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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.

Double rf systems are used to lengthen the beam bunches in synchrotron light sources. In such a system, the performance of the bunch lengthening is limited by the transient beam-loading effect, which is induced by gaps in the fill pattern. To improve its performance, we investigate an application of a normal-conducting harmonic cavity, which is based on the TM020 resonant mode. By using the TM020 mode with low $R/Q$ and high Q, fluctuation of the rf voltage due to the transient beam loading can be reduced significantly. The remaining small fluctuation of the rf voltage can be compensated by using an active feedforward technique. Using these measures, we expect to realize a bunch-lengthening performance that is comparable to that obtained with superconducting cavities under realistic operational parameters of a proposed 3-GeV next-generation light source. We estimate the bunch-lengthening performances using macroparticle tracking simulations together with semianalytical calculations.

For a future synchrotron light source based on a linac, e.g. an X-ray free electron laser and an energy recovery linac (ERL), an injector is a key component to generate a high brightness electron beam. For the acceleration and transportation of the electron beam in the injector, the adjustment of beam orbit inside the cavity is important to avoid the deterioration of the beam quality due to the transverse electric field of it, which causes the transverse emittance growth. To adjust the beam orbit, an investigation of the electromagnetic center of the cavity is required in the beam operation. This paper shows a new method for measuring the electromagnetic center of the cavity, and describes an analytical model of emittance growth due to a combination of transverse electric field and orbit offset. The validation of the method was confirmed by the emittance measurement in the compact ERL (cERL) injector at KEK.

The field emission electron currents from the main superconducting cavities (Cavities #3 and #4) of compact ERL at KEK, Japan, were estimated indirectly from photon dose rates measured around the cavities and on the roof of the compact ERL room. The field emission electron currents estimated from the photon dose rates measured around the cavities are in good agreement with those on the roof of the compact ERL room. The field emission electron currents increased steeply with the applied voltage. The field emission electron currents corresponding to the applied voltages were different between Cavity #3 and Cavity #4. We found that the field emission electron current exceeded 1 μA at 13.5 MV for Cavity #3 and at 15.5 MV for Cavity #4. This result was used in considering unexpected loss of field emission electrons.

In order to protect 3.4-m-long rf cavities from impinging intense synchrotron radiation (SR), we developed a unique SR mask for use in the Photon Factory Advanced Ring (PF-AR). To obtain a wide aperture that is required for beam injection, we designed the mask so that the head of the mask can be moved over a horizontal range of 30 mm. To avoid the harmful excitation of resonance modes in this mask structure, a microwave absorber was built into it. Two of the designed masks were installed in the PF-AR, and they showed satisfactory performance during routine single-bunch operations under beam currents of up to 60 mA. We report the design of this movable mask and its performance during operations.

For energy recovery linacs, excellent damping of higher-order modes (HOMs) in superconducting cavities is required for preventing beam breakup. As a promising method for this purpose, we propose to damp the HOMs using a radial transmission line attached to the sides of the cavity. The HOM powers are dissipated in RF absorbers while an accelerating field is rejected with a choke filter. An initial design of this radial-line HOM damper was carried out with simulations. Then, we measured the damping of HOMs by means of this method using a model cavity of the TESLA type. Investigations on a single-cell cavity showed a promising performance of HOM damping.

In order to investigate the possibility of exciting transverse higher-mode bunch oscillations for electron beams, we carried out a trial experiment in the Photon Factory (PF) storage ring at the High Energy Accelerator Research Organization (KEK). While storing a single bunch of electrons, we modulated the betatron tune at a frequency of two-times the fractional betatron frequency, using a high-frequency quadrupole magnet. The bunch oscillations excited were measured by detecting visible synchrotron light with a streak camera. As a result, a transverse quadrupole-mode oscillation of the bunch was clearly detected. This technique will be useful for studying the properties of the transverse higher-mode bunch oscillations in the electron storage rings.

A protection system for five high-power wiggler/undulator beam lines has been built to protect the Ultra-High-Vacuum (UHV) ( −8 Pa) of the 2.5 GeV storage ring at the Photon Factory. At an instantaneous vacuum failure downstream of a beam line, the closing titanium-alloy blade of the Fast-Closing Valve (FCV) is exposed to an intense synchrotron radiation beam from a wiggler/undulator, causing a meltdown of the blade within 80 ms. This results in fatal damage to components of the storage ring and a long shutdown time. The protection system comprises UHV detectors, FCVs, beam line control computers, and a dedicated high-speed optical communication link that is connected to four high-power RF klystrons (500 MHz, 180 kWmax, each) at the storage ring. Constant communication is maintained between the high-power beam lines and the RF klystrons at a transmission rate of 70.4 MHz using high-speed optical-fiber links. Upon detecting a vacuum failure at any beam lines, the system can dump the positron beam within 95 μs by tuning off the RF power before initiating closure of the FCV blade. The system allows the FCV blade to close within 12.4 ms before the insuring gas deteriorates the UHV of the storage ring. The result of an actual accidental vacuum failure at a wiggler beam line proved that the protection system could successfully protect the UHV of the storage ring.

The results of calculations and simulations of the vertical emittance in the KEK Accelerator Test Facility damping ring are described. Both systematic and random alignment tolerances and a skew correction scheme are presented which limit the normalized vertical emittance to γϵ y −8 m rad. The effect of intrabeam scattering on the vertical emittance is calculated.

A high frequency quadrupole magnet (HFQM), which can split betatron tunes from bunch to bunch, was developed at the Photon Factory storage ring at KEK. The effect of introducing a bunch-to-bunch tune spread on transverse coupled-bunch instabilities was investigated in detail. A remarkable rise in the threshold currents of transverse coupled-bunch instabilities was observed using the HFQM.

The loss parameters of the ducts in the Photon Factory (PF) storage ring were evaluated using the wire method and the code tbci. Both the measurement and the calculation were done for different bunch length σ ranging from 25 to 80 ps. The ring impedance was estimated to be ‖Z/n‖=3.19 Ω using the broadband impedance model. The measurements in frequency domain were also carried out to investigate the resonances of some duct components such as the gate valves without rf shield and the flange gaps. The results are presented.

The finite-element method was used to calculate a rise in temperature at a fast-closing valve (FCV) blade during exposure to intense synchrotron radiation from a 53-pole wiggler at the 2.5-GeV Photon Factory storage ring. The results indicate a possible meltdown of the titanium-alloy blade within 0.1 s at a maximum beam current of 500 mA, making the vacuum protection function of the FCV ineffective for an instantaneous vacuum failure downstream of the wiggler beam line. In order to prevent the blade from melting, the FCV control system has been interlocked with RF klystrons so as to initiate blade closure after dumping the electron beam by turning off the RF power. Performance tests have shown that the system could dump the electron beam within 95 mu s and then close the blade within 12.4 ms after being triggered. >

The Photon Factory Advanced Ring (PF‐AR) is a 6.5‐GeV synchrotron light source at the High Energy Accelerator Research Organization (KEK). It can provide high‐flux hard X‐rays for such research as the materials science, structural biology and medical applications. The PF‐AR has five insertion devices including four in‐vacuum undulators. It is operated with a single bunch which fits for time‐resolved experiments. A special 5‐GeV operation is partly provided for a clinical application. An initial beam current and a beam lifetime are 60 mA and approximately 14 hours, respectively. Recent developments include an installation of a new in‐vacuum undulator (U♯NW14‐36) to the west rf section, which was accompanied by transferring two rf cavities to other section. The undulator has been successfully operated at a minimum magnetic gap of 10 mm. We also carried out such accelerator studies as a successful test of beam injection using a pulsed quadrupole magnet, a study of low emittance optics, an establishment of tw...

The ultrahigh-vacuum (UHV) protection system at the 2.5 GeV synchrotron radiation source (positron storage ring) at the Photon Factory allows the intense-photon-flux wiggler beamlines to operate safely during synchrotron radiation experiments for a long time. There are six high-power wiggler/undulator beamlines that provide intense photon-flux beams to the experimental hall. In the case of a possible instantaneous vