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The 1st report was delivered in May, 2018 at the IPAC 18 in Vancouver, Canada. The 2nd report was delivered in September, 2018 at the LINAC 18 in Beijing, China. We will make our 3rd report in July, 2019 at the SRF-19 in Dresden, Germany. There will be two main points this time. The first is that by using our improved Ninja Electrode Premium, we can out-perform our number one and number two competitors in terms of uniform electropolishing of the interior of the 9-cell cavity. The second point is that we can remove hydrogen gas, reacted during electropolishing, from the cavity chambers in a manner that has not been successfully achieved by 1st report, May 2018 and 2nd report, September 2018. We will report our 9-cell vertical polishing revolver-type unit that solves the above two problems., Proceedings of the 19th International Conference on RF Superconductivity, SRF2019, Dresden, Germany

Marui Galvanizing Co.Ltd., and CEA have been studying Vertical Electro-Polishing (VEP) on Nb single-cell accelerating superconducting accelerator cavity with the goal of mass-production and cost-reduction, in collaboration with KEK within TYL-FJPPL Particle Physics Laboratory. Marui has invented and patented a rotative cathode called ‘i-Ninja'. The version 5 has been tested for the first time in Europe at CEA Saclay. The four wings of the cathode remove efficiently, bubbles of hydrogen, and the chosen parameters make it possible to achieve better surface and uniform material removal compared to VEP with a fixed cathode. The effect of the temperature of the cavity walls on current oscillations has been precisely studied. Two single-cell cavities have been electro-polished and tested at 2 K in vertical cryostat and the results will be presented., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Vertical electropolishing (VEP) technique have been successfully developed for 1.3 GHz niobium (Nb) single cell cavity to achieve a smooth surface with uniform removal and better RF performance as achieved after horizontal EP (HEP) process. VEP parameters for 1.3 GHz Nb nine-cell cavities are being studied using a nine-cell coupon cavity and our unique Ninja cathode. The investigated VEP parameters heretofore were applied on a 1.3 GHz Tesla shape nine-cell superconducting RF cavity for bulk removal of 100 µm followed by fine removal of 20 and 10 µm. The interior surface was found to be smooth and shiny after the VEP process. Our recently developed dual flow technique, in which the EP acid is flown separately in the Ninja cathode housing and cavity, yielded lower asymmetry in removal along the cavity length. The cavity was tested in a vertical cryostat after the final VEP process. The cavity achieved 28.3 MV/m at Q0 value of 6.7x109. The cavity performance was almost the same as in the baseline vertical test performed after the HEP process., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

Marui Galvanizing Co., Ltd. has been developing Nb cavity vertical electropolishing (VEP) technologies in collaboration with KEK. Until now, we reported that inner surface state and removal thickness distribution were improved in VEP with Ninja cathode and coupon cavity. This time, a 1.3GHz Nb single-cell cavity VEP with Ninja cathode was performed in Marui and vertical test was performed in KEK. The inner surface state and removal thickness distribution were satisfactory. And as a result of vertical test, the accelerating gradient of 32MV/m (Q0=8.0E9) was achieved., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Marui Galvanizing Co., Ltd. has been developing niobium 9-cell cavity vertical electropolishing (VEP) technologies with Ninja cathode in collaboration with KEK. Conventional 9-cell cavity VEP had a serious problem, which was asymmetry of removal thickness distribution. Usually removal thickness of upper side became larger than lower side in case of both in-cell and inter-cell. So far, as one solution, we proposed bubble diffusion prevention method and proved it was effective for uniform removal. This time, as other new solution, we tried cavity flip upside down and Ninja cathode masking VEP methods. In this article we will report the purpose, intention and VEP experiment result of these methods., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

Horizontal electropolishing (HEP) is an established tech-nique for surface treatment of niobium accelerating cavi-ties. Vertical electropolishing (VEP), in which the cavity is electropolished in the vertical posture, is in R&D phase for parameter optimization. We performed HEP and VEP of a niobium single-cell coupon cavity to compare the effect of both the methods on surface state and removal at different positions of the cavity. HEP was performed at STF, KEK with the standard EP parameters. VEP was performed at Marui Galvanizing Company with a cathode called 'Ninja cathode' that can be rotated during the VEP process. The optimized cathode design and VEP parame-ters resulted in symmetric removal as obtained with the HEP technique and yielded a smooth inner surface of the entire cavity, Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Authors report a study on vertical electropolishing (VEP) carried out for a 1.3 GHz 9-cell niobium (Nb) coupon cavity using a unique cathode namely 'Ninja Cathode'. The design of the cathode for VEP of a 9-cell cavity was based on the Ninja cathode used for 1-cell cavity since the 1-cell Ninja cathode was found effective to reduce longitudinal asymmetry in material removal and to obtain a smooth surface of a 1-cell cavity. Moreover, 1-cell Nb cavities after being treated in VEP using the Ninja cathode showed good performance in vertical RF tests. The 9-cell coupon cavity used in this study was designed to have totally nine coupons set on the iris and equator positions of the first, fifth and ninth cells. These three cells contain viewports as well at their upper and lower iris positions. Measurement of currents from the individual coupons and in-situ observation are possible using the cavity to understand EP phenomenon at different locations of the cavity. VEP results, which include removal thicknesses at different positions of the cavity and surface study of the coupons, are discussed., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

In IPAC18 (Vancouver, Canada), we reported our first step of development of niobium 9-cell cavity vertical electropolishing (VEP) facility. In this article, we will report the method, system for uniform polishing for niobium 9-cell cavities and the current situation of our 9-cell cavity VEP facility (The result of polishing uniformity, vertical test will be presented in other posters of this conference). In addition, we will show the movie of experiments of VEP-3 with Ninja cathode. This facility aims not only for test VEP but also for mass production and long-time operation., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

In our previous studies, we have reported parameter investigation for vertical electropolishing (VEP) of 1-cell niobium (Nb) tesla/ILC type cavities using a Ninja cathode. A 1-cell coupon cavity containing six Nb disk coupons at its different positions was found effective to reduce time and cost to establish an optimized VEP recipe. In this work, we present surface analyses of VEPed Nb coupon surfaces using scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). Surfaces contained micro- and nano-sized particles which were found with random distributions and different number densities on the beam pipe and iris coupons. Surfaces of equator coupons were found to have relatively less number of particles or almost clean. To analyze particles, a few particles were picked-up from a coupon surface using a tungsten tip under SEM and analyzed with EDX while the coupon was moved out from the SEM chamber to avoid its effect in EDX spectra. The particles were confirmed as oxygen-rich niobium and contained fluorine and carbon also. XPS analysis of the coupon surfaces was also carried out for further study of surface chemistry., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

Our Nb accelerating cavity vertical electropolishing (VEP) facility development group which was led by KEK started single-cell VEP facility development from 2014. This is based on horizontal electropolishing (HEP) techniques developed by KEK over 10 years and stainless steel electropolishing techniques developed by Marui over 30 years. We have reported results of Nb cavity VEP with Ninja cathode so far. In order to achieve international linear collider (ILC) construction, it is said that cost reduction and productivity improvement are necessary, however in case of 9-cell cavity, uniform inner surface polishing is difficult, as well known to predecessors. In this article, we will present the first report of VEP facility development from initial transparent plastic mock-up to improvement for Nb 9-cell cavity., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

VEP parameters and process have been already optimized with single-cell 1.3 GHz niobium cavity at Marui Galva-nizing Company working in collaboration with KEK. A unique cathode called 'Ninja cathode' with an optimized shape was applied to single-cell cavities. The cathode was effective to stop the bubble accumulation in the upper half-cell of the cavity and yielded smooth surface and uniform removal in the cell. This work shows parameter study with the Ninja cathode and a 9-cell coupon cavity which contains totally 9 coupons and viewports in the first, fifth, and ninth cells. Effects of temperature and acid flow in the cathode housing were studied using coupon currents and by observing bubbles through the viewports. The adequate parameters found with 9-cell coupon cavity were applied on a 9-cell cavity to be tested in vertical cryostat. The VEP and vertical test results are reported., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

Marui Galvanizing Co. Ltd. has been improving Vertical Electro-Polishing (VEP) technology for Nb superconducting RF cavity in collaboration with KEK. In this collaboration, we developed a unique cathode namely Ninja cathode for VEP treatment of Nb cavities. We have already reported that longitudinal symmetry in niobium removal and surface state of a single cell cavity were improved after VEP using the Ninja cathode. In this article, we report a result of accelerating gradient evaluation for 1.3 GHz single cell RF cavity after VEP with Ninja cathode in collaboration with KEK and CEA Saclay., Proceedings of the 18th Int. Conf. on RF Superconductivity, SRF2017, Lanzhou, China

A 1.3 GHz single-cell niobium (Nb) coupon cavity was vertically electropolished (VEPed) with three different Ninja cathodes which were specially designed for VEP of 1.3 GHz superconducting RF elliptical (ILC/Tesla type) cavities. The cathodes were fabricated to have different surface areas and different distances between cathode surface and the equator. The Ninja cathode prepared with an enhanced cathode surface area was covered with a meshed shield to avoid bubble attack on the surface of the cavity cell. It has been turned out that the anode-cathode distance and the cathode area affect surface morphology of the equator. A smooth equator surface was obtained in the cases in which the cathode surface was geometrically close to the equator or instead the cathode surface area was sufficiently larger. Two 1.3 GHz ILC/Tesla type single-cell cavities VEPed with the Ninja cathodes and using optimized conditions showed good performance in vertical tests., Proceedings of the 28th Linear Accelerator Conf., LINAC2016, East Lansing, MI, USA

Marui Galvanizing Co. Ltd. has been improving Vertical Electro-Polishing (VEP) technologies and facilities for Nb 9-cell superconducting accelerator cavity for International Linear Collider (ILC) in collaboration with KEK. This time, we developed new type 'Ninja' cathode in order to improve VEP uniformity of Nb 9-cell cavity inner surface based on the results of 1-cell cavity VEP experiment. In this article, we will report construction of new type "Ninja" cathode for Nb 9-cell cavity and experiment of VEP using this., Proceedings of the 28th Linear Accelerator Conf., LINAC2016, East Lansing, MI, USA

We have been using single cell coupon cavities to establish vertical electropolishing (VEP) process for a couple of years. A series of in-situ measurements of an EP current at an individual coupon in a coupon cavity can help determination of appropriate EP conditions. VEPed coupons which are surface analysed with XPS, SEM and the other tools can also bring lot information and expertise to development of VEP cathode and optimization of VEP conditions. This time we fabricated the world first 9-cell coupon cavity where 3 sample coupons at the equators and 6 sample coupons at positions close to the irises can be installed. VEP of this coupon cavity with a newly developed Ninja cathode brought useful information for improvement of the VEP facility and optimization of the VEP conditions., Proceedings of the 28th Linear Accelerator Conf., LINAC2016, East Lansing, MI, USA

Superconducting accelerating cavities used in high-energy accelerators can generate high electric fields of several 10 MV/m by supplying radio frequency waves (RF) with frequencies matched with resonant frequencies of the cavities. Generally, frequencies of input RFs are fixed, and resonant frequencies of cavities that are fluctuated by Lorentz force detuning and Microphonics are corrected by feedbacks of cavity frequency tuners and input RF power. Now, we aim to develop the cavity frequency tracking type RF control system where the frequency of input RF is not fixed and consistently modulated to match the varying resonant frequency of the cavity. In KEK (Tsukuba, Japan), we are developing SRF-TEM that is a new type of transmission electron microscope using special-shaped superconducting cavity. By applying our new RF control system to the SRF-TEM, it is expected to obtain stable accelerating fields so that we can acquire good spatial resolution. In this presentation, we will explain the required stabilities of accelerating fields for SRF-TEM and the feasibility of SRF-TEM in the case of applying the cavity frequency tracking type RF control system., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

Horizontal electropolishing (HEP) is being used for final surface treatment of niobium SRF cavities. However a HEP system is equipped with complicated mechanism that makes it expensive and enhances cost of surface treatment of cavities especially when mass production is considered. Vertical electropolishing (VEP) has been introduced by other labs and the research is being carried out to establish the VEP technique. The VEP system requires simple mechanism and has advantages over HEP setup. Positive results have been obtained from the VEPed cavities also as shown by other labs. However further improvement in a VEP setup, cathode and VEP parameters is required. Marui Galvanizing Co., Ltd in collaboration with KEK has been working for development of VEP system, optimization of cathode and VEP parameters to obtain uniform Nb removal with a smooth surface of a cavity. Here we report our recent development of VEP system, unique Ninja cathode and parameter optimization with a 1-cell coupon cavity containing 6 Nb disk coupons at the beam pipes, irises and equator. The coupon surfaces were analyzed to obtain detail of the cavity surface., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

Marui Galvanizing Co.,Ltd. has been developing Nb 9-cell SRF cavity vertical electro-polishing (VEP) facility and technique for mass production in collaboration with KEK. Our first 9-cell cavity VEP facility was not enough to control temperature during VEP, so the polishing quality was not so high. In this article, we will report the progress of temperature distribution and polishing quality due to the improvement of temperature control system of electrolyte and cavity during VEP., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

Vertical electropolishing (VEP) leads several advantages over horizontal EP in respect of easy operation and mechanism of an EP system resulting in lower cost. However, till yet VEP always resulted inhomogeneous removal of a niobium (Nb) cavity along its length and bubble traces especially on the top iris of a vertically set cavity. In this work we performed lab EP and VEP experiments in order to study and solve these two problems. A coupon cavity which contains 6 disk type Nb coupons positioned at beam pipes, irises and equator was vertically electropolished to optimize VEP parameters so as to get almost uniform removal of Nb and a smooth surface of the cavity without bubble traces. Our patented unique i-Ninja cathode having 4 wings was used with an optimized rotation speed to get homogeneous removal of Nb. The homogeneous removal and the surface without bubble traces might be result of a uniform thickness of a viscous layer on the surface of the cavity cell and no accumulation of hydrogen bubbles on the top iris surface. The surfaces of the coupons were studied in detail with surface analytical tools., Proceedings of the 17th International Conference on RF Superconductivity, SRF2015, Whistler, BC, Canada

We have constructed an Electro-polishing (EP) Facility in the Superconducting RF Test Facility (STF) at KEK in 2008. The EP facility has been used for the EP process of Superconducting RF (SRF) 9-cell cavities for more than one year. In the EP facility, the capacity of the EP-electrolyte reservoir tank is 2,000 L. This size is relatively large if compared with EP facilities in other laboratories. It means that the quality control of EP electrolyte is more difficult because the status of EP-electrolyte changes as the aging of EP-electrolyte proceeds. In the real EP-process operations, we circulated the EP electrolyte of 1,100 L which was firstly delivered into the tank in January 2008 and was disposed in May 2009. During this period, we performed the EP processes 40 times and periodically measured the concentration of Nb ,Al ,HF in the EP electrolyte. In this article, we report the detailed results of the EP-electrolyte monitoring as well as the observation of changing electronic current oscillation in the EP processes during this period in the EP facility at STF/KEK., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

In development of superconducting radio-frequency niobium cavities, there are problems in low performances of electro-polished (EP) cavities with a fresh EP solution due to stains on the surfaces with discoloration. Although the stain problems have been known from the past researches, the detailed study with surface analysis has not been carried out. In this study, the stains on the niobium surfaces were observed with x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy and scanning electron microscope. According to results of XPS, there are some differences in atomic components at the stained and non-stained surfaces, ex, a little amount of fluorine and no metal oxide were found only at the stained surface. In this article, we will describe the detail of the XPS results., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Buffered chemical polishing (BCP) has been widely used as a final recipe of the surface treatment for niobium cavities and there is still much room to improve this technology since it is environment friendly, cheaper and simpler than electro-polishing. To examine BCPed surface in detail, we carried out BCP experiment followed by various rinsing methods on a series of niobium samples at KEK. As a result of the BCP process some contaminants like fluorine, carbon, etc. have been detected at the surfaces which may be the prominent cause of limiting the performance of SRF cavities. To remove these contaminants, various rinsing processes such as ultra pure water rinse, ultrasonic pure water rinse, alcoholic rinse, detergent rinse, high pressure water rinse (HPR) had been tested after the BCP. The preliminary results show that, only HPR had potential to mitigate these contaminants. In this article, we describe the surface analysis results using X-ray photo electron spectroscopy etc and a comparative study of niobium BCPed samples followed by above mentioned rinsing processes., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

In an attempt at demonstrating an average field gradient of 31.5 MV/m as per the design accelerating gradient for ILC, a program called S1-Global is in progress as an international research collaboration among KEK, INFN, FNAL, DESY and SLAC. The S1-Global cryomodule will contain eight superconducting cavities from FNAL, DESY and KEK. The cryomodule will be constructed by joining two half-size cryomodules, each 6 m in length. The module containing four cavities from FNAL and DESY has been constructed by INFN. The module for four KEK cavities is being modified at present. The assembly of the cryomodules is scheduled from January 2010, and the operation of the system is scheduled from June 2010 at the KEK-STF. In this paper, the construction of the S1-Global cryomodule will be presented., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan