RF Cavities Research Articles

Design and vertical tests of double-quarter wave cavity prototypes for the high-luminosity LHC crab cavity system

Crab crossing is essential for high-luminosity colliders. The High Luminosity Large Hadron Collider (HL-LHC) will equip one of its Interaction Points (IP1) with Double-Quarter Wave (DQW) crab cavities. A DQW cavity is a new generation of deflecting RF cavities that stands out for its compactness and broad frequency separation between fundamental and first high-order modes. The deflecting kick is provided by its fundamental mode. Each HL-LHC DQW cavity shall provide a nominal deflecting voltage of 3.4 MV, although up to 5.0 MV may be required. A Proof-of-Principle (PoP) DQW cavity was limited by quench at 4.6 MV. This paper describes a new, highly optimized cavity, designated DQW SPS-series, which satisfies dimensional, cryogenic, manufacturing and impedance requirements for beam tests at SPS and operation in LHC. Two prototypes of this DQW SPS-series were fabricated by US industry and cold tested after following conventional SRF surface treatment. Both units outperformed the PoP cavity, reaching a deflecting voltage of 5.3-5.9 MV. This voltage - the highest reached by a DQW cavity - is well beyond the nominal voltage of 3.4 MV and may even operate at the ultimate voltage of 5.0MVwith sufficient margin. This paper covers fabrication, surface preparation and cryogenic RF test results and implications.

The fast linear accelerator modeling engine for FRIB online model service

Commissioning of a large accelerator facility like FRIB needs support from an online beam dynamics model. Considering the new physics challenges of FRIB such as modeling of non-axisymmetric superconducting RF cavities and multi-charge state acceleration, there is no readily available online beam tuning code. The design code of FRIB super-conducting linac, IMPACT-Z, is not suitable for online tuning because of its code design and running speed. Therefore, the Fast Linear Accelerator Modeling Engine (FLAME), specifically designed to fulfill FRIB’s online modeling challenges, is proposed. The physics model of FLAME, especially its novel way of modeling non-axisymmetric superconducting RF cavities using a multipole expansion based thin-lens kick model, is discussed in detail, and the benchmark results against FRIB design code is presented, after which the software design strategy of FLAME and its execution speed is presented.

Conceptual design for a novel pre-injector applied in a high power terahertz free electron laser

To reduce design pressure for the upstream DC gun, this paper proposes a challenging task of improving bunching capacity for an independent tunable cavity (ITC) RF gun employed in a high power terahertz (THz) free electron laser (FEL). Based on a one-dimensional model for various ITCs, comparisons of their bunching capacities are conducted according to motion and momentum trajectories of the electrons passing through. Since the bunching performance is determined by longitudinal properties of the first cell, special axial electrical fields are designed for it. Furthermore, in the process of structure design, besides inheriting merits of the original configuration, a new idea called “duty sharing” — each part is in charge of a different function for designing such a gun — is applied. Additionally, physical design for the new configuration shows that the proposed task can be accomplished, while the FEL gain length is almost the same as in the previous design.

High order mode power loss evaluation in future circular electron-positron collider cavities

The high order mode (HOM) power losses induced by a particle beam in accelerator components should be taken into account during their design and the choice of HOM couplers. In this work the power losses due to HOMs in the proposed 400.79 MHz radio frequency (rf) cavity designs were evaluated for various energy options (Z, W, H, and $\mathrm{t}\overline{\mathrm{t}}$) of the future circular electron-positron collider (FCC-ee). The constraints for the rf cavity design are dictated by the Z machine with the lowest beam energy, but the highest beam current. Still certain beam filling patterns (bunch and train spacings) are not feasible and should be avoided in beam operation. For other energy options the power loss are sufficiently low and can be extracted by conventional HOM couplers.

Fluid Flow and Current Distribution during Pulse-Reverse Electropolishing of Niobium Superconducting RF Cavities

