Overview of SRF Deflecting and Crabbing Cavities

Developments over the past few years on novel superconducting deflecting and crabbing cavities have introduced advanced rf geometries with improved performance, in comparison to the typical squashed elliptical cavities operating in TM110 type mode. These new structures are compact geometries operating in either TEM type or TE11-like mode. One of the key applications of such cavities is the use of crabbing systems for circular colliders in increasing the luminosity. Crabbing systems are an essential component in future colliders with intense beams and proposed electron-ion colliders. High luminosity upgrade of LHC is planned to implement crabbing systems at two interaction points. Recently, a two-cavity cryomodule with double quarter wave crabbing cavity was installed in SPS at CERN and successfully tested with the proton beam. We present the details of different superconducting deflecting and crabbing cavities and their applications, as well as the recent results of the crabbing systems test at SPS

Measurement of Surface Resistance Properties With Coaxial Resonators - Review

Achieving ever decreasing surface resistance at higher field in superconducting RF accelerating structures is one of most outstanding developments in modern accelerators. The BCS theory has been used widely to estimate the surface resistance and to direct the technology. However, recent research results show that the behavior of the surface resistance further deviates from the BCS theory. So far the study on surface resistance was performed usually with cavities of single frequency which limited the study of frequency dependent surface resistance. The Center for Accelerator Science at Old Dominion University has designed and built several half wave coaxial cavities to study the frequency, temperature, and RF field dependence of surface resistance. TRIUMF in Canada also joined this line of research using such multi frequency quarter wave and half wave coaxial cavities. This type of multi mode cavity will allow us to systematically study the parameters affecting surface resistance on the same cavity surface. In this paper, we review the results ODU and TRIUMF collected so far and proper analysis methods

Temperature Mapping of Nitrogen-Doped Niobium Superconducting Radiofrequency Cavities

It was recently shown that diffusing nitrogen on the inner surface of superconducting radiofrequency (SRF) cavities at high temperature can improve the quality factor of the niobium cavity. However, a reduction of the quench field is also typically found. To better understand the location of rf losses and quench, we used a thermometry system to map the temperature of the outer surface of ingot Nb cavities after nitrogen doping and electropolishing. Surface temperature of the cavities was recorded while increasing the rf power and also during the quenching. The results of thermal mapping showed no precursor heating on the cavities and quenching to be ignited near the equator where the surface magnetic field is maximum. Hot-spots at the equator area during multipacting were also detected by thermal mapping

Measurement of the Magnetic Field Penetration Into Superconducting Thin Films

The magnetic field at which first flux penetrates is a fundamental parameter characterizing superconducting materials for SRF cavities. Therefore, an accurate technique is needed to measure the penetration of the magnetic field directly. The conventional magnetometers are inconvenient for thin superconducting film measurements because these measurements are strongly influenced by orientation, edge and shape effects. In order to measure the onset of field penetration in bulk, thin films and multi-layered superconductors, we have designed, built and calibrated a system combining a small superconducting solenoid capable of generating surface magnetic field higher than 500 mT and Hall probe to detect the first entry of vortices. This setup can be used to study various promising alternative materials to Nb, especially SIS multilayer coatings on Nb that have been recently proposed to delay the vortex penetration in Nb surface. In this paper, the system will be described and calibration will be presented

Measurements of Frequency, Temperature, RF Field Dependent Surface Resistance Using Superconducting Half-Wave Cavity

A theory of surface resistance of superconductor was rigorously formulated by Bardeen, Cooper, Schrieffer more than 50 years ago. Since then the accelerator community has been used the theory as a guideline to improve the surface resistance of the superconducting cavity. It has been observed that the surface resistance is dependent on frequency, temperature and rf field strength, and surface preparation. To verify these dependences, a well-controlled study is required. Although many different types of cavities have been tested, the typical superconducting cavities are built for specific frequencies of their application. They do not provide data other than at its own frequency. A superconducting half wave cavity is a cavity that enables us to collect the surface resistance data across frequencies of interest for particle accelerators and evaluate preparation techniques. This paper will present the design of the half wave cavity, its electromagnetic mode characteristics and experimental results

Cryogenic Probe Station at Old Dominion University Center for Accelerator Science

