Superconducting Niobium Cavities Research Articles

Successful Al2O3 coating of superconducting niobium cavities with thermal ALD

The surface modification of superconducting radio frequency (SRF) cavities is mandatory to further push the limits in future accelerators. One strategy is the deposition of multilayer superconducting and insulating materials on top of the inner surface of an SRF cavity. Here, we report on a successful low-temperature coating of an SRF cavity with insulating by thermal atomic layer deposition (ALD), without mitigating its maximum achievable accelerating field of more than 40 MV m−1. Furthermore, an improvement of the surface resistance above 30 MV m−1 has been observed, which is likely caused by an enhanced oxygen diffusion during the deposition process. Our results show that ALD is perfectly suited to provide a conformal coat to the interior of the cavity and to even modify and improve the properties of such devices.

Surface resistance effects of medium temperature baking of buffered chemical polished 1.3 GHz nine-cell large-grain cavities

Three 1.3 GHz nine-cell large-grain superconducting niobium cavities were investigated with medium-temperature baking, using buffered chemical polishing to remove the impurity profiles from previous heat treatments. The average intrinsic quality factor in 2 K vertical tests of the cavities was 2.7 × 1010 at 16 MV m−1 with maximum accelerating gradients of 20–22 MV m−1. These promising values confirmed the effectiveness of an improved medium-temperature baking recipe for niobium cavities, beneficial for superconducting radiofrequency applications. Furthermore, resistance analysis demonstrated that medium-temperature baking reduced both the Bardeen–Cooper–Schrieffer (BCS) resistance and the residual resistance of the cavities. Impurity analysis on niobium samples provided some proof that the reduction in BCS resistance was due to the shortened electron mean free path while the reduction in residual resistance was probably associated with mitigation of the increase in interstitial impurity atoms.

Frequency sensitivity of the passive third harmonic superconducting cavity for SSRF

A 1.5 GHz passive third harmonic superconducting cavity was proposed to improve the beam quality and lifetime in the Shanghai Synchrotron Radiation Facility Phase-II beamline project. Lifetime improvement highly depends on the resonant frequency of the passive third harmonic superconducting cavity. It is important that the operating frequency of the cavity is within the design range and the cavity has reasonable mechanical stability. A simulation method for the multiphysics coupled analysis has been developed based on the ANSYS code. Multiphysics coupled simulations have been performed under different conditions, such as etching, evacuation, cooling, and preloading. Analyses of mechanical modes and structural stress have been executed. A possible stiffening ring method for the two-cell superconducting niobium cavity has been investigated. In this paper, we present a multiphysics coupled analysis of the third harmonic cavity using a finite element analysis code. The results of the analysis show that a reliable frequency for the cavity after electron beam welding is 1498.033 MHz, and the corresponding frequency of the pre-tuning goal is 1496.163 MHz. A naked cavity is a reasonable option based on structural stress and mechanical modal analyses. A frequency range of ± 500 kHz and limiting tolerable displacement of ± 0.35 mm are proposed for the design of the frequency tuner.

Vertical electropolishing for 1.3 GHz single- and nine-cell superconducting niobium cavities: A parametric optimization and rf performance

Electropolishing (EP) is a standard process for the treatment of the interior surface of niobium (Nb) superconducting radio-frequency (SRF) cavities used in particle accelerators. We previously conducted a study on vertical electropolishing (VEP), in which the cavity was set vertically during EP of a 1.3 GHz Nb single-cell cavity. In that study, we showed that the major cause of asymmetric removal in VEP is the accumulation of hydrogen gas bubbles, generated during the EP process, on the upper half cell of the cavity. Three types of unique rotating cathodes were tested with the aim to resolve the issues of asymmetric removal and rough surface. Appropriate cathode design and parameters reduced the asymmetric removal in the cell and yielded a smooth surface of the cavity. In this study, we have performed an extensive investigation on VEP of both single- and nine-cell Nb 1.3 GHz cavities with a modified cathode. The VEP parameters with single- and nine-cell coupon cavities were investigated, which facilitate an in situ study of the coupon currents at various positions of the cavities and observation from the viewports located near the iris and equator positions of the cavities. The modified cathode and optimized VEP process for the single-cell cavity yielded uniform removal in the cavity cell and a smooth surface of the interior of the cavity. A novel acid flow method that allows the separation of the acid flow in the cavity and cathode housing was employed in the nine-cell cavity to efficiently remove the gas bubbles from the cavity. The optimized acid flow rates in the cavity and cathode housing, along with other adequate parameters including temperature, cathode rotation speed, and applied voltage, made the surface smooth and significantly reduced the removal nonuniformity in the nine-cell cavity. The single- and nine-cell cavities after an average removal of 46 and $130\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, respectively, were tested in a vertical cryostat. The single- and nine-cell cavities achieved $31\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a ${Q}_{0}$ value of $9\ifmmode\times\else\texttimes\fi{}{10}^{9}$ and $28.3\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a ${Q}_{0}$ value of $6.7\ifmmode\times\else\texttimes\fi{}{10}^{9}$, respectively, in the rf tests at the temperature of 2 K. The achieved SRF performances were as good as the baseline performances achieved after the horizontal electropolishing process of the same cavities.

Microwave Characterization of 122-Pnictides by a Superconducting Niobium Cavity

Microwave Characterization of 122-Pnictides by a Superconducting Niobium Cavity

N-Doped Surfaces of Superconducting Niobium Cavities as a Disordered Composite

The Q-factor of superconducting accelerating cavities can be substantially improved by a special heat treatment under N2 atmosphere (N-doping). Recent experiments at Fermi National Laboratory investigated the dependence of Q on the RF frequency and showed, unexpectedly, both an increase and a decrease with the RF field amplitude. This paper shall explain this finding by extending a previously proposed model founded on the two fluid model of RF losses, percolation and the proximity effect in a disordered composite.

High field Q slope and the effect of low-temperature baking at 3 GHz

A strong degradation of the unloaded quality factor with field, called high field $Q$ slope, is commonly observed above ${\mathrm{B}}_{p}\ensuremath{\cong}100\text{ }\text{ }\mathrm{mT}$ in elliptical superconducting niobium cavities at 1.3 and 1.5 GHz. In the present experiments several 3 GHz niobium cavities were measured up to and above ${\mathrm{B}}_{p}\ensuremath{\cong}100\text{ }\text{ }\mathrm{mT}$. The measurements show that a high field $Q$ slope phenomenon limits the field reach at this frequency, that the high field $Q$ slope onset field depends weakly on the frequency, and that the high field $Q$ slope can be removed by the typical empirical solution of electropolishing followed by heating to 120\ifmmode^\circ\else\textdegree\fi{}C for 48 hrs. In addition, one of the cavities reached a quench field of 174 mT and its field dependence of the quality factor was compared against global heating predicted by a thermal feedback model.

Understanding Quality Factor Degradation in Superconducting Niobium Cavities at Low Microwave Field Amplitudes.

In niobium superconducting radio frequency (SRF) cavities for particle acceleration, a decrease of the quality factor at lower fields-a so-called low field Q slope or LFQS-has been a long-standing unexplained effect. By extending the high Q measurement techniques to ultralow fields, we discover two previously unknown features of the effect: (i)saturation at rf fields lower than E_{acc}∼0.1 MV/m; (ii)strong degradation enhancement by growing thicker niobium pentoxide. Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner cavity surface, thereby identifying a new source of residual resistance and providing guidance for potential nonaccelerator low-field applications of SRF cavities.

Process simulations for the LCLS-II cryogenic systems

Linac Coherent Light Source II (LCLS-II), a 4 GeV continuous-wave (CW) superconducting electron linear accelerator, is to be constructed in the existing two mile Linac facility at the SLAC National Accelerator Laboratory. The first light from the new facility is scheduled to be in 2020. The LCLS-II Linac consists of thirty-five 1.3 GHz and two 3.9 GHz superconducting cryomodules. The Linac cryomodules require cryogenic cooling for the super-conducting niobium cavities at 2.0 K, low temperature thermal intercept at 5.5-7.5 K, and a thermal shield at 35-55 K. The equivalent 4.5 K refrigeration capacity needed for the Linac operations range from a minimum of 11 kW to a maximum of 24 kW. Two cryogenic plants with 18 kW of equivalent 4.5 K refrigeration capacity will be used for supporting the Linac cryogenic cooling requirements. The cryogenic plants are based on the Jefferson Lab’s CHL-II cryogenic plant design which uses the “Floating Pressure” design to support a wide variation in the cooling load. In this paper, the cryogenic process for the integrated LCLS-II cryogenic system and the process simulation for a 4.5 K cryoplant in combination with a 2 K cold compressor box, and the Linac cryomodules are described.

