GHz Cavity Research Articles

Dark photon constraints from a 7.139 GHz cavity haloscope experiment

The dark photon is a promising candidate for the dark matter which comprises most of the matter in our visible Universe. Via kinetic mixing with the Standard Model it can also be resonantly converted to photons in an electromagnetic cavity, offering novel experimental possibilities for the discovery and study of dark matter. We report the results of a pathfinder dark photon dark matter cavity search experiment performed at Hunan Normal University and the Institute of Physics, Chinese Academy of Sciences, representing the first stage of the Axion and dark Photon EXperiment program. Finding no statistically significant excess, we place an upper limit on the kinetic mixing parameter |χ|<3.7×10−13 around mA≃29.5 μeV at 90% confidence level. This result exceeds other constraints on dark photon dark matter in this frequency range by roughly an order of magnitude. Published by the American Physical Society 2024

Improving Fabrication and Performance of Additively Manufactured RF Cavities by Employing Co-Printed Support Structures and Their Subsequent Removal

The enormous potential of additive manufacturing (AM), particularly laser powder bed fusion (L-PBF), to produce radiofrequency cavities (cavities) has already been demonstrated. However, the required geometrical accuracy for GHz TM010 cavities is currently only achieved by (a) avoiding downskin angles <40∘, which in turn leads to a cavity geometry with reduced performance, or (b) co-printed support structures, which are difficult to remove for small GHz cavities. We have developed an L-PBF-based manufacturing routine to overcome this limitation. To enable arbitrary geometries, co-printed support structures are used that are designed in such a way that they can be removed after printing by electrochemical post-processing, which simultaneously reduces the surface roughness and thus maximizes the quality factor Q0. The manufacturing approach is evaluated on two TM010 single cavities printed entirely from high-purity copper. Both cavities achieve the desired resonance frequency and a Q0 of approximately 8300.

Temperature mapping on a niobium-coated copper superconducting radio-frequency cavity

Since the late ’80s, CERN has pioneered the development of niobium thin film radio-frequency (RF) cavities deposited on copper substrates for several particle accelerator applications. However, niobium thin film cavities historically feature a progressive performance degradation as the accelerating field increases. In this study, we describe a temperature mapping system based on contact thermometry, specially designed to obtain temperature maps of niobium-coated copper cavities and, consequently, study the mechanisms responsible for performance degradation. The first temperature maps on a niobium/copper 1.3 GHz cavity are reported along with its RF performance. In addition to some hotspots displayed in the temperature maps, we surprisingly observed a temperature decrease in a limited portion of the cavity cell as the accelerating field increased. This may shed new light on understanding the heat dissipation of niobium thin film cavities in liquid helium-I, which might be exploited to improve the RF cavity performance.

Monolithically integrated femtosecond optical parametric oscillators

Synchronously pumped optical parametric oscillators (OPOs) are highly efficient sources of long-wavelength pulses and nonclassical light, making them invaluable for applications in spectroscopy, metrology, multi-photon microscopy, and quantum computation. Typical systems based on free-space cavities either operate non-degenerately, which limits their efficiency, or use active feedback control to achieve degenerate operation, which limits these systems to dedicated low-noise environments. In this work, we demonstrate a femtosecond monolithically integrated OPO. In contrast with bulk OPOs, our monolithic 10 GHz cavity, based on reverse-proton-exchanged lithium niobate, operates stably without active locking. By detuning the repetition rate of the free-running pump laser from the cavity free spectral range, we control the intracavity pulse dynamics and observe many of the operating regimes previously encountered in free-space degenerate OPOs, such as box-pulsing and quadratic bright-dark solitons (simultons), in addition to non-degenerate operation. When operated in the simulton regime and pumped with 125 fs pulses at 1 µm, this monolithic OPO chip outputs broadband sech2 pulses (63 nm, 3 dB) with tens of milliwatts of average power.

Electropolishing parameters study for surface smoothening of low-[formula omitted] 650 MHz five-cell niobium superconducting radio frequency cavity

Electropolishing (EP) is applied to niobium (Nb) superconducting radio frequency (SRF) cavities, which are used in particle accelerators for their surface treatment. The EP process for 1.3 GHz cavities has been extensively studied earlier. In this work, a parametric study on EP of low-β (0.61) 650 MHz Nb SRF cavities (LB650), which will be used in pre-production cryomodule for proton improvement plan-II (PIP-II) linear accelerator, was conducted to determine adequate EP conditions for attaining a smooth surface of the cavities. EP performed with the standard parameters and an initial cathode (cathode-I) having a cathode surface area of ∼5% of the cavity surface area yielded a rough equator surface of the cavity. The grain step height on the equator weld position was measured to be ∼32μm. The rough surface was attributed to preferential grain etching confirmed by a polarization curve showing a linear relationship between the EP current and voltage and the absence of the current plateau region. The cathode was modified to make its surface area twice that of cathode-I. The modified cathode (cathode-M) provided a current plateau region in the corresponding I-V curves measured at different cavity temperatures. The onset voltage for the plateau was found to be higher at higher cavity temperatures. This study revealed that even with cathode-M, the standard 18 V was low for EP of such large-sized cavities when the cavity temperature was ≥18 °C. EP performed at a higher voltage of 22–24.5 V with cathode-M yielded a smooth surface with a grain step height of only ∼0.6μm. The applied EP conditions also improved removal uniformity along the cavity length. In contrast to the cavity treated with cathode-I, the cavities treated with cathode-M achieved a significantly higher accelerating gradient (Eacc) in vertical tests conducted in a cryostat at 2 K. The modified EP was found efficient to produce the cavities that achieved Eacc of 22.4 MV/m, required by PIP II project in the baseline RF tests, to qualify for further surface processing used to enhance their quality factors.

High resolution diagnostic tools for superconducting radio frequency cavities.

Superconducting radio-frequency (SRF) cavities are one of the fundamental building blocks of modern particle accelerators. To achieve the highest quality factors (1010-1011), SRF cavities are operated at liquid helium temperatures. Magnetic flux trapped on the surface of SRF cavities during cool-down below the critical temperature is one of the leading sources of residual RF losses. Instruments capable of detecting the distribution of trapped flux on the cavity surface are in high demand in order to better understand its relation to the cavity material, surface treatments and environmental conditions. We have designed, developed, and commissioned two high-resolution diagnostic tools to measure the distribution of trapped flux at the surface of SRF cavities. One is a magnetic field scanning system, which uses cryogenic Hall probes and anisotropic magnetoresistance sensors that fit the contour of a 1.3GHz cavity. This setup has a spatial resolution of ∼13μm in the azimuthal direction and ∼1 cm along the cavity contour. The second setup is a stationary, combined magnetic and temperature mapping system, which uses anisotropic magnetoresistance sensors and carbon resistor temperature sensors, covering the surface of a 3GHz SRF cavity. This system has a spatial resolution of 5mm close to the iris and 11mm at the equator. Initial results show a non-uniform distribution of trapped flux on the cavities' surfaces, dependent on the magnitude of the applied magnetic field during field-cooling below the critical temperature.

A three-dimensional Josephson parametric amplifier

A Josephson parametric amplifier (JPA) is executed in a three-dimensional (3D) microwave cavity by coupling it to a superconducting quantum interference device (SQUID) that is embedded in a two-dimensional resonator. The JPA is activated in a three-wave mixing configuration by injecting ac magnetic flux, at twice the 3D cavity frequency, into the SQUID. An 8.3 GHz cavity is measured in a non-degenerate phase-insensitive configuration which yields gains in excess of 40 dB, where a 20 dB gain results in an operational bandwidth of 0.4 MHz, a 1 dB compression point of −115 dBm with half a quantum of added noise.

Quality Factor Enhancement of 650 MHz Superconducting Radio-Frequency Cavity for CEPC

Medium-temperature (mid-T) furnace baking was conducted at 650 MHz superconducting radio-frequency (SRF) cavity for circular electron positron collider (CEPC), which enhanced the cavity unloaded quality factor (Q0) significantly. In the vertical test (2.0 K), Q0 of 650 MHz cavity reached 6.4 × 1010 at 30 MV/m, which is remarkably high at this unexplored frequency. Additionally, the cavity quenched at 31.2 MV/m finally. There was no anti-Q-slope behavior after mid-T furnace baking, which is characteristic of 1.3 GHz cavities. The microwave surface resistance (RS) was also studied, which indicated both very low Bardeen–Cooper–Schrieffer (BCS) and residual resistance. The recipe of cavity process in this paper is simplified and easy to duplicate, which may benefit the SRF community.

Electrodeposition of copper applied to the manufacture of seamless superconducting rf cavities

Niobium thin film coated copper superconducting radio frequency elliptical cavities have demonstrated for many years their strong potential as an alternative to bulk niobium cavities. The thin film lower performance at high rf field is often attributed to the defects observed in the elaborated Nb layer, sometimes originated from defects inherited from the substrate itself. The currently used methods of manufacturing the copper elliptical substrates include several steps of electron-beam welding in order to join the half cells and the cutoffs which can contribute to defects and porosities. Seamless methods are nowadays developed in order to avoid welding steps and to decrease the global manufacturing cost of the cavities. We propose in this study an innovative alternative route in which the cavity is formed by electrodeposition of copper on a sacrificial aluminum mandrel. The strength of the process relies on the total absence of welding joints. Two different electroforming techniques using either direct current or pulsed plating have been investigated. The electroformed copper exhibited similar mechanical robustness, cryogenic properties and purity as the oxygen-free copper. In addition, the fabrication process was validated on test mandrels which mimic the geometry of 1.3 GHz cavities.