With a growing effort in research and development of an alternative material to bulk Nb for a superconducting radiofrequency (SRF) cavity, it is important to have a cost effective method to benchmark new materials of choice. At Old Dominion University\u27s Center for Accelerator Science, a cryogenic probe station (CPS) will be used to measure the response of superconductor samples under RF fields. The setup consists of a closed-cycle refrigerator for cooling a sample wafer to a cryogenic temperature, a superconducting magnet providing a field parallel to the sample, and DC probes in addition to RF probes. The RF probes will extract a quality factor from a sample patterned in a coplanar waveguide resonator structure on a 2\u27 wafer. From the measured quality factor, the surface resistance and the penetration depth as a function of temperature and magnetic field will be calculated. This paper will discuss the design and measurement procedures of the current CPS setup

Compact Superconducting RF-Dipole Cavity Designs for Deflecting and Crabbing Applications

Over the years the superconducting parallel-bar design has evolved into an rf-dipole cavity with improved properties. The new rf-dipole design is considered for a number of deflecting and crabbing applications. Some of those applications are the 499 MHz rf separator system for the Jefferson Lab 12 GeV upgrade, the 400 MHz crabbing cavity system for the proposed LHC high luminosity upgrade, and the 750 MHz crabbing cavity for the medium energy electron-ion collider in Jefferson Lab. In this paper we present the optimized rf design in terms of rf performance including rf properties, higher order modes (HOM) properties, multipacting, and multipole expansion for the above mentioned applications

Lessons learned from RF-Dipole Prototype Cavities for LHC High Luminosity Upgrade

The RF-Dipole Crabbing Cavity designed for the LHC High Luminosity Upgrade includes two higher order mode (HOM) couplers. One of the HOM couplers is an rf filter, which is a high pass filter designed to couple to the horizontal dipole modes and accelerating modes up to 2 GHz, while rejecting the fundamental operating mode at 400 MHz. The coupler consists of a high pass filter circuit where the rejection of the operating mode and transmission of HOMs are sensitive to dimensional deviations. An rf test box has been designed to measure the transmission of the rf filter in order to qualify the fabricated HOM coupler and to tune the coupler. This paper presents the measurements of the HOM coupler with the rf test box

Investigation of the Surface Resistance of Niobium Between 325 MHz and 1300 MHz Using a Coaxial Half-wave Cavity

The Center for Accelerator Science at Old Dominion University has built a half-wave coaxial cavity (*) to measure the surface resistance of niobium as a function of frequency, temperature, rf field, preparation techniques, over a wide range of frequencies of interest for particle accelerators. The characteristics of the half-wave coaxial cavity provide these information on a same surface. The preliminary results showed clearly the frequency dependence of residual surface resistance (**). After establishing baseline, we have conducted a study of low temperature baking effect on the surface resistance under controlled environment. This paper will describe the details of the test procedure, results and we will explore underlying physics of the phenomenon. * H. Park et al., MOPB003, Proc. SRF2015, http://jacow.org/** H. Park et al., THPB080, Proc. SRF2017, http://jacow.org

Magnetic Field Penetration Technique to Study Field Shielding of Multilayered Superconductors

The SIS structure which consists of alternative thin layers of superconductors and insulators on a bulk niobium has been proposed to shield niobium cavity surface from high magnetic field and hence increase the accelerating gradient. The study of the behavior of multilayer superconductors in an external magnetic field is essential to optimize their SRF performance. In this work we report the development of a simple and efficient technique to measure penetration of magnetic field into bulk, thin film and multilayer superconductors. Experimental setup contains a small superconducting solenoid which can produce a parallel surface magnetic field up to 0.5 T and Hall probes to detect penetrated magnetic field across the superconduct- ing sample. This system was calibrated and used to study the effect of niobium sample thickness on the field of full magnetic flux penetration. We determined the optimum thickness of the niobium substrate to fabricate the multi-layer structure for the measurements in our setup. This technique was used to measure penetration fields of Nb3Sn thin films and Nb3Sn/Al2O3 multilayers deposited on Al2O3 wafers. The system was optimized to mitigate thermo- magnetic flux jumps at low temperatures