The International Linear Collider (ILC) is a 200–500 GeV center-of-mass linear electron-positron collider, based on 1.3 GHz superconducting radio-frequency accelerating technology. This installation will require ~16,000 RF superconducting cavities operating within two linear accelerators at near absolute zero [[1]]. These SRF cavities are fabricated from pure Nb; to take full advantage of the Nb superconducting properties, the inner surface must be polished to a microscale roughness, and cleaned to be free of impurities that could degrade performance. Current methods use high viscosity electrolytes containing hydrofluoric acid, which is not conducive to low-cost, high volume manufacturing and is potentially harmful to workers. Faraday is developing an electropolishing process for niobium SRF cavities, based on a new and evolving paradigm of non-viscous dilute acid processing, enabled by a pulse-reverse electric field. Based on our understanding to date [[2]], we have speculated that the process works via oxide film formation controlled during a designed anodic pulse, followed by an off-time to remove heat and waste byproducts, followed by a cathodic pulse that removes the oxide film from the surface. This cycle is repeated many times per second, effectively removing niobium. The waveform design is such that the niobium is preferentially removed from the peaks on the surface, thus smoothing the surface. This talk will describe two recent efforts undertaken to improve understanding of various factors influencing the uniformity and speed of pulse-reverse electropolishing of niobium SRF cavities. The first is a flow study performed in a transparent plastic model of a single-cell (single-bell) cavity (Figure 1, left), to examine the flow dynamics in the absence and presence of an axisymmetric baffle fixed to the rod counter-electrode within the cavity bell. High-speed photography clearly shows the presence of a slow-moving eddy in the equatorial region of the bell (Figure 1, right), which is appreciably reduced in size when the baffle is present. Furthermore, rapid clearance of electrolysis gases and niobium oxide precipitates from the bell is expected to be strongly dependent on a proper configuration of flow throughout the bell. The second effort to be discussed comprises multiphysics modeling of the actual distribution of material removal in the EP process, as a function of position within the cavity. Modeling of EP of passive materials is complex, as numerous coupled phenomena must be accounted for, including: primary, secondary and tertiary current distributions; multi-phase effects, including fluid flow; and oxide formation/removal at the working surface. The strongest effects appear to be the primary and secondary current distributions, along with the surface oxide dynamics, inclusion of these physics (or semi-empirical approximations thereof) provides a significantly improved match between the model to the experimentally observed distribution of material removal, as compared to simulations incorporating only the primary current distribution. Figure 1 Caption (Left) Photograph of the transparent SRF cavity model, showing flow direction and early dye injection streaming pattern. (Right) High-speed photograph with annotations for bypass and eddy flows observed in a representative flow study test. References [[1]] http://www.linearcollider.org/ILC/What-is-the-ILC/The-project [[2]] M. Inman et al “Electropolishing of Passive Materials in HF-Free Low Viscosity Aqueous Electrolytes, J. Electrochem. Soc., 160 (9) E94-E98 (2013). Figure 1

Vacuum Brazing Of Titanium/316L Stainless Steel Transition Joint For Application In Helium Vessel Of Superconducting RF Cavities

Superconducting radiofrequency (SRF) cavities would form important part of the proposed Indian Spallation Neutron Source and Accelerator Driven System. The high beta 650 MHz SRF cavity would be enclosed in a cylindrical vessel to hold liquid helium (LHe). Titanium (Ti) is being considered as the material of construction of helium vessel. The LHe inlet supply line and return helium gas pipe line of the helium vessel would be made of 316L stainless steel (SS). This requires a bi-metallic tubular transition joint between Ti and 316LSS, operating at LHe temperature. Vacuum brazing and explosive welding are potential processes for joining these two dissimilar metals, as fusion welding leads to extensive cracking. In the present experimental study, vacuum brazing was performed to join Ti-pipe/316LSS flange with BVAg8 braze filler metal (BFM). Notable features of the process include use of (i) 5-10 μm nickel electro-plating as a diffusion barrier on SS part for preventing possible iron migration towards titanium while improving surface wettability for BFM and (ii) use of 304 SS plug, shrunk fit into Ti-pipe, for achieving dimensional and profile accuracy of Ti-pipe during machining while also controlling joint gap during brazing process. The brazed joint displayed uniform joint thickness and acceptable level of hermeticity (helium leak rate 2 x 10-10mbar.lit/s) and also sustained six thermal cycles between 293 K and 77 K. Shear strength of Ti-SS brazed specimens, made in sandwich configuration, was found to be in the range of 50-60 MPa, with failure occurring at Ti/braze interface.

Resonance frequency tuning of an RF cavity through sliding mode extremum seeking

Tuning of the natural resonance frequency of an RF cavity is crucial to achieve maximum power transfer from the cavity to the beam, to reduce power requirements, and to guarantee a predictable particle speed gain within each accelerator structure. So far, most operational cavities are frequency tuned through a phase comparison technique, which is sensitive to temperature variations if the facility or signal transportation cables are not temperature controlled. Temperature induced phase errors render this technique labor and time intensive. This paper presents an RF cavity tuning scheme, based on reflected power measurements and sliding mode extremum seeking control, to eliminate the phase measurement related difficulties. A stability analysis provides conditions on how to choose the controller parameters to guarantee stable system operation up to twice the cavity bandwidth, which is a controllable bandwidth improvement of a factor of two. The system has been implemented in two of TRIUMF’s (Canada’s particle accelerator centre) room temperature resonators and long term measurements show that the system effectively counteracts the RF heating effect as well as diurnal temperature variations.

