RF Cavities Research Articles

Characterization of dissipative regions of a N-doped superconducting radio-frequency cavity

We report radio-frequency measurements of quality factors and temperature mapping of a nitrogen doped Nb superconducting RF cavity. Cavity cutouts of hot and cold spots were studied with low temperature scanning tunneling microscopy and spectroscopy, X-ray photoelectron spectroscopy and secondary electron microscopy. Temperature mapping revealed a substantial reduction of the residual resistance upon cooling the cavity with a greater temperature gradient and hysteretic losses at the quench location, pointing to trapped vortices as the dominant source of residual surface resistance. Analysis of the tunneling spectra in the framework of a proximity effect theory shows that hot spots have a reduced pair potential and a wider distribution of the contact resistance between the Nb and the top Nb oxide. Alone, these degraded superconducting properties account for a much weaker excess dissipation as compared with the vortex contribution. Based on the correlation between the quasiparticle density of states and temperature mapping, we suggest that degraded superconducting properties may facilitate vortex nucleation or settling of trapped flux during cooling the cavity through the critical temperature.

Design Study of a Superconducting Quadrupole Magnet System Sustainable Under Dark Current Heating in ILC Main Linac

The International Linear Collider (ILC) main linac comprises a series of 12 m-long cryomodules. The cryomodule contains eight 9-cell superconducting (SC) RF (SRF) cavities and an SC quadrupole magnet combined with dipole correctors to focus and steer electron and positron beams. The magnets are installed between the SRF cavity string and at the longitudinal center of a common cryomodule/cryostat. These magnets are conductively cooled by pure aluminum channels thermally connected to a 2 K two-phase helium pipe for cooling the SRF cavities. A recent study shows that field-emitted electrons, so-called “dark current” initiated in the SRF cavities, are transmitted through the SRF cavity string and reach the SC magnet. The energy is inevitably absorbed in the SC coil due to the magnetic field, resulting in risks of a quench caused by the coil heating. We are investigating alternate magnet designs by using Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{MgB}_{2}$</tex-math></inline-formula> SC and by adding the dark current absorber surrounding the beam-pipe to realize sustainable magnet operation under the dark current heating. We report the design study of the magnet system and interfaces to the cryomodule accommodating it with the SRF cavities.

Construction of a New MgB2 Coating System for 1.3-GHz Superconducting RF Cavities at LANL

After many years of evaluating MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films prepared with various techniques for the application to superconducting radio-frequency (SRF) cavities, we have decided to build a system to coat full-size 1.3-GHz elliptical cavities. This paper describes the design and construction of the system. Additionally, we briefly describe experimental results with a small system and first tests with the new large system. We were able to obtain superconducting samples with a T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of up to 38 K with the small system, but we have not been able to get any superconducting samples with the new system yet.

Autonomous RF Cavity Filter Tuning

Autonomous RF Cavity Filter Tuning

Longitudinal parameters measurement at BEPC II storage rings

A new upgrade to BEPC II has been deployed to improve the luminosity and maximum energy. After the upgrade, the maximum beam energy will reach to 2.8 GeV, and the optimized luminosity at 2.35 GeV will be tripled through squeezing the bunch length and the vertical beta function at the interaction point β y *. Therefore, one more RF cavity will be added to each ring to increase the RF voltage to 3.3 MV. In order to study the feasibility of this upgrade and the longitudinal beam characteristics after the upgrade, some beam-based experiments were carried out at the electron and positron rings of BEPC II. Bunch length and its variation with RF gap voltage are measured, shows that it can be squeezed by approximately 30% at 3.3 MV compared to the normal 1.65 MV operation voltage. Bunch synchronous phases at different RF voltages are also obtained by fitting the variation of bunch centroid with RF voltage at low current. The longitudinal broadband coupling impedance is measured with streak camera, real part impedance is obtained from synchronous phase shift with current, and the imaginary part from bunch lengthening with current. Energy loss per turn at 2.474 GeV is 363.2 ± 14.5 keV by measuring synchrotron tune varies with RF voltages.

