrast imaging, to determine how slip affects the dislocation substructure.
JLAB, Newport News, Virginia, USA
The Hebrew University of Jerusalem, The Racah Institute of Physics, Jerusalem, Israel
In an attempt to correlate the SRF performance of niobium cavities with the superconducting properties, we present the results of the magnetization and ac susceptibility of the niobium used in the superconducting radiofrequency cavity fabrications which were subjected to buffer chemical polishing surface and high temperature heat treatments, typically applied to the SRF cavities fabrications. The analysis of the results show the different surface and bulk ac conductivity for the samples subjected to BCP and HT. Furthermore, the RF surface impedance is measured on the sample using the TE011 microwave cavity for a comparison to the low frequency measurements.
Fermilab, Batavia, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Ultramet, California, USA
A seamless 3 GHz bulk niobium cavity constructed by Ultramet using rapid chemical vapor deposition (CVD) techniques has been tested on the vertical SRF test stand at Cornell. The cavity received a 25 um buffered chemical polish (BCP) and 700 C heat treatment for 4 days. First test results gave an intrinsic quality factor of Q0 = (1.55 ± 0.12) x 107 and (2.00 ± 0.15) x 107 at 4.2 K and 1.5 K, respectively. A second BCP removed 100 um of material, after which test results improved to Q0 = (7.59 ± 1.52) x 107 and (4.16 ± 0.31) x 108 at 4.2 K and 1.5 K. During the first test poor coupling to the input amplifier impeded tests at accelerating fields >0.2 MV/m, while during the second test the cavity quenched at 1.3 MV/m when operating at 1.5 K. An optical inspection of the cavity after the second test revealed the presence of at least 4 pits on the upper hemisphere suggesting an area of higher than average surface resistance that may have contributed to the low field quench via thermal runaway. The potential of CVD as a construction method for SRF cavities is discussed.
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
One of the most important properties of Superconducting Radio Frequency (SRF) cavities is their ability to disperse generated heat from the internal cavity wall to the external super fluid helium bath. When the generated heat is not removed fast enough, an effect known as thermal feedback dominates, resulting in medium field Q-slope. This medium field Q-slope has the ability to reduce the Q factor should it become strong enough. To determine what physical factors affect the creation of the medium field Q-slope we will be computationally modeling the medium field Q-slope with varying parameters, such as Kapitza conductivity, wall thickness, RF frequency, bath temperature, residual resistivity ratio, residual resistance, and phonon mean path. Our results show that the medium-field Q slope is highly dependent on the Kapitza conductivity and that by doubling the Kapitza conductivity the medium field Q-slope reduces significantly. Understanding and controlling the medium field Q-slope will benefit future continuous wave (CW) applications such as the Energy Recovery Linacs (ERL) where cryogenics costs dominate due to CW operation at medium fields (< 20 MV/m).
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
JLab, Newport News, Virginia, USA
TU, Philadelphia, USA
Due to the very shallow penetration depth of the RF fields, SRF properties are inherently a surface phenomenon involving a material thickness of less than 1 micron thus opening up the possibility of using thin film coatings to achieve a desired performance. The challenge has been to understand the dependence of the SRF properties on the detailed characteristics of real surfaces and then to employ appropriate techniques to tailor these surface properties for greatest benefit. Our aim is to achieve gradients >100 MV/m and no simple material is known to be capable of sustaining this performance. A theoretical framework has been proposed which could yield such behavior [1] and it requires creation of thin film layered structures. I will present our systematic studies on such proof-of-principle samples. Our overarching goal has been to build a basic understanding of key nano-scale film growth parameters for materials that show promise for SRF cavity multilayer coatings and to demonstrate the ability to elevate the barrier for vortex entry in such layered structures above the bulk value of Hc1 for type-II superconductors and thus to sustain higher accelerating fields.
[1]. A. Gurevich, Appl. Phys. Lett. 88, 012511 (2006).