Submacropulse electron-beam dynamics correlated with higher-order modes in Tesla-type superconducting rf cavities

We report the direct observations of sub-macropulse beam centroid oscillations correlated with higher order modes (HOMs) which were generated by off-axis electron beam steering in TESLA-type superconducting RF cavities. The experiments were performed at the Fermilab Accelerator Science and Technology (FAST) facility using its unique configuration of a photocathode rf gun injecting beam into two separated 9-cell cavities in series with corrector magnets and beam position monitors (BPMs) located before, between, and after them. Oscillations of ~100 kHz in the vertical plane and ~380 kHz in the horizontal plane with up to 600-{\mu}m amplitudes were observed in a 3-MHz micropulse repetition rate beam with charges of 100, 300, 500, and 1000 pC/b. However, the effects were much reduced at 100 pC/b. The measurements were based on HOM detector circuitry targeting the first and second dipole passbands, rf BPM bunch-by-bunch array data, imaging cameras, and a framing camera. Calculations reproduced the oscillation frequencies of the phenomena in the vertical case. In principle, these fundamental results may be scaled to cryomodule configurations of major accelerator facilities.

Spectrometer design for diagnostics in a wide range of energy at a linear accelerator

A spectrometer design has been presented for the electron linear accelerator at Institute for Research in Fundamental Sciences (IPM), Tehran, Iran. The energy of the linac is 8 MeV (upgradable to 11 MeV) with a maximum current of 10 mA. The design and construction of the machine is nearly finished and it is in the commissioning stage. The linac consists of a thermionic electron gun followed by a pre-buncher and some travelling wave tubes connected together via appropriate flanges. The commissioning process is foreseen in stages of different energies. At each stage an RF cavity will be appended in order to investigate its performance. Therefore, the energy diagnostics is demanded in a wide range—from 50 keV to 8 MeV . A spectrometer based on a dipole magnet, a movable view screen and a focusing solenoid is proposed for this purpose. The challenge is to keep the resolution in the wide range of measurements. This paper first describes the beam dynamics design of the spectrometer with the focus on the measurement resolution. Then the magnetic design of the dipole is presented in details. Two and three dimensional simulations have been carried out in order to optimize the dipole parameters such as the magnetic flux density, multipole components, and the integrated field quality. Finally, magnetic design features of the solenoid have been discussed.

Design and High-Power Test of an Internal Coupler to HE11 Mode in Corrugated Waveguide for High-Power Gyrotrons

Gyrotrons produce power in high-order, transverse electric modes that are typically converted to a Gaussian beam inside the tube. In fusion plasma heating applications, the power is transmitted through a diamond window to an external mirror optical unit where it is coupled into an HE11 mode in corrugated waveguide. A new coupler is described that couples the cavity RF power directly into corrugated waveguide inside the gyrotron. A high-power gyrotron with the improved coupler was tested using 1-MW, 3- $\mu \text{s}$ pulses and demonstrated equivalent power and efficiency as a traditional coupler with HE11 mode output in corrugated waveguide. In addition, higher HE11 mode purity was achieved at the gyrotron output.

Toward Optimization of Centrifugal Barrel Polishing Procedure for Treatment of Niobium Cavities

Centrifugal barrel polishing (CBP) is a simple and environmentally friendly method that can be applied for mechanical abrasion of the cavity interior in order to remove the mechanically damaged surface after its production. The CBP recipes described in the literature, however, require CBP to be performed in many stages, require long processing times and nevertheless are unable to provide good cavity RF performance without additional chemical processing. Here, we report new results on characterization of cavity surfaces treated with a typical CBP recipe, including the contamination with abrasive particles, plastic deformation and hydrogen contamination, and critically evaluate it. Methods to reduce the depth of significant plastic deformation as well as the modified commercially viable CBP procedure followed by final electropolishing are proposed and tested on samples.

Simulation of beam–beam interaction with crab cavities for LHC upgrade

The crab cavities are a critical component in the high luminosity LHC upgrade project to compensate the luminosity loss from large crossing angle collision. However, these crab cavities will not be perfect in the real accelerator. In this paper, we studied the effects of crab cavity imperfections on colliding beam luminosity lifetime degradation for the LHC upgrade through detailed numerical simulations. Our simulation results suggest that the white noise jitter in the crab cavity RF phase and voltage rms amplitudes should be kept below a few 10−5 for a good luminosity lifetime, while with frequency-dependent jitter, the amplitudes should be kept below a few 10−4 for a good lifetime. The RF multipole errors in the current crab cavity designs are small enough and would not cause extra luminosity degradation.

Magnetometric mapping of superconducting RF cavities.

A scalable mapping system for superconducting RF (SRF) cavities is presented. Currently, it combines local temperature measurement with 3D magnetic field mapping along the outer surface of the resonator. This allows for the observation of dynamic effects that have an impact on the superconducting properties of a cavity, such as the normal to superconducting phase transition or a quench. The system was developed for a single cell 1.3 GHz TESLA-type cavity, but can be easily adopted to arbitrary other cavity types. A data acquisition rate of 500 Hz for all channels simultaneously (i.e., 2 ms acquisition time for a complete map) and a magnetic field resolution of currently up to 14 mA/m/μ0 = 17 nT have been implemented. While temperature mapping is a well known technique in SRF research, the integration of magnetic field mapping opens the possibility of detailed studies of trapped magnetic flux and its impact on the surface resistance. It is shown that magnetic field sensors based on the anisotropic magnetoresistance effect can be used in the cryogenic environment with improved sensitivity compared to room temperature. Furthermore, examples of first successful combined temperature and magnetic-field maps are presented.

Erratum to: Measurement of Characteristic of X-band RF Cavity for 6 MeV Electron Linac

The online version of the original article can be found at http://dx.doi.org/10.3938/jkps.72.818

Processing of a Nigerian columbite-rich ilmenite ore for improved industrial application by sulphuric acid solution

Due to increasing technological advancements, the demand for pure niobium products emanated from columbite ore processing has led to the wide array of its application in capacitors, nuclear uses, superconducting RF cavities, an electromagnetic radiation detector, medical devices, and aerospace among others. Therefore, this paper focuses on the kinetics of dissolution of a Nigerian Columbite mineral in sulphuric acid solution for improved industrial applications. The effects of acid concentration, reaction temperature and particle size on the ore dissolution rate were examined. Experimental results showed that the ore dissolution increases with increasing acid concentration and temperature and decreases with increasing particle size fraction. With 2 mol/L H 2 SO 4 solution, 60.2% of the ore dissolved within 120 minutes using <90µm particle size at a temperature of 75°C with moderate stirring (300 rpm). The unleached products were analyzed and found to contain siliceous compounds applicable as industrial by-products. The calculated activation energy (Ea) deduced to be 30.3 kJ/mol supported the proposed diffusion reaction mechanism for the first order dissolution process with a collision factor of 0.12 per minute. Hence, the dissolution data obtained in this study is an indication of thermodynamic feasibility for Nigeria Columbite ore fitness in improving science and technology for economic growth and development.

Surface characterization of nitrogen-doped Nb (100) large-grain superconducting RF cavity material

(100) Oriented niobium (Nb) crystals annealed in the vacuum conditions close to that used in mass production of 1.3 GHz superconducting radio frequency cavities for linear accelerators and treated in nitrogen at a partial pressure of 0.04 mbar at temperatures of 800 and 900 °C have been studied. The surfaces of the nitrogen-treated samples were investigated by means of various surface-sensitive techniques, including grazing-incidence X-ray diffraction, X-ray photoemission spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy in planar view and on cross-sections prepared by a focused ion beam. The appearance of a dense layer of epitaxial rectangular precipitates has been observed for the Niobium nitrided at 900 °C. Increased nitrogen concentration in the near-surface region was detected by glow-discharge optical-emission spectroscopy, focused ion-beam cross-sectional images and X-ray photoelectron spectroscopy. Crystalline phases of NbO and β-Nb2N were identified by X-ray diffraction. This information was confirmed by X-ray photoelectron measurements, which in addition revealed the presence of Nb2O5, NbON, NbN, and NbN x O y components on the surface. These results establish the near-surface Nb phase composition after high-temperature nitrogen treatment, which is important for obtaining a better understanding of the improved RF cavity performance.