Improved calibration of rf cavities for relativistic electron beams: Effects of secondary corrections and experimental verification

In the aspect of longitudinal beam bunching, the bunching strength can be controlled by the rf cavity phase and voltage. However, these machine parameters are different from those that interact with the beam itself. In order to gain control of the beam-cavity interaction, cavity calibration must be performed. Furthermore, it relies on fitting the beam energy gain versus cavity phase to a calibration function. Under the conventional assumption of relativistic beam conditions, the calibration function is a first harmonic sinusoidal function (a sinusoidal function with a period of $2\ensuremath{\pi}$). However, this expression is insufficient for a high-voltage bunching cavity. Due to beam acceleration inside the cavity, an energy bias and a second harmonic function should be included to modify the conventional calibration function, even for a relativistic electron beam. In this paper, we will derive this modification and provide a comparison to both the Coherent Electron Cooling Experiment and the impact-t simulation, respectively.

Investigating the Superconducting Properties and Surface Morphology of Sputtered Nb Films on Cu Due to Laser Treatment

Bulk niobium is currently the material of choice for superconducting radio frequency (SRF) cavities and is a well matured process. However, it is possible that SRF cavities could be further improved beyond bulk Nb by sputtering thin Nb films onto Cu cavities. Copper has a greater thermal conductivity than Nb and is also easier to machine, whilst sputtering films on the surface reduces the amount of Nb used to fabricate the whole cavity. However, sputtering Nb on Cu produces other issues, for example, the surface quality of the Cu affects the quality of the Nb deposited on the surface and therefore the superconducting parameters. As the Nb on the surface is not perfect, the magnetic field produced by the RF can enter the cavity earlier than expected, producing RF losses, which can in turn lead to a quench. One approach is to treat the Nb post deposition by irradiating the surface using a laser to polish the surface of the Nb and increase the surface magnetic field that the cavity can maintain whilst remaining in the Meissner state. A magnetic field penetration experiment designed and built at Daresbury Laboratory has been used to measure the field of full flux penetration to characterise the effect of the laser treatment on the superconducting properties of the Nb. Surface characterisation and the response of the Nb in a DC magnetic field have also been performed to try and provide an explanation for the change in the superconducting properties. The results demonstrate that the laser treatment can lead to an increase in the magnetic field at which the flux penetrates from one side of the sample to the other, thus it could potentially improve the performance of Nb coated RF cavities.

11–20 MeV Alvarez Drift Tube Linac for low energy proton accelerator

Bhabha Atomic Research Centre is developing Low Energy High Intensity Proton Accelerator (LEHIPA) as a pre-injector for proposed Accelerator Driven Sub-Critical Reactor Systems (ADSS). Drift Tube Linac is one of the two major accelerating structures of LEHIPA, the other being the Radio Frequency Quadrupole (RFQ). Like all particle accelerating structures, DTL needs multi-disciplinary expertise for design, production, qualification, and installation. This paper describes the comprehensive design, production, qualification, and installation of the ∼ 11–20 MeV section of DTL. It details the design of all sub-components which form part of the DTL RF cavity. Several novel concepts were implemented in course of this development. Important among them are DT-DTL alignment, referencing methodology, robust vacuum design and hermetically sealed DTs. This paper is a comprehensive coverage of design, development and installation aspects of Alvarez DTL. At present, the DTL is ready for beam commissioning.

New collimating shielded bellow for the connection of superconducting rf cavities in a high current storage ring

New collimating shielded bellow for the connection of superconducting rf cavities in a high current storage ring

Optimal design and fabrication of a RF cavity for accelerator mass spectrometry system cyclotron

Optimal design and fabrication of a RF cavity for accelerator mass spectrometry system cyclotron

Innovative research and development activities on high energy and high current isochronous proton accelerator

