We demonstrate fiber optic, multiplexible temperature sensing using a fiber Bragg grating (FBG) with an operational range of 2-400 K, and a temperature resolution better than 10 mK for temperatures <;12 K. This represents a significant reduction in the lowest usable temperature as well as a significant increase in sensitivity at cryogenic temperatures compared with previously reported multiplexible solutions. This is accomplished by mounting the section of the fiber with a FBG on a polytetrafluoroethylene coupon, which has a non-negligible coefficient of thermal expansion down to <;4 K. The sensors exhibit a good stability over multiple temperature cycles and acceptable sensor-to-sensor repeatability. Possible applications for this sensor include distributed temperature sensing across superconducting elements and cryogenic temperature measurements in environments where electrical measurements are impractical or unsafe.

A Fiber Bragg Grating Temperature Sensor for 2-400 K

Recent results on electron loading in superconducting cavities are presented and are compared with observations in DC field emission. It is shown that electron loading is the main obstacle to achieving accelerating fields higher than 10 MV/m. Multipactor electron loading, which may limit the accelerating fields, can be processed away, but processing times might be long. There exists experimental evidence that accelerating fields of 10 MV/m can be reproducibly obtained in storage-ring accelerating multicell structures. New diagnostic tools developed for superconducting RF cavities have contributed to the knowledge of electron emission in broad areas. Experimental results in DC and RF field emission are very similar. A simple statistical model, with an exponential increase of the number of emitters toward lower beta values, may explain the frequency dependence of field-emission electron loading observed so far. >

/pdf/on-electrical-breakdown-in-superconducting-accelerating-ngj7cnnuyk.pdf

On electrical breakdown in superconducting accelerating cavities

The surface resistance R s of superconducting materials can be obtained by measuring the quality factor of an elliptical cavity excited in a transverse magnetic mode (TM010). The value obtained has however to be taken as averaged over the whole surface. A more convenient way to obtain R s, especially of materials which are not yet technologically ready for cavity production, is to measure small samples instead. These can be easily manufactured at low cost, duplicated and placed in film deposition and surface analytical tools. A commonly used design for a device to measure R s consists of a cylindrical cavity excited in a transverse electric (TE110) mode with the sample under test serving as one replaceable endplate. Such a cavity has two drawbacks. For reasonably small samples the resonant frequency will be larger than frequencies of interest concerning SRF application and it requires a reference sample of known R s. In this article we review several devices which have been designed to overcome these limitations, reaching sub-nΩ resolution in some cases. Some of these devices also comprise a parameter space in frequency and temperature which is inaccessible to standard cavity tests, making them ideal tools to test theoretical surface resistance models.

https://iopscience.iop.org/article/10.1088/0953-2048/30/1/013001/pdf

Devices for SRF material characterization

In order to measure surface resistance of new materials other than niobium such as Nb3Sn and MgB2, two sample host niobium cavities operating at TE modes have been developed at Cornell University. The first one is a 6GHz pillbox TE011 cavity modified from an older vision enabling testing 2.75 inch diameter flat sample plates. The second one is an optimized mushroom-shape niobium cavity operating at both 4GHz TE012 and 6GHz TE013 modes for 10cm inch diameter flat sample plates. First results from the commissioning of the two TE cavities will be reported.

TE Sample Host Cavities Development at Cornell

Superconducting radio frequency (SRF) technology is widely adopted in particle accelerators. There remain many open questions, however, in developing a systematic understanding of the fundamental behavior of SRF materials, including niobium treated in different ways and various other bulk/thin film materials that are fabricated with different methods under assorted conditions. A facility that can measure the SRF properties of small samples in a range of 2~40 K temperature is needed in order to fully answer these questions. The Jefferson Lab surface impedance characterization (SIC) system has been designed to attempt to meet this requirement. It consists of a sapphire-loaded cylindrical Nb TE011 cavity at 7.4 GHz with a 50 mm diameter flat sample placed on a noncontacting end plate and uses a calorimetric technique to measure the radio frequency (RF) induced heat on the sample. Driving the resonance to a known field on this surface enables one to derive the surface resistance of a relatively small localized area. TE011 mode identification has been done at room temperature and 4 K, and has been compared with Microwave Studio® and SuperFish simulation results. The variable input and transmitted RF couplers have been characterized. RF loss mechanisms in the SIC system are under investigation. A VCO phase lock loop system has been used in both CW and pulsed mode. Two calorimeters, with stainless steel and Cu as the thermal path material for high precision and high power versions, respectively, have been designed and commissioned for the SIC system to provide low temperature control and measurement. A power compensation method has been developed to measure the RF induced power on the sample. Simulation and experimental results show that with these two calorimeters, the whole thermal range of interest for superconducting radiofrequency (SRF) materials has been covered. The power measurement error in the interested power range is within 1.2% and 2.7% for the high precision and high power versions, respectively. Temperature distributions on the sample surface for both versions have been simulated and the accuracy of sample temperature measurements have been analysed. Both versions have the ability to accept bulk superconductors and thin film superconducting samples with a variety of substrate materials such as Al, Al2O3, Cu, MgO, Nb and Si. Tests with polycrystalline and large grain bulk Nb samples have been done at <15 mT magnetic field. Based on BCS surface impedance, least-squares fittings have been done using SuperFit2.0, a code developed by G. Ciovati and the author. Microstructure analyses and SRF measurements of large scale epitaxial MgB2 films have been reported. MgB2 films on 5 cm dia. sapphire disks were fabricated by a Hybrid Physical Chemical Vapor Deposition (HPCVD) technique. The electron-beam backscattering diffraction (EBSD) results suggest that the film is a single crystal complying with a MgB2(0001)//Al2O3(0001) epitaxial relationship. The SRF properties of different film thicknesses (200 nm and 350 nm) were evaluated using SIC system under different temperatures and applied fields at 7.4 GHz. A surface resistance of 9±2 μΩ has been observed at 2.2 K. Based on BCS theory with moving Cooper pairs, the electron states distribution at 0K and the probability of electron occupation with finite temperature have been derived and applied to anomalous skin effect theory to obtain the surface impedance of a superconductor with moving Cooper pairs. We present the numerical results for Nb.

/pdf/surface-impedance-of-superconducting-radio-frequency-srf-tmkyzmm4gh.pdf

Surface Impedance of Superconducting Radio Frequency (SRF) Materials

For future application in high energy linear colliders superconducting accelerator structures must exhibit gradients of at least E acc = 30 MV/m at Q-values above 1010. Even though Q-values as high as 1011have been obtained in smaller assemblies, the Q-value of niobium accelerator structures is usually limited by temperature independent residual losses. The corresponding average residual resistance is typically a few orders of magnitude higher than the theoretical surface resistance at 1.5 K predicted by the BCS-theory. In order to systematically investigate the spatial distribution of the residual resistance and its dependence on surface conditions of the niobium cavity surface, we have built a TE 011 - S-band cavity and a thermometry system, which allows the detection of losses as low as a few microwatt in superfluid helium using a lock-in technique. The thermometry system consists of 100 Allen-Bradley carbon resistors, which are pressed against the endplate of the cavity and which are well shielded against the surrounding helium bath. A computerized data acquisition system is used to measure the loss distribution on the endplate of the cavity. First measurements indicate the presence of three kinds of losses: joint losses caused by poor rf contact at the indium gasket between cavity body and endplate, large patches of moderate resistance and a few localized spots of enhanced losses. With the present system we have been able to measure at a bath temperature of 1.5 K surface resistances of about 50 nOhm at an rf magnetic field of about 20 Oersted, which in turn indicates that at some higher field levels the system will be capable of detecting loss distributions equivalent to Q-values above 1011.

Investigation of the surface resistance of superconducting niobium using thermometry in superfluid helium

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Investigation of the surface resistance of superconducting niobium using thermometry in superfluid helium