Vertical-longitudinal coupling induced by wakefields at the Cornell Electron-Positron Storage Ring Test Accelerator

Transverse vertical wakefields can increase vertical emittance and distort the phase space of a bunch in a storage ring. Here we report recent measurements and simulations of these effects from wakefields from movable scrapers at the Cornell electron-positron storage ring test accelerator. Charge-dependent vertical beam size growth was observed with a single scraper inserted through the top of the storage ring vacuum chamber. No change in the beam size was observed with top and bottom scrapers inserted symmetrically. The apparent growth in the vertical beam size was due in large part to the $yz$ coupling (vertical crabbing) induced by the transverse monopole wake of the asymmetric scraper configuration. We explored this $yz$ coupling and thus the orientation at the observation point by varying the vertical betatron phase advance between the vertical beam size monitor and the scrapers. In addition, we found that existing residual, current-independent $yz$ coupling, perhaps due to nonzero vertical dispersion in the rf cavities, could be compensated by the scraper wake. Predictions from tracking simulations are in good agreement with the measurements. Moreover, the vertical beam size as a function of vertical displacement at narrow-gap chambers (closed scrapers and undulator chamber) was measured. We found the transverse wakefield produced by the off-axis beam could also introduce a $yz$ tilt as well as dilute the beam emittance.

Muon Acceleration Concepts for NuMAX: “Dual-use” Linac and “Dogbone” RLA

We summarize the current state of a concept for muon acceleration aimed at a future Neutrino Factory. The main thrust of these studies was to reduce the overall cost while maintaining performance by exploring the interplay between the complexity of the cooling systems and the acceptance of the accelerator complex. To ensure adequate survival for the short-lived muons, acceleration must occur at high average gradient. The need for large transverse and longitudinal acceptances drives the design of the acceleration system to an initially low RF frequency, e.g., 325 MHz, which is then increased to 650 MHz as the transverse size shrinks with increasing energy. High-gradient normal conducting RF cavities at these frequencies require extremely high peak-power RF sources. Hence superconducting RF (SRF) cavities are chosen. We consider two cost effective schemes for accelerating muon beams for a stageable Neutrino Factory: exploration of the so-called “dual-use” linac concept, where the same linac structure is used for acceleration of both H− and muons and, alternatively, an SRF-efficient design based on a multi-pass (4.5) “dogbone” RLA, extendable to multi-pass FFAG-like arcs.

Elliptical superconducting RF cavities for FRIB energy upgrade

The multi-physics design of a five cell, βG=0.61, 644 MHz superconducting elliptical cavity being developed for an energy upgrade in the Facility for Rare Isotope Beams (FRIB) is presented. The FRIB energy upgrade from 200 MeV/u to 400 MeV/u for heaviest uranium ions will increase the intensities of rare isotope beams by nearly an order of magnitude. After studying three different frequencies, 1288 MHz, 805 MHz, and 644 MHz, the 644 MHz cavity was shown to provide the highest energy gain per cavity for both uranium and protons. The FRIB upgrade will include 11 cryomodules containing 5 cavities each and installed in 80-meter available space in the tunnel. The cavity development included extensive multi-physics optimization, mechanical and engineering analysis. The development of a niobium cavity is complete and two cavities are being fabricated in industry. The detailed design of the cavity sub-systems such as fundamental power coupler and dynamic tuner are currently being pursued. In the overall design of the cavity and its sub-systems we extensively applied experience gained during the development of 650 MHz low-beta cavities at Fermi National Accelerator Laboratory (FNAL) for the Proton Improvement Plan (PIP) II.

Exact cancellation of emittance growth due to coupled transverse dynamics in solenoids and rf couplers

Small, stray magnetic and RF fields in electron guns and injectors can perturb an electron beam and introduce correlations between the otherwise orthogonal transverse trajectories. These correlations couple the x and y dynamics which increases the transverse emittance. If the correlation becomes "diluted" or randomized in the beam transport then as the correlation disappears the 4D emittance increases. This paper discusses two important correlations common to most electron injectors. The first results from the coupling of a weak quadrupole field with beam rotation in a solenoid, and the second x-y coupling is generated by an asymmetric on-axis RF field due to a high-power RF coupler or cavity port. This paper shows that a small quadrupole field combined with solenoidal focusing can result in significant emittance growth due to coupled transverse dynamics. It also shows how adding a skewed quadrupole field can exactly cancel this correlation and its emittance. Similar emittance cancellation is demonstrated for asymmetric RF fields, with the degree of cancellation limited by the electron bunch length. Analytic expressions are derived and compared with simulations and experiments.