The MW class proton accelerators are expected to play important roles in many fields, attracting institutions to continue researching and tackling key problems. The continuous wave (CW) isochronous accelerator obtains a high-power beam with higher energy efficiency, which is very attractive to many applications. Scholars generally believe that the energy limitation of the isochronous cyclotron is ∼1 GeV. To get higher beam power by the isochronous machine, enhancing the beam focusing become the most important issue.Adjusting the radial gradient of the average magnetic field makes the field distribution match the isochronism. When we adjust the radial gradient of the peak field, the first-order gradient is equivalent to the quadrupole field, the second-order, the hexapole field, and so on. Just like the synchrotron, there are quadrupoles, hexapole magnets, and so on, along the orbits to get higher energy, as all we know.If we adjust the radial gradient for the peak field of an FFA's FDF lattice and cooperate with the angular width (azimuth flutter) and spiral angle (edge focusing) of the traditional cyclotron pole, we can manipulate the working path in the tune diagram very flexibly. During enhancing the axial focusing, both the beam intensity and the energy of the isochronous accelerator are significantly increased. Here a 2 GeV CW FFA with 3 mA of average beam intensity design is presented. It is essentially an isochronous cyclotron although we use 10 FDF lattices. The key difficulty is that the magnetic field and each order of gradient should be accurately adjusted in a large radius range.As a high-power proton accelerator with high energy efficiency, we adopt high-temperature superconducting (HTS) technology for the magnets. 15 RF cavities with a Q value of 90000 provide energy gain per turn of ∼15 MeV to ensure the CW beam intensity reaches 3 mA. A 1:4 scale, 15-ton HTS magnet, and a 1:4 scale, 177 MHz cavity have been completed. The results of such R&D will also be presented in this paper.

Three-dimensional modeling and simulation of RF cavity for a transmitter with bandwidth of 40–100 MHz at 100-kilowatt level

A 1.5 MW final power amplifier is to be developed for ion cyclotron resonance heating (ICRH) system for the China fusion engineering testing reactor (CFETR) project. First, the feasibility of driver power amplifier with input matching circuit and output resonator cavity and a newly developed tetrode was cross-verified through three-dimensional (3D) simulations and experiments. A single coaxial cavity in parallel with the tetrode covered the frequency range of 40 to 100 MHz at a 100-kilowatt level. In contrast to the previous method of deducing matching parameters using RF circuit and transmission line theory, the use of 3D transmitter simulations provided more accurate matching parameters of tuning components at different operating frequencies. A new phenomenon of high frequency oscillation between the short strip inductor and the shield cavity was found in the input simulation results, which was subsequently verified by experiments. Specific reasons for dividing the output into two parts of high and low frequency were found through output circuit simulations. One is that the length of short stub decreases with the increase of operating frequency, thus decreasing the frequency resolution if all frequencies were using low frequency port. The other is that the coupling capacitor increases sharply with the increase of frequency due to parasitic effect in the low output. Further, the simulation facilitated the study of parasitic effects on matching parameters caused by strip-inductance, inter electrode capacitances of tetrode, and RF cavity. Thus, facilitating the design of final power amplifier in the future. Finally, the amplifier installed with the new tetrode DB968 achieved the design objectives in the test experiment with 150 and 110 kW power outputs at 48 and 100 MHz, respectively.

Anomaly detection at the European X-ray Free Electron Laser using a parity-space-based method

A novel approach to detect anomalies in superconducting radio-frequency (rf) cavities is presented, based on the parity space method with the goal to detect quenches and distinguish them from other anomalies. The model-based parity space method relies on analytical redundancy and generates a residual signal computed from measurable rf waveforms. The residual is a sensitive indicator of deviation from the model and provides different signatures for different types of anomalies. This new method not only helps with detecting faults but also provides a catalog of unique signatures, based on the detected fault. The method was experimentally verified at the European X-ray Free Electron Laser (EuXFEL). Various types of anomalies incorrectly detected as quenches by the current quench detection system are analyzed using this new approach.9 MoreReceived 28 February 2022Accepted 5 December 2022DOI:https://doi.org/10.1103/PhysRevAccelBeams.26.012801Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMethods in electromagnetismRadio frequency calculationsAccelerators & Beams

Multipacting study for fundamental power coupler of a superconducting cavity

The research work of Hefei Advanced Light Facility (HALF) is in progress. A 499.8 MHz superconducting cavity will be used in the HALF RF system. The preliminary RF design and thermal analysis for the fundamental power coupler (FPC) of HALF 499.8 MHz superconducting cavity have been completed. Multipacting (MP) is one of the major causes of the breakdown in many high power input couplers and RF cavities, thus, MP simulation for our FPC is necessary. In this paper, the MP simulation for the FPC was performed by using the CST. The MP performance of the FPC was studied. The formation process of the MP in the FPC was investigated and the physical mechanism of the delay of the MP establishment was analyzed. The simulation results show that MP will occur when the power level is higher than 175 kW, and the MP location is near the ceramic. Therefore, MP suppression is required in the FPC. In order to suppress the MP, the effect of the biasing DC electric voltage to MP in the FPC was analyzed in this paper. The simulation results indicates that the biasing DC electric voltage not only can suppress the MP, but also can excites new MP in the coupler. Thus, it is crucial to select the appropriate biasing DC electric voltage for the MP suppression in the coupler. The results show that the MP in the FPC can be suppressed very well by using 2 kV or 3 kV biasing DC electric voltage.

Comparative Investigations on Detuning Estimator for Experimental RF Cavity

Comparative Investigations on Detuning Estimator for Experimental RF Cavity

6D Phase space diagnostics based on adaptively tuned physics-informed generative convolutional neural networks

A physics-informed generative convolutional neural network (CNN)-based 6D phase space diagnostic is presented which generates all 15 unique 2D projections (x, y), (x, y′),...,(z, E) of a charged particle beam’s 6D phase space (x, y, z, x′, y′, E). The CNN is trained by supervised learning over a wide range of input beam distributions, accelerator parameters, and the associated 6D beam phase spaces at multiple accelerator locations. The CNN is applied in an un-supervised adaptive manner without knowledge of the input beam distribution or accelerator parameters and is robust to their unknown time variation. Adaptive feedback automatically tunes the low-dimensional latent space of the encoder-decoder CNN to predict the 6D phase space based only on 2D (z, E) longitudinal phase space measurements from a device such as a transverse deflecting RF cavity (TCAV). This method has the potential to provide diagnostics beyond the existing state of the art at many accelerator facilities. Studies are presented for two very different accelerators: the 5-meter-long ultra-fast electron diffraction (UED) HiRES compact accelerator at LBNL and the kilometer long plasma wakefield accelerator FACET-II at SLAC.

Excitation of the σl_l = 90° resonance by the cavity rf accelerating fields

In rf linacs the longitudinal focalization is done by si-nusoidal forces and at high accelerating fields the zero-current longitudinal phase advance per longitudinal fo-cusing period σ0l_l can be high. The nonlinear compo-nents of the sinusoidal rf field (sextupolar, octupolar and higher order components) can then excite parametric resonances, including the 4th-order resonance (σl_l = 90°) when σ0l_l is higher than 90°, inducing strong longitudinal emittance growths and acceptance reductions. As pointed out in previous papers, the longitudinal beam dynamics is therefore complex, even when the space-charge forces are ignored. The parametric resonance excitation by the rf field is analyzed before discussing the additional effect of the space-charge forces, in particular to explain why the zero-current longitudinal phase advance per trans-verse focusing period σ0l_t is not a relevant parameter. Examples are given in the SPIRAL2 linac case.

Design of beam optics for RF-separated kaon and antiproton beams in the M2 beam line of the CERN North Area

The present contribution reviews the principle of RF separation and explains its dependence on different parameters of beam optics and hardware. The first examination of potential showstoppers for the RF-separated beam implementation for Phase-2 of the AMBER experiment in the M2 beam line of the North Area is presented. Different beam optics settings have been examined, providing either focused or parallel beams inside the RF cavities. The separation and transmission capability of the different optics settings for realistic characteristics of RF cavities are discussed and the preliminary results of the potential purity and intensity of the RF-separated beam are presented. These show that a trade-off between the overall beam intensity and the share of the required particle type in the overall beam needs to be established. No showstoppers have been identified for achieving the beam parameters required for AMBER’s kaonic Primakoff reactions, kaon spectroscopy, prompt-photon production and kaon charge-radius programs. However, the high beam intensity requirements of the AMBER Drell–Yan programme cannot be satisfied with an RF-separated beam.

Compact and efficient radio frequency digital feedback control system for accelerator applications

The digital feedback (DFB) setup employed in a low-level radio frequency (LLRF) control system is crucial to ensure RF field stability in accelerating cavities, thus guaranteeing successful operation. To this end, a novel in-house and multi-purpose prototype DFB setup was developed for multiple applications at the Japan Proton Accelerator Research Complex (J-PARC). Being efficient, low-cost, and compact is key to achieving a cost-effective system that fulfills the performance specifications. It also reduces the production and maintenance costs of future DFB setups. A field-programmable gate array (FPGA)-based design with a digital signal processing (DSP) function uses an analog in-phase/quadrature-phase (I/Q) demodulator for RF-to-baseband signal conversion and an I/Q modulator to generate RF power. This arrangement compensates for RF phase fluctuations and amplitude modulations by employing a proportional and integral (PI) feedback controller. In addition, systematic errors are minimized by applying a hardware-error-compensation process. The system was utilized to conduct high-power tests on the interdigital H-mode drift-tube linac (IH-DTL) cavity of a muon linac for short RF pulses. The setup was also tested on a buncher cavity (Buncher-2) in the J-PARC linac, achieving efficient performance for longer RF pulses. The stability of the RF accelerating field in the IH-DTL was achieved at ±0.25% peak-to-peak (pp) in amplitude and ±0.36 degree pp in phase. For the Buncher-2, the amplitude stability of ±0.18% pp and phase stability of ±0.13 degree pp were obtained. Therefore, this DFB setup can be used to conduct high-power tests of RF cavities and klystrons. This study discusses the design aspects of a cost-effective DFB system and reports high-power measurements.

Analyzing the filamentation of MeV-range proton bunches in a laser-driven ion beamline and optimizing their peak intensity

In this article, we report on the latest investigations and achievements in proton beam shaping with our laser-driven ion beamline at GSI Helmholtzzentrum f\"ur Schwerionenforschung GmbH. This beamline was realized within the framework of the Laser Ion Generation, Handling, and Transport (LIGHT) collaboration to study the combination of laser-driven ion beams with conventional accelerator components. At its current state, the ions are accelerated by the high-power laser PHELIX via target normal sheath acceleration, and two pulsed high-magnetic solenoids are used for energy selection, transport, and transverse focusing. In between the two solenoids, there is a rf cavity that gives the LIGHT beamline the capability to longitudinally manipulate and temporally compress ion bunches to sub-nanosecond durations. To get optimal results, the rf cavity has to be synchronized with the PHELIX laser and therefore a reliable measurement of the temporal ion beam profile is necessary. In the past, these measurements showed unexpected correlations between the temporal beam profile and the phase as well as the electric field strength of the cavity. In this article, we present a numerical simulation of the beam transport through the LIGHT beamline which explains this behavior by a beam filamentation. We also report on our latest experimental campaigns, in which we combined transverse and longitudinal focusing for the first time. This led to proton bunches with a peak intensity of $(3.28\ifmmode\pm\else\textpm\fi{}0.24)\ifmmode\times\else\texttimes\fi{}{10}^{8}\text{ }\text{ }\mathrm{protons}/({\mathrm{ns}\text{ }\mathrm{mm}}^{2})$ at a central energy of ($7.72\ifmmode\pm\else\textpm\fi{}0.14$) MeV. The intensity refers to a circle with a diameter of ($1.38\ifmmode\pm\else\textpm\fi{}0.02$) mm that encloses 50% of the protons in the focal spot at the end of the beamline. The temporal bunch width at this position was ($742\ifmmode\pm\else\textpm\fi{}40$) ps (FWHM).