A high-current, long-pulsed beam operation without beam loss was demonstrated at the Superconducting rf Test Facility (STF) of the High Energy Accelerator Research Organization. The primary purpose of the STF was to validate the superconducting radio-frequency (rf) acceleration technology for the International Linear Collider (ILC) using accelerator components in line with the ILC design. Thus, the most critical objective of STF was to perform beam operations with a pulse length (726−μsec) and current (5.8 mA) equal to those of the ILC. Severe radiation caused by beam loss hinders stable beam operation. In a 100−μsec pulsed beam operation, a gradient drop in the accelerating cavities owing to beam loading causes beam loss. This beam loading was suppressed by imposing an additional driving power as feedforward control (BeamFF) to compensate for the gradient drop. BeamFF can completely suppress the gradient variation from a flattop to a 0.4% level with an amplitude stability of 0.3%–0.4%. Based on the knowledge obtained in the 100−μsec beam operation, we achieved a stable 726−μsec, 5.8-mA beam operation, and a beam power of 5.6 kW. The maximum energy variation of the pulsed beam was 1.5%. This value is significantly less than the threshold of 4%, which causes beam loss. We thus verified the stable beam acceleration technology for the ILC at the STF. Published by the American Physical Society 2025

We conducted electropolishing (EP) of a Nb superconducting RF coupon cavity containing coupons at various positions of the cavity, and laboratory EP of Nb samples. The surface of the electropolished coupons was studied by using XPS, SEM, AFM, and EDX. The XPS analysis revealed that the atomic compositions and Nb 3d peak shapes (intensity ratio of the Nb5+ and Nb-0 peaks) varied for the different coupons, indicating that the chemical state was not the same over the entire cavity surface. These differences might originate from the particle clusters observed on the coupon surfaces, which mainly existed as Nb2O5 and its hydrated form in different number densities. An individual particle cluster was isolated from the surface for EDX analysis, revealing the presence of C and F impurities, where F existed mostly as CFx with traces of niobium fluoride. These particle clusters were possibly synthesized during the post-EP water rinsing process by the hydrolysis of the niobium fluoride species (H2NbOF5) that were produced in the EP process and existing in the acid viscous layer on the surface. The post-EP water rinsing conditions affected the number density of particle clusters, and strong agitation of the acid reduced the particle content on the laboratory electropolished samples. An individual particle cluster emitted electrons at a DC field of 64 MV/m after being activated at a high field of similar to 150 MV/m, possibly limiting the performance of the SRF cavity. An optimized post-EP water rinsing followed by HF rinsing or prolonged ultrasonic rinsing of the cavity was effective at removing undesired particles from the surface.

We develop a lower critical field (Hc1) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. Parameter Hc1 is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally applied to one side of the sample mimics the magnetic field within superconducting radio-frequency (SRF) cavities and prevents edge effects of the superconducting sample. With this approach, our measurement system can potentially characterize surface-engineered SRF materials such as Superconductor-Insulator-Superconductor multilayer structure (S-I-S structure). As a validation test, we measure the temperature dependence of Hc1 of two high-RRR bulk Nb samples and obtain results consistent with the literature. We also confirm that our system can apply magnetic fields of at least 120 mT at 4-5 K without any problem of heat generation of the coil. This field value is higher than those reported in previous works and makes it possible to more accurately estimate Hc1 at lower temperatures.

We designed a niobium-based mushroom-shaped cavity for evaluating the critical magnetic field of superconducting multilayer thin films. The distribution of the magnetic field strength at a radio frequency (RF) of 5.2 GHz was calculated in a sample region, and the maximum magnetic field strength at 1 W RF power in the cavity was calculated to be 60.0 kA m−1 (75.4 mT). Subsequently, we tested the production of a miniature hemisphere cavity to confirm the accuracy of its curvature radius. We measured the slope profile of the hemisphere using a three-dimensional shape measurement system and evaluated the accuracy of fabrication.

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 mu m, respectively, were tested in a vertical cryostat. The single- and nine-cell cavities achieved 31 MV/mat a Q(0) value of 9 x 10(9) and 28.3 MV/m at a Q(0) value of 6.7 x 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.

In order to achieve a maximum acceleration gradient of superconducting radio frequency cavities beyond the physical limit of conventional Nb cavities, a method of reducing the magnetic field reaching Nb base material by alternately layering superconducting thin films and insulating thin films on the inner surface of that has been proposed. We investigated the optimal deposition condition of NbN and SiO2 layers and made multilayered samples consisting of NbNSiO2 thin films and bulk Nb substrates by using reactive sputtering method. We will report the detail evaluation results of the correlation between sputtering conditions and film properties such as critical temperature in this article.

We designed a mushroom-shaped Al test cavity for measurement of the critical magnetic field at a radio frequency microwave with a frequency of 5.2 GHz. The characteristics of the Al test cavity are characterized toward the evaluation of the superconducting multilayer thin films under high-power radio frequency microwaves at cryogenic temperatures by the Nb-based cavity. We evaluated a target resonant frequency, the separations of neighboring modes, and the electromagnetic field distribution of the target mode. The calculated frequency change was in good agreement with that obtained experimentally.

Electron Positron Linear Collider is the only way to realize annihilation of elementary particles with more than 350 GeV center of mass energy in the current technology. In Linear Collider, the beam spot should be flat, i.e. tiny in one axis and moderate in another axis to maximize luminosity suppressing beam-beam effect. Instead of radiation damping with a storage ring, we propose to generate the flat beamwith phase-space rotation technique in the injector. We plan to carry out beam tests at STF(Superconducting Test Facility)KEK and WFA (Wake Field Accelerator)-ANL(Argonne National Laboratory). According to our simulation, the flat beam required at IP in ILC can be made with this technique. INTRODUCTION Electron Positron Collider is the only way to realize annihilation of elementary particles with controlled conditions with the current technology. Because there has been no any significant evidence of Super-symmetry in LHC experiments, the significance of detail studies of Higgs boson and searching inconsistency in the standard model with electron positron collider is maximized. ILC (International Linear Collider) [1] is an e+elinear collider based on superconducting accelerator with CME from 250 to 1000 GeV. It would be constructed in Iwate, Japan, as the main project of High energy physics. Luminosity L is the index showing the performance of colliders. It can be expressed as L = f nbN 4πσxσy , (1) where f is repetition of pulse, nb is number of bunches in a pulse, N is number of particles in a bunch, σx,y is transverse beam size. The numerator can’t be too large, because it increases the wall plug power as Pwall = ηeE f nbN, (2) where η is power efficiency and eE is beam energy. One way to enhance the luminosity is minimize σx,y , but it causes a ∗ mkuriki@hiroshima-u.ac.jp large energy spread by Beamstrahlung as ∆E ∝ 1 σz ( 2 σx + σy )2 . (3) A practical way to enhance the luminosity and suppress Beamstrahlung simultaneously is squeezing the beam in one of the transverse direction, e.g. σx σy . For ILC, The beam size at IP is 640 nm in horizontal direction and 5.7 nm in vertical direction. Emittances are 10 and 0.04 mm mrad in horizontal and vertical directions, respectively. This asymmetric emittance beam is made by radiation damping in a storage ring in the current design. We propose to generate the flat beam for ILC only with the injector by employing the emittance exchange technique. As the phase-space rotation technique, there are two methods. One is RFBT (Round to Flat Beam Transformation) [2] generating the flat beam from an angular-momentum dominated beam produced by beam emission in a solenoid field. Another is TLEX (Transverse to Longitudinal Emittance eXchange) exchanging the phase-spaces between longitudinal and transverse directions by dipole mode cavity in a dispersive beam line [3]. EMITTANCE EXCHANGE TECHNIQUES In this section, RFBT and TLEX are briefly explained. RFBT RFBT generates the asymmetric emittance beam between two transverse directions from an angular momentum dominated beam. A proof of principle experiment of RFBT was performed demonstrating 100 emittance ratio [4]. The angular momentum dominated beam is made by the beam emission in a solenoid field. Vector potential of the solenoid field A makes a canonical angular momentum Pc as

Superconducting Radio-Frequency (SRF) cavities are used to accelerate particles in particle accelerators. In this application, the cavity is made of niobium (Nb) material and the inner surface of cavity should be polished so as to obtain the high performance of particle accelerator. The Electro-Polishing (EP) process is one of the best methods to polish the inner-surface of Nb cavity where the electrolyte is the mixture of sulfuric and hydrofluoric acids. However, this EP electrolyte is very dangerous in the operation and thus the operators need rigorous protection. Furthermore, all parts of instrument for the EP process must be made of high density polyethylene or fluorocarbon resin because the corrosion of metal occurs by hydrofluoric acid. These are the reasons why the cost of EP process becomes very expensive. In addition, the EP process produces sulfur compound on the Nb surface as a by-product. This sulfur compound can be the sources of field-emission on the inner surface of cavity and degrades the accelerating performance. In this article, we report a novel EP method using periodic reverse current and sodium hydroxide solution. The reaction produces no sulfur content and the instrument is less expensive because it is made of usual plastic material. As the result of experiments with Nb-coupon samples, we found that the surface roughness is equivalent to the conventional EP method. And some experiments with a Nb single-cell cavity will be reported.

A detailed study on vertical electropolishing (VEP) of a 1.3-GHz single-cell niobium coupon cavity, which contains six coupons and four viewports at different positions, is reported. The cavity was vertically electropolished using a conventional rod and three types of unique cathodes named as Ninja cathodes, which were designed to have four retractable blades made of either an insulator or a metal or a combination of both. This study reveals the effect of the cathodes and their rotation speed on uniformity in removal thickness and surface morphology at different positions inside the cavity. Removal thickness was measured at several positions of the cavity using an ultrasonic thickness gauge and the surface features of the coupons were examined by an optical microscope and a surface profiler. The Ninja cathode with partial metallic blades was found to be effective not only in reducing asymmetric removal, which is one of the major problems in VEP and might be caused by the accumulation of hydrogen (H-2) gas bubbles on the top iris of the cavity, but also in yielding a smooth surface of the entire cavity. A higher rotation speed of the Ninja cathode prevents bubble accumulation on the upper iris, and might result in a viscous layer of similar thickness in the cavity cell. Moreover, a higher electric field at the equator owing to the proximity of partial metallic blades to the equator surface resulted in a smooth surface. The effects of H-2 gas bubbles and stirring were also observed in lab EP experiments.

We have developed a method to obtain mechanical centers of nine cell superconducting radio frequency (SRF) cavities from localized dipole modes, that is one of the higher order modes (HOM) induced by low-energy beams. It is to be noted that low-energy beams, which are used as alignment probes, are easy to bend in fringe fields of accelerator cavities. The estimation of the beam passing orbit is important because only information about the beam positions measured by beam position monitors outside the cavities is available. In this case, the alignment information about the cavities can be obtained by optimizing the parameters of the acceleration components over the beam orbit simulation to consistently represent the position of the beam position monitors measured at every beam sweep. We discuss details of the orbit estimation method, and estimate the mechanical center of the localized modes through experiments performed at the STF accelerator. The mechanical center is determined as x,y=(0.44±0.56mm,−1.95±0.40mm). We also discuss the error and the applicable range of this method.

Chemical etching or electropolishing of Nb superconducting radio-frequency (SRF) cavities to achieve a clean and smooth internal surface require the removal of the passive Nb2O5 film. Typically, an aggressive chemical, such as hydrofluoric acid is included in the solution to accomplish this. The problem with using HF is that it is dangerous both to people and to the environment. As an alternative, in order to depassivate the surface, cathodic pulses may be interspersed between anodic pulses which passivate the Nb surface forming Nb2O5 film, in place of or in conjunction with off-times. Use of such cathodic pulses eliminates the need for HF and/or fluoride salts or other chemicals to actively remove the surface oxide. A present understanding of a pulse reverse electropolishing of Nb in alkaline electrolyte-sodium hydroxide will be reported. In this study, 10~30 % concentration of NaOH has been used as the electrolyte. In order to better understand the mechanism of a pulse reverse electropolishing of Nb with sodium hydroxide, and optimize the process parameters for Nb cavity process, the effect of cathodic pulse duration and amplitude, off time, and anodic pulse duration have been systematically studied, the surface of Nb samples have been characterized chemically and morphologically. We expect the results will enable application of this safer and greener process to future accelerator projects using Nb SRF cavities.

We report the successful demonstration of an ILC-like high-gradient near-quench-limit operation at the Superconducting RF Test Facility at the High Energy Accelerator Research Organization (KEK) in Japan. Preparation procedures necessary for the accelerator operation were conducted, such as rf phase calibration, beam-based gradient calibration, and automated beam compensation. Test runs were performed successfully for nominal operation, high-loaded Q (QL) operation, and automated P(k)Q(L) operation. The results are described in terms of the achieved precision and stabilities of gradients and phases.

A superconducting quadrupole magnet with splittable yoke has been designed for use in ILC Main Linac (ML) cryomodules. The splittable yoke allows assembly around the beam pipe, to avoid potential contamination of the superconducting RF cavities. The magnet is cooled by conduction and covers the full range of required ML field gradients. A critical requirement is stability of the magnetic center, at the level of 5 μm, for a 20% variation of the operating field. We report here the results of thermal, quench, and magnetic performance tests of a prototype splittable quadrupole, that were made up to the maximum design operating gradient in a conduction-cooled test cryostat.

Quasi-monochromatic X-rays with high brightness have a broad range of applications in fields such as life sciences, bio-, medical applications, and microlithography. One method for generating such X-rays is via inverse Compton scattering (ICS). X-ray generation experiments using ICS were carried out at the superconducting RF test facility (STF) accelerator at KEK. A new beam line, newly developed four-mirror optical cavity system, and new X-ray detector system were prepared for experiments downstream section of the STF electron accelerator. Amplified pulsed photons were accumulated into a four-mirror optical cavity and collided with an incoming 40MeV electron beam. The generated X-rays were detected using a microchannel plate (MCP) detector for X-ray yield measurements and a new silicon-on-insulator (SOI) detector system for energy measurements. The detected X-ray yield by the MCP detector was 1756.8±272.2 photons/(244 electron bunches). To extrapolate this result to 1ms train length under 5Hz operations, 4.60×105 photons/1%-bandwidth were obtained. The peak X-ray energy, which was confirmed by the SOI detector, was 29keV, and this is consistent with ICS X-rays.