Mechanical losses in low loss materials studied by Cryogenic Resonant Acoustic spectroscopy of bulk materials (CRA spectroscopy)

Mechanical losses of crystalline silicon and calcium fluoride have been analyzed in the temperature range from 5 to 300 K by our novel mechanical spectroscopy method, cryogenic resonant acoustic spectroscopy of bulk materials (CRA spectrocopy). The focus lies on the interpretation of the measured data according to phonon-phonon interactions and defect induced losses in consideration of the excited mode shape.Comment: 4 pages, 4 figures, proceedings of the PHONONS 2007, submitted to Journal of Physics: Conference Serie

Heat source localisation by trilateration of helium II second sound detected with transition edge sensors thermometry

The detection of second sound in He-II can be exploited during superconducting cavity testing to locate the germ of a quench. The sudden appearance of a hotspot generates this wave in the helium II bath, which is routinely detected by Oscillating Superleak Transducers (OST) reacting to the first arrived inter-component velocity front. Recently, we have developed Transition Edge Sensors (TES) that are able to detect second sound by measuring directly the temperature fluctuation of second sound (below milli-Kelvin, in sub-millisecond time scale) with a good native signal-to-noise ratio. We present the current state of development of second sound detectors based on TES, experiments aiming to characterize more thoroughly their behaviour as second sound detectors by thermometry, and the capabilities they provide in terms of localisation of the heat source in the case of direct sight

On the mechanical quality factors of cryogenic test masses from fused silica and crystalline quartz

Current interferometric gravitational wave detectors (IGWDs) are operated at room temperature with test masses made from fused silica. Fused silica shows very low absorption at the laser wavelength of 1064 nm. It is also well suited to realize low thermal noise floors in the detector signal band since it offers low mechanical loss, i. e. high quality factors (Q factors) at room temperature. However, for a further reduction of thermal noise, cooling the test masses to cryogenic temperatures may prove an interesting technique. Here we compare the results of Q factor measurements at cryogenic temperatures of acoustic eigenmodes of test masses from fused silica and its crystalline counterpart. Our results show that the mechanical loss of fused silica increases with lower temperature and reaches a maximum at 30 K for frequencies of slightly above 10 kHz. The losses of crystalline quartz generally show lower values and even fall below the room temperature values of fused silica below 10 K. Our results show that in comparison to fused silica, crystalline quartz has a considerably narrower and lower dissipation peak on cooling and thus has more promise as a test mass material for IGDWs operated at cryogenic temperatures. The origin of the different Q factor versus temperature behavior of the two materials is discussed.Comment: 11 pages, 2 figures, submitted to Class. Quantum Gra

Conceptual Design of the Cryostat for a Highly Radiation Transparent 2 T Superconducting Detector Solenoid for FCC-ee⁺

The Future Circular Collider electron-positron version (FCC-ee$^{+}$) may be the next step towards a next generation of particle colliders. It may include an Experiment for probing ee$^{+}$ collisions using the IDEA (International Detector for Electron-positron Accelerator), or a similar detector, requiring a solenoid enclosing the inner tracking detector. An innovative 2 T superconducting solenoid with 4 m bore and 6 m long has been accepted as baseline. Positioning the solenoid in between tracker and calorimeter requires an ultra-thin and highly radiation transparent cold mass. Likewise, a thin and radiation transparent cryostat is needed. The set value for the solenoid's maximum radiation length is 1 × X0. The cryostat is designed as a sandwich of thin Aluminum alloy inner and outer shells, eventually locally reinforced, for achieving vacuum tightness, and layers of innovative insulation material providing lowest thermal conductivity and sufficient mechanical resistance. Cryogel Z, a composite blanket of silica aerogel and reinforcing fibers, has a density of 160 kg/m$^{3}$ and would allow a 250 mm cryostat thickness. As an alternative, glass spheres (e.g., K1 type, manufactured by 3M, with a 65μm diameter and a 125 kg/m$^{3}$ density), or similar material, can be dispersed between the vacuum vessel thin-walls providing structural support. Besides the cryostat conceptual design, we outline the setup developed at CERN to represent the real-case cryostat and to measure the heat load transferred through the above-mentioned materials and we present the test results for Cryogel Z

Towards robust design of thin film transition edge sensors for use in the next-generation superconducting radio frequency cavities

In order to increase the accelerating gradient, the next-generation of Superconducting Radio Frequency (SRF) cavities will be operated with superfluid helium cooling. This upgrade requires the development of a state-of-the-art cryogenic temperature mapping system, which can be used to identify quench initiation in new cavities, and thereby assess their suitability for installation. This paper presents a new mapping system based on an array of Transition Edge Sensors (TESs): electrical devices that exploit the superconducting transition of a thin film to identify temperature changes. The TES array is manufactured using photolithography to deposit a thin film on a 100 mm diameter glass wafer. Two different designs of Au-Sn TES have been assessed; Design 1 was composed of a 10 nm Cr adhesive layer, followed by 20 nm of Au and 100 nm of Sn, and Design 2 was identical except that the Cr layer was not applied. Design 1 showed excellent film adherence, however no superconducting transition was observed. In contrast, Design 2 showed poor film bonding but a superconducting transition. These insights are being used to design a new cryogenic temperature mapping device that combines Design 1 for robust electrical contacts and Design 2 for second sound detection