SNS proton power upgrade

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) is preparing for the Proton Power Upgrade (PPU) project to increase the output energy of the accelerator from 1.0 GeV to 1.3 GeV. As part of this project with the combination of increasing the output energy and beam current, the beam power capability will be doubled from 1.4MW to 2.8MW. In this project, seven new high beta cryomodules housing 28 superconducting niobium cavities will be added to the LINAC tunnel. Lessons learned from over ten years of operation will be incorporated into the new cryomodule and cavity design. The design and the fabrication of these cryomodules and how these will be integrated into the existing accelerator will be detailed in this paper.

Measurement of Microwave Parameters of a Superconducting Niobium Cavity

This paper describes a method for direct measurement of the amplitude–frequency characteristics and the Q factor of empty superconducting niobium radio frequency Tesla-type cavities. An automated measuring complex that permits recording the superconductivity effect and measuring high Q values has been developed. Measurements have been made of the Q factors of the investigated objects (the first domestic 1.3-GHz niobium cavities) at a level no lower than 0.1·109 (with a maximum value of ~1.2·1010) and a level of relative losses lower than ~130 dB (with a minimum factor of ~139.7 dB) at liquid nitrogen temperature.

Effect of non-uniform surface resistance on the quality factor of superconducting niobium cavity

The formula Rs=G/Q0is commonly used in the calculation of the surface resistance of radio frequency niobium superconducting cavities. The applying of such equation is under the assumption that surface resistance is consistent over the cavity. However, the distribution of the magnetic field varies over the cavity. The magnetic field in the equator is much higher than that in the iris. According to Thermal Feedback Theory, it leads non-uniform distribution of the density of heat flux, which results in a different temperature distribution along the cavity inter surface. The BCS surface resistance, which depends largely on the temperature, is different in each local inner surface. In this paper, the effect of surface non-uniform resistance on the quality factor has been studied, through the calculation of Q0 in the original definition of it. The results show that it is necessary to consider the non-uniform distribution of magnetic field when the accelerating field is above 20MV/m for TESLA cavities. Also, the effect of inhomogeneity of residual resistance on the quality factor is discussed. Its distribution barely affects the quality factor.

Manufacture of superconducting niobium cavity parts by hydropercussion punching

A complete cycle of manufacturing all parts of superconducting niobium cavities which comprise two half-cells, two drift tubes, and two flanges each has been developed. The cavity half-cells are manufactured by hydropercussion punching, which has a number of advantages over the instrumental stamping technique. For the first time, the diagram of formability of ultrapure niobium has been experimentally obtained for hydropercussion punching and the key parameters of the process have been determined that ensure complete deep-drawing of workpieces, viz., the value of the limit drawing ratio of ultrapure niobium that is 1.92 at a specific impact energy of 0.42 MJ/m2. The deviations of the half-cell dimensions from the rated values do not exceed 0.1 mm. Production tools required for all manufacturing steps have been created. The parameters of machining of the niobium cavity parts prior to welding have been experimentally established.

Developing of superconducting niobium cavities for accelerators

The results of a study of structure and mechanical properties of welding joints, superconducting characteristics of the material after joining of welded components of superconducting radio frequency cavities are presented. The paper also describes the results of testing of the RF 1.3 GHz single-cell niobium cavity manufactured in the PTI NAS Belarus.

High sensitivity niobium parametric transducer for the Mario Schenberg gravitational wave detector

Parametric transducers can work below the quantum limit of sensitivity for resonant mass gravitational wave detectors. This makes them a promising alternative for electromechanical transductance for such detectors. These transducers consist of a reentrant superconducting niobium cavity coupled to a mass-spring system with three mechanical modes. These cavities have a central post responsible for creating a narrow axial gap between its top and the cavity wall, which is a resonant membrane. Their displacement sensitivity (df/dx) increases as the gap spacing decreases. However, this is not a linear relationship and the dimensioning of the cavity becomes critical if the gap is of the order of a few microns. In this paper, we describe how to obtain a gap spacing of ∼ 3 μ m and also the development of parametric transducers that will be employed in the coming experimental runs of the Schenberg gravitational wave antenna. Mechanical thinning methods were performed followed by mechanical and electrical frequency measurements to tune the device to operate at the required frequencies. The main results present better frequency stability and an improvement of df/dx by one order of magnitude higher than the preceding models. These results will allow us to reach the quantum limit of detector sensitivity of ∼ 10−22 Hz−1/2 in the near future, making it possible to search for gravitational waves around 3.2 kHz.

Ultra-high quality factors in superconducting niobium cavities in ambient magnetic fields up to 190 mG

Ambient magnetic field, if trapped in the penetration depth, leads to the residual resistance and therefore sets the limit for the achievable quality factors in superconducting niobium resonators for particle accelerators. Here, we show that a complete expulsion of the magnetic flux can be performed and leads to: (1) record quality factors Q > 2 × 1011 up to accelerating gradient of 22 MV/m; (2) Q ∼ 3 × 1010 at 2 K and 16 MV/m in up to 190 mG magnetic fields. This is achieved by large thermal gradients at the normal/superconducting phase front during the cooldown. Our findings open up a way to ultra-high quality factors at low temperatures and show an alternative to the sophisticated magnetic shielding implemented in modern superconducting accelerators.

High-Q superconducting niobium cavities for gravitational wave detectors

The main purpose of this work is to optimize the electricQ-factor of superconducting niobium klystron cavities to be used inparametric transducers of the Mario Schenberg gravitational wave detector.Many cavities were manufactured from niobium with relatively high tantalumimpurities (1420 ppm) and they were cryogenically tested to determine theirresonance frequencies, unloaded electrical quality factors (Q0) andelectromagnetic couplings. These cavities were closed with a flat niobiumplate with tantalum impurities below 1000 ppm and an unloaded electricalquality factors of the order of 105 have been obtained. AC conductivityof the order of 1012 S/m has been found for niobiumcavities when matching experimental results with computational simulations.These values for the Q-factor would allow the detector to reach the quantumlimit of sensitivity of ∼ 10−22 Hz−1/2 in the nearfuture, making it possible to search for gravitational waves around 3.2 kHz.The experimental tests were performed at the laboratories of the NationalInstitute for Space Research (INPE) and at the Institute for AdvancedStudies (IEAv - CTA).

Dependence of the residual surface resistance of superconducting radio frequency cavities on the cooling dynamics around Tc

We report a strong effect of the cooling dynamics through Tc on the amount of trapped external magnetic flux in superconducting niobium cavities. The effect is similar for fine grain and single crystal niobium and all surface treatments including electropolishing with and without 120 °C baking and nitrogen doping. Direct magnetic field measurements on the cavity walls show that the effect stems from changes in the flux trapping efficiency: slow cooling leads to almost complete flux trapping and higher residual resistance, while fast cooling leads to the much more efficient flux expulsion and lower residual resistance.

Feedback Control of Loaded Q Values of the Superconducting Cavities at FLASH

The well-established technology of superconducting niobium cavities (TESLA - TeV-Energy Superconducting Linear Accelerator) finds an increasing number of applications for linear accelerators in high-energy physics experiments. Together with cavity design and manufacturing, control systems for accelerating field parameters were developed. The digital control system developed for cryomodule operation at the Free Electron Laser in Hamburg (FLASH) is able to perform field amplitude and phase regulation with the precision requested for the multiple user experiments carried at FLASH and according to the required laser light parameters. While new experiments (like the International Linear Collider or the European X-FEL) are based on the same technology (and similar control systems), they require additional levels of controlling superconducting structures parameters in order to comply with tighter power budget overheads and finer field regulation requirements. Consequently, the possibility of controlling such parameters as cavity loaded quality factor or resonance frequency is becoming more attractive. The necessity and the benefits of regulating these parameters have been described in and . In this paper, we describe the algorithm for feedback control of the loaded Q in TESLA cavities by means of motor control of the fundamental power coupler.

The effect of vacancies on the microwave surface resistance of niobium revealed by positron annihilation spectroscopy

Using variable-energy positron annihilation spectroscopy, we demonstrate that a different near-surface vacancy concentration accompanies drastic differences in surface resistance of superconducting niobium cavities for particle acceleration. Our data suggest that vacuum baking at 120 °C leads to the doping of a near-surface layer with vacancy-hydrogen complexes, and that higher vacancy-type defect concentration distinguishes electropolished from chemically etched cavities. Our findings may help to explain a strong dependence of cavity performance on heat and chemical treatments, and may be of interest to other physics fields including cavity quantum electrodynamics (QED), microresonators, and single photon detectors.