RF design optimization for the SHINE 3.9 GHz cavity

The Shanghai high-repetition-rate XFEL and extreme light facility (SHINE) under construction is designed to be one of the most advanced free electron laser (FEL) facilities in the world. The main part of the SHINE facility is an 8 GeV superconducting Linac operating in continuous wave (CW) mode. The Linac consists of seventy-five 1.3 GHz and two 3.9 GHz cryomodules. Based on the design and experience of 3.9 GHz cavities in the European X-ray free electron laser (E-XFEL) and the Linac coherent light source-II (LCLS-II) projects, we optimize the SHINE 3.9 GHz cavity design to adapt it for CW mode operation. In this paper, we present a particular redesign of the end group for the SHINE 3.9 GHz superconducting cavity that includes a redesign of the end cell and beam pipe to shift away the potentially troublesome lowest high-order modes (HOMs), a modification of the main coupler antenna, and a tuning of the HOM notch filter to meet the cavity requirements. RF losses calculations on the HOM coupler antennas show that the overheating on the inner conductor at the operating mode is diminished significantly. Furthermore, we have also studied the HOMs to ensure there are no dangerously trapped modes in the optimized cavity design.

Simulation of Quench Detection Algorithms for Helmholtz Zentrum Berlin SRF Cavities

The Helmholtz Zentrum Berlin is carrying out two accelerator projects which make use of high gradient Continuous Wave (CW) Superconducting Radiofrequency (SRF) cavities: bERLinPro and BESSY-VSR. In both projects, a prompt detection of a quench is crucial to avoid damages in the cryomodules and cavities themselves. In this paper, the response of real time estimation of the cavity parameters using the transmitted and forward RF signals is simulated, in order to perform the quench detection. The time response of the estimated half bandwidth is compared with the dissipated energy in the cavity walls for the different type of SRF cavities used in both projects, i.e., bERLinPro's photoinjector, booster and linac, and BESSY-VSR 1.5 GHz and 1.75 GHz cavities. As an intermediate step prior to the implementation in an mTCA.4 system together with the LLRF control and test with a real cavity, the algorithm has been implemented using a National Instruments FPGA board to check its proper behavior.

Simulation of quench detection algorithms for Helmholtz Zentrum Berlin SRF cavities

The Helmholtz Zentrum Berlin is carrying out two accelerator projects which make use of high gradient Continuous Wave (CW) Superconducting Radiofrequency (SRF) cavities: bERLinPro and BESSY-VSR. In both projects, a prompt detection of a quench is crucial to avoid damages in the cryomodules and cavities themselves. In this paper, the response of real time estimation of the cavity parameters using the transmitted and forward RF signals is simulated, in order to perform the quench detection. The time response of the estimated half bandwidth is compared with the dissipated energy in the cavity walls for the different type of SRF cavities used in both projects, i.e., bERLinPro’s photoinjector, booster and linac, and BESSY-VSR 1.5 GHz and 1.75 GHz cavities. As an intermediate step prior to the implementation in an mTCA.4 system together with the LLRF control and test with a real cavity, the algorithm has been implemented using a National Instruments FPGA board to check its proper behavior.

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.

Magnetic resonance probing of ground state in the mixed valence correlated topological insulator SmB6

Introducing of topological insulator concept for fluctuating valence compound – samarium hexaboride – has recently initiated a new round of studies aimed to clarify the nature of the ground state in this extraordinary system with strong electron correlations. Here we discuss the data of magnetic resonance in the pristine single crystals of SmB6 measured in 60 GHz cavity experiments at temperatures 1.8–300 K. The microwave study as well as the DC resistivity and Hall effect measurements performed for the different states of SmB6 [110] surface prove definitely the existence of the layer with metallic conductivity increasing under lowering temperature below 5 K. Four lines with the g-factors g ≈ 2 are found to contribute to the ESR-like absorption spectrum that may be attributed to intrinsic paramagnetic centers on the sample’s surface, which are robust with respect to the surface treatment. The temperature dependence of integrated intensity I(T) for main paramagnetic signal is found to demonstrate anomalous critical behavior I(T) ~ (T* − T)ν with characteristic temperature T* = 5.34 ± 0.05 K and exponent ν = 0.38 ± 0.03 indicating possible magnetic transition at the SmB6 [110] surface. Additional resonant magnetoabsorption line, which may be associated with either donor-like defects or cyclotron resonance mode corresponding to the mass mc ~ 1.2m0, is reported.