High-temperature superconducting screens for magnetic field-error cancellation in accelerator magnets

Accelerators magnets must have minimal magnetic field imperfections to reduce particle-beam instabilities. In the case of coils made of high-temperature superconducting (HTS) tapes, the magnetization due to persistent currents adds an undesired field contribution, potentially degrading the magnetic field quality. In this paper we study the use of superconducting screens based on HTS tapes for reducing the magnetic field imperfections in accelerator magnets. The screens exploit the magnetization by persistent currents to cancel out the magnetic field error. The screens are aligned with the main field component, such that only the undesired field components are compensated. The screens are self-regulating, and do not require any externally applied source of energy. Measurements in liquid nitrogen at 77 K show for dipole-field configurations a significant reduction of the magnetic field error up to a factor of four. The residual error is explained via numerical simulations accounting for the geometric defects in the HTS screens, achieving satisfactory agreement with experimental results. Simulations show that if screens are increased in width and thickness, and operated at 4.5 K, field errors may be eliminated almost entirely for the typical excitation cycles of accelerator magnets

Heat Extraction From the LHC Main Dipole, Main Quadrupole, and MQXA Superconducting Cables

The forthcoming operation of the CERN Large Hadron Collider (LHC) at 13-14 TeV requires a deep understanding of the heat transfer mechanisms in the most critical superconducting magnets. This is aimed at determining their steady-state quench limits and constitutes an input to compute the magnets stability in transient conditions as well, to prevent beam induced quenches. Heat extraction capability of the LHC Nb-Ti magnets relies on the significant contribution provided by superfluid helium (He II). Due to lack of knowledge of the He II distribution in the cable and in the compressed insulation, experimental investigations are necessary. In this work we present an experimental study aimed at reproducing the thermal behavior of superconducting coils using short length samples. With respect to previous studies, a new instrumentation technique was developed and an in-situ calibration of the thermocouples was performed. The study was conducted on different types of instrumented cables-stack reproducing the main bending dipole (MB), the main quadrupole (MQ), and the MQXA, which is one of the two low-β interaction region quadrupoles. The heat extraction was determined as a function of the cable temperature, of the bath temperature and of the beam loss scenario

High flux three dimensional heat transport in superfluid helium and its application to a trilateration algorithm for quench localization with OSTs

Oscillating superleak transducers of second sound can be used to localize quench spots on superconducting cavities by trilateration. However propagation speeds faster than the velocity of second sound are usually observed imped- ing the localization. Dedicated experiments show that the fast propagation cannot be correlated to the dependence of the velocity on the heat flux density, but rather to boiling effects in the vicinity of the hot spot. 17 OSTs were used to detect quenches on a 704MHz one-cell elliptical cavity. Two different algorithms for quench localization have been tested and implemented in a computer program enabling direct crosschecks. The new algorithm gives more consis- tent results for different OST signals analyzed for the same quench spot

Semi-Dry Cooling Solutions for Future Superconducting Accelerator Structures

Cryogenic dry cooling solutions for Superconducting RF cavities allow for synergies with modern cavity production methods based on vacuum coating and A15 SC layer deposition methods. A proof-of-concept cryogenic performance test stand will study relevant parameters of low-temperature cooling loops with respect to the RF performance of a 1.3 GHz prototype cavity. The paper describes cooling options concerning helium forced flow heat transfer at the capillary surface plus the conduction pathway and the consequent temperature distribution in the cavity itself. Such novel and drastically reduced helium content cooling schemes can be applied to a wide variety of SC cavity or magnet assemblies. The results of the numerical evaluation of heat transfer and pressure drop relations for two-phase and single-phase supercritical flow are cited. These data form the basis for future experimental validation campaigns in the temperature range of 4.2 K to 25 K and helium pressures of up to 2.2 MPa. Special attention is paid to possible new introduced effects of mechanical vibrations or temperature gradients along the cooling capillary when compared to a stagnant He bath cooling

Thermal Mapping of SRF Cavities by Second Sound Detection With Transition Edge Sensors and Oscillating Superleak Transducers

The SRF cavity testing facilities at CERN include four vertical cryostat stations in SM18 and a cryostat for small cavities in the Cryolab. A large range of structures are tested, from Nb thin film cavities for HIE-Isolde and LHC, to bulk Nb crab cavities for HiLumi or 704 MHz 5-cell high-gradient cavities. To cope with different shapes and small series tests, thermal mapping diagnostics is deployed by sensing second sound in superfluid helium. A new type of Transition Edge Sensors (TES) has been developed in the last 2 years. These are miniature resistors of thin-film superconducting alloys, micro-produced on insulating wafers. An extensive campaign of optimization of design, fabrication process and composition was accompanied by qualification in a calibration cryostat. Reproducibility, stability, then intensity, distance and angular dependence of the response were assessed and compared to Oscillating Superleak Transducers (OST). The TES were then installed in a vertical cryostat for tests of a prototype crab cavity for HiLumi. TES are now applied to quench localization on high gradient cavities, for which the most recent results will be presented, together with the OST results