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Energy-recovery linacs (ERLs) are promising for advanced synchrotron light sources, high-power free electron lasers (FELs), high-brightness gamma-ray sources , and electron–ion colliders. To demonstrate the critical technology of ERL-based light sources, we have designed and constructed a test accelerator, the compact ERL (cERL). Using advanced technology that includes a photocathode direct current (DC) electron gun and two types of 1.3-GHz-frequency superconducting cavities, the cERL was designed to be capable of recirculating low emittance ( ≤ 1 mm ⋅ mrad) and high average-current ( ≥ 10 mA) electron beams while recovering the beam energy. During initial commissioning, the cERL demonstrated successful recirculation of high-quality beams with normalized transverse emittance of ∼ 0.14 mm ⋅ mrad and momentum spread of ∼ 1.2 × 10−4 (rms) at a beam energy of 20 MeV and bunch charge below 100 fC. Energy recovery in the superconducting main linac was also demonstrated for high-average-current continuous-wave beams. These results constitute an important milestone toward realizing ERL-based light sources.

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 40 MeV 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 1 ms train length under 5 Hz operations, 4.60×10 5 photons/1%-bandwidth were obtained. The peak X-ray energy, which was confirmed by the SOI detector, was 29 keV, and this is consistent with ICS X-rays.

The high power test for STF -2 cryomodule with twelve cavities has completed successfully in 2015. It became clear the average accelerating gradient was 30.3 MV/m, eight cavities achi eved above 31.5 MV/m as the ILC specification, and three cavities had the significant performance degradation by heavy field emission due to the additional clean room work in t he STF tunnel. As the following next steps, there are the LFD (Lorenz Force Detuning) measurem ent, LFD compensation by piezo, and long ru n for check of stable operation at high gradient. In th e long run around 32 MV/m, each cavity without degradation showed the stable operation with the successful LFD compensation by piezo and RF feedback system. On the other hand, from the view point of pl ug-compatibility for the input coupler in the ILC, new STF input couplers with 40mm of input port were re -designed, fabricated and high -power -tested successfully. Moreover, for the lower cost study, another input couplers with coating -free ceramic will be also tested soon in the collaboration of CERN and KEK. In this presentation, the detailed results for STF -2 Cryomodule t est and 40mm input couplers are reported.

The injector cryomodule for the compact energy recovery linac (cERL) is under development at KEK. The cryomodule with 3L-band 2-cell cavities was built in June 2012. A prototype 2-cell cavity and three other 2-cell cavities with five higher-order-mode (HOM) couplers for actual operation were fabricated in May 2011. The vertical tests of these cavities were carried out after standard surface preparation at the KEK Superconducting accelerator Test Facility (KEK-STF) from October 2010 to March 2012. Radio-frequency feedthroughs with high thermal conductivity for the HOM coupler were also developed to achieve 12.5 MV/m CW operation in the cryomodule. A Kyocera NR-type connector was modified to connect to this target. The results of vertical tests of the 2-cell cavities to measure their feedthrough performance will be reported in this paper.

Since 2008, vertical tests of 1.3 GHz 9-cell cavities at the superconducting RF test facility (STF) in KEK have been carried out for the International Linear Collider (ILC). The tested cavities are mainly KEK-05 through KEK-22. Test results showed that KEK-12, KEK-13, KEK-17, and KEK-21 attained the unloaded Q value (Q0) of 0.8×1010 at the accelerating gradient (Eacc) of 35 MV/m (ILC specification) only by the electro-polishing (EP) process. These four cavities had no problematic defect on any electron beam welding seams at their equators; however, field emission was still observed in the vertical tests. This paper reports on the recent progress of the cavity performance at KEK-STF, along with a detailed analysis of problematic defects and the effect of the local mechanical grinding.

An injector cryomodule for the compact energy recovery linac (cERL) is under development at KEK. This injector cryomodule has 3 L-band 2-cell superconducting rf cavities. The cERL is required to accelerate a 10-mA CW electron beam to 5 MeV. The required accelerating gradient per cavity is 7.5–12.5 MV/m at ∼30 kW input power to the cavity and the beam. The operational frequency is 1300 MHz at 2 K and the mode of operation is CW. In this application, the critical hardware components are not the cavities, but the rf input couplers and higher-order-mode (HOM) dampers. Initially, a TESLA-style coaxial HOM coupler was chosen for HOM damping of the injector cavities. However, this HOM coupler had a heating problem at low gradients (a few MV/m) in CW operation. The components heated in the accelerating mode were the HOM body and the feedthrough that extracts HOM power from the cavity. To control the heating problem, a new HOM coupler was designed based on a TESLA-style coaxial HOM coupler, and the feedthrough was also modified based on a Kyocera N-R type connector to have better thermal conductivity. A prototype 2-cell cavity and 3 other 2-cell cavities with 5 new HOM couplers for actual operation were fabricated through May 2011. Vertical tests of these cavities were carried out after standard surface preparation at the KEK Superconducting Accelerator Test Facility (KEK-STF) through March 2012. The accelerating gradient achieved exceeded 50 MV/m without quenching during the vertical test using the prototype 2-cell cavity and feedthroughs. The magnetic field at the cell equator was 2127 Oe. Three 2-cell cavities passing the criteria of the High Pressure Gas Safety Institute of Japan exceeded 25 MV/m without field emissions. The cavities with the best performance were prepared in March 2012 for the cERL injector. The designs of the HOM couplers and feedthroughs and the results of the vertical tests to evaluate their performance are reported here.

Four superconducting cavities have been developed at the superconducting RF test facility (STF) at KEK as candidates for the International Linear Collider (ILC) accelerating cavity. These cavities, which are termed TESLA-type STF baseline cavities, are equipped with newly designed higher-order-mode (HOM) couplers that have a short outer cylinder and a single welding point on the antenna that acts as the inner conductor. Each STF baseline cavity is equipped with two types of these new HOM couplers: STF I-type and STF L-type. The HOM couplers were designed and developed, and their RF performance was evaluated by using a Cooper model. Then, niobium models were fabricated and cool down tests were conducted with these cavities. In all, 14 cool down tests were performed for the four cavities using a vertical setup that involved the newly designed HOM couplers. Electrochemical polishing of the inner surface of the cavity and frequency tuning of the accelerating mode were performed before each cool down test in order to maintain the RF performance and obtain a higher gradient. These new HOM couplers were operated up to a gradient of 32 MV/m in the accelerating field of the end cell without any serious problem. Their damping performance is comparable to that of the TESLA cavity except for the TM 011 mode, whose performance can be easily corrected by changing the installation angle of the next fabricated cavity. In this paper, the development of the new HOM couplers for STF baseline cavities is described.

A prototype cryomodule containing two 9-cell superconducting cavities of β = 0.725 and f 0 = 972 MHz had been constructed under the collaboration of Japan Atomic Energy Research Institute (JAERI) and High Energy Accelerator Research Organization (KEK) on the development of superconducting LINAC for Accelerator Driven System (ADS). Cool-down tests to 2 K of the cryomodule and high power rf tests with a 972 MHz pulsed klystron have been carried out. Rf power of 350 kW in pulsed operation of 3 m s and 25 Hz was transferred to a 9-cell cavity through an input coupler. Accelerating gradients of 14 MV/m (compared to the specification of 10 MV/m) have been achieved in both cavities. Compensation of Lorentz-force detuning by a piezo tuner was successfully demonstrated.

For the future large scale application of superconducting (SC) RF cavities like TESLA, one has to fabricate the cavities cheaply to achieve high gradients and high Q values (E/sub acc/>25 MV/m, Q>1/spl times/10/sup 10/). The amount of niobium material has to be decreased and the complex welding structure should be eliminated to reduce the fabrication cost. Here, the authors propose to fabricate seamless clad cavities of niobium and copper material and report the results of a feasibility study.

The Japan Atomic Energy Research Institute (JAERI) is proposing a high intensity proton accelerator for the Neutron Science Project. A superconducting (SC) RF structure is the main option for the high energy part of the linac. Design and development work of the SC proton linac is in progress in JAERI. Three single-cell SC prototype cavities were fabricated and their performances were tested. The test results were good enough to apply to the SC linac. This paper describes the present status of the design and development work for the SC proton linac.

The STF-Baseline superconducting cavity system, which includes four TESLA-type 9-cell cavities, input couplers and frequency tuners, has been developed for the future ILC project. A main improvement in the cavity system is a very stiff design in a He vessel and a cavity tuning system, which can relax a cavity deformation due to Lorentz force. Vertical tests of four 9-cell cavities have been carried out repeatedly, and the attained accelerating gradients have reached to 20 ~ 29 MV/m with Qo values higher than 1times1010. Peculiar phenomena, like increasing of Qo values with higher Eacc or very slow degradation of Qo values, were observed in the vertical tests. RF processing of input couplers was successfully carried out up to 1.0 MW with a 1.5 msec and 5 Hz operation .

A prototype cryomodule containing two 9-cell superconducting cavities of β=0.725 and fo=972MHz had been constructed under the collaboration of Japan Atomic Energy Research Institute (JAERI) and High Energy Accelerator Research Organization (KEK) on the development of superconducting LINAC for Accelerator Driven System (ADS). Cool-down tests to 2K of the cryomodule and high power rf tests with a 972MHz pulsed klystron have been carried out. Rf power of 350kW in pulsed operation of 3msec and 25Hz was transferred to a nine-cell cavity through an input coupler. Accelerating gradients of 14MV/m higher than the specification of 10MV/m have been achieved in both cavities. Compensation of Lorentz-force detuning by a piezo tuner was successfully demonstrated.

Prototype coaxial input couplers with a planar rf window and a high power test stand were designed and fabricated. The high power tests with a 972 MHz pulsed klystron had been successfully carried out. Input rf power of 2.2 MW in a pulsed operation of 0.6 msec and 25 Hz (370 kW in 3.0 msec and 25 Hz) was transferred to the input couplers without any problem.

A steep drop of the cavity quality factor (Q/sub 0/) at high electric field has been observed frequently in chemically-polished and high-pressure-rinsed Saclay cavities, even in the absence of field emission. Three single-cell cavities from Saclay were tested at KEK following electropolishing and high pressure rinsing. This treatment eliminated the steep Q/sub 0/ drop, the quench field increased to an accelerating gradient of more than 30 MV/m with a quality factor above 2/spl times/10/sup 10/. No dependence of the quench field on the niobium RRR was observed between RRR=200 and 320 in electropolished cavities.

The TRISTAN superconducting RF system has been operated since 1988. The remaining operational problem is the trip of the cavity caused by synchrotron radiation. In order to reduce the trip rate, we made realignment of Q-magnets in the straight section and put a movable radiation mask near the end of the are section. The effect of these improvement, operating status and long term performance are reported. >

An attractive future application of superconducting cavities is a TeV energy superconducting linear collider (TESLA). The substantial merits of TESLA compared to a normal conducting linear collider are to loosening of alignment tolerance and less wake field due to the lower frequency (1.3 GHz). The final focus also is easier to obtain by having the electron/positron population in bunches. TESLA demands upgrading field gradients to over 25 MV/m. KEK has been carrying out R&D on 1.3 GHz niobium superconducting cavities since 1990. So far, seven single cell cavities and two 9-cell cavities have been fabricated and tested. In parallel, input coupler design and niobium material studies are being conducted. This paper reports the present status of KEK's efforts for TESLA. >

A superconducting RF system including 32 5-cell cavities has been operated in TRISTAN for three years. The authors describe cavity performance in the ring, a study of frequent trips caused by a discharge in the cavity, and the way in which certain problems were solved. >

The development of superconducting cavity for high accelerating fields has been continued at KEK on the basis developed for TRISTAN superconducting cavity. Many attempts have been pursued to achieve high field and to understand the phenomena that limit the cavity performance at high field; investigations of niobium material, surface treatment, heat treatment, surface condition, cavity-shape and cavity-forming. Accelerating fields of more than 25 MV/m have been achieved repeatedly in the several single-cell cavities, even though we do not yet fully understand what happens at high fields. The present status will be reported.

The microwave surface resistance of YBaCuO thick films was measured using a demountable copper cavity operated at 3 GHz in the TE011 mode over a temperature range from 4.2 K to 300 K. The films were deposited onto end plates and cylinder walls of silver substrates or nickel-buffered copper substrates by a low-pressure plasma-spraying technique combined with a melt-reaction method. The area of the end plate was 177 cm2 and that of the cylinder wall was 396 cm2. The surface resistance was obtained from the Q values measured before and after substituting either the end plate or the cylinder wall for one covered with the film. The ratio of the surface resistance of the film to that of the copper was lower than 0.08 at 5.2 K and 0.65 at 77.3 K for the end plate and was 0.15 at 5.1 K and 1.91 at 77.3 K for the cylinder wall.

Two important factors that contributed to Japan's economic success were government investment in industrial development and the early recognition that a good educational system is a prerequisite to technological progress. Government policies promoted the importation of technologies from Europe and North America and encouraged the education of students abroad. This facilitated the rapid development of Japanese industry and the adaptation of foreign technologies to local conditions. Many of the methods used to develop industry in Japan could be used to advantage in developing countries today.

In summary it appears that a program of synthesized supply and acreage controls probably could assist materially to (1) hold production to desired levels over the short and long run, (2) keep production from year to year within reasonable limits, (3) shift storage costs of surpluses back to the producer where they belong, (4) stabilize agricultural income by having an administered market supply-a system long used by most major manufacturing industries in our "free enterprise" system, (5) return prices to a free market determination, within the framework of administered supply control, (6) provide a continued incentive for progress in production technology on the farm, (7) permit production, over time, to shift toward the more efficient producers rather than tending to freeze allotment patterns as in the past, and (8) retain livestock and livestock products production and prices in a free market determination framework.14 While the program outlined above no doubt has its share of imperfections and costs, there is no way out of the present enigmatic situation without having some concurrent costs attached. Also, some avenues for further useful agricultural policy formulation by our profession are suggested and they are sorely needed. The need for brevity prevents giving a more detailed working model here, or mentioning various other flexibilities that might be incorporated into such a program. As John D. Black once so wisely remarked, no control program should be without re-evaluating correction mechanisms, be it a local area milk marketing order program or a national effort.

TRISTAN was operated with an average beam current of 12.0 mA at a beam energy of 29.0 GeV in 1990. The 32 5-cell superconducting RF cavities had been kept cold for about 6 months. No irrecoverable degradation of the cavity performance had been observed in the long-term operation. Changes of the cavity performance, problems, and improvements of the superconducting RF system are described. >