Detuning related coupler kick variation of a superconducting nine-cell 1.3 GHz cavity

Superconducting TESLA-type cavities are widely used to accelerate electrons in long bunch trains, such as in high repetition rate free electron lasers. The TESLA cavity is equipped with two higher order mode couplers and a fundamental power coupler (FPC), which break the axial symmetry of the cavity. The passing electrons therefore experience axially asymmetrical coupler kicks, which depend on the transverse beam position at the couplers and the rf phase. The resulting emittance dilution has been studied in detail in the literature. However, the kick induced by the FPC depends explicitly on the ratio of the forward to the backward traveling waves at the coupler, which has received little attention. The intention of this paper is to present the concept of discrete coupler kicks with a novel approach of separating the field disturbances related to the standing wave and a reflection dependent part. Particular attention is directed to the role of the penetration depth of the FPC antenna, which determines the loaded quality factor of the cavity. The developed beam transport model is compared to dedicated experiments at FLASH and European XFEL. Both the observed transverse coupling and detuning related coupler kick variations are in good agreement with the model. Finally, the expected trajectory variations due to coupler kick variations at European XFEL are investigated and results of numerical studies are presented.

Resonant excitation of high order modes in the 3.9 GHz cavity of the Linac Coherent Light Source

Construction of the Linac Coherent Light Source II (LCLS-II) is underway for the world's first hard x-ray free-electron laser. A central part of the LCLS-II project is a 4 GeV superconducting radio frequency electron linac that will operate in the continuous wave (cw) mode. The linac is segmented into four sections named as $L0$, $L1$, $L2$, and $L3$. Two 3.9 GHz cryomodules, each housing of eight third-harmonic cavities similar to the cavities developed for the European X-ray Free Electron Laser (XFEL), will be used in section $L1$ of the linac for linearizing the longitudinal beam profile. In this paper, we present a study of trapped high order modes (HOMs) excited by a cw electron beam in the third-harmonic cavities of the LCLS-II linac. A detailed comparison of the original XFEL design and the LCLS-II design with a modified end group is performed in order to estimate the effect of a reduced beam pipe aperture on the efficiency of HOM damping. Furthermore, we apply a statistical analysis of the eigenmode spectrum for the estimation of the probability of resonant HOM losses and influence of HOMs on beam dynamics.

Influence of tests results on serial production of EXFEL superconducting cavities

The European XFEL (EXFEL) 1.3 GHz cavities were produced without any final cold RF performance guarantee for characteristics like accelerating gradient, quality factor or field emission. In the case of low-performance, DESY provided all re-treatment procedures in order to improve these characteristics getting closer to specification levels. But EXFEL cavities have very tight tolerances for the parameters like cavity length, cells eccentricity, fundamental mode frequency and field distribution on the operational mode. Usage of modern infrastructure, analytical methods and powerful tools of the database analysis allows us not only keeping these characteristics in specified ranges, but also investigate the cavity shape uncertainties and improve the damping of the higher order modes.

Magnesium diboride on inner wall of copper tube: A test case for superconducting radio frequency cavities

Superconductor magnesium diboride is considered one of the viable materials to substitute bulk niobium for superconducting radio frequency cavities. Utilizing a ${\mathrm{MgB}}_{2}$ coating on the inner wall of a copper cavity will allow operation at higher temperatures (20--25 K) than Nb cavities due to the high transition temperature of ${\mathrm{MgB}}_{2}$ (39 K) and the high thermal conductivity of Cu. In this paper, we present results of ${\mathrm{MgB}}_{2}$ coating on Cu tubes with similar dimensions to a 3 GHz cavity, as the first step towards coating the actual cavity, using the hybrid physical chemical vapor deposition technique. The results show successful coating of a uniform ${\mathrm{MgB}}_{2}$ layer on the inner wall of the Cu tubes with ${T}_{c}$ as high as 37 K.

Performance in the vertical test of the 832 nine-cell 1.3 GHz cavities for the European X-ray Free Electron Laser

Comprehensive description of the fabrication, surface treatment, and performance of all superconducting cavities for the European X-ray Free Electron Laser project.

Validation of the superconducting 3.9 GHz cavity package for the European X-ray Free Electron Laser

A full test of the cavity package concept under realistic operating condition was a necessary step before the assembly of the European XFEL (EXFEL) 3.9 GHz superconducting system and its installation in the accelerator. One cavity, equipped with magnetic shielding, power coupler and frequency tuner has been tested in a specially designed single cavity cryostat in one of the test benches of the DESY Accelerator Module Test Facility (AMTF). The cavity was operated at high pulsed power up to an accelerating field of $24\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, above the quench accelerating field of $21\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ achieved during the continuous wave (CW) vertical qualification test and with a large margin with respect to the EXFEL maximum operating specification of $15\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ for the 3.9 GHz system. All subsystems under test---coupler, tuner, waveguide tuners, low level radio-frequency (LLRF) system---were qualified to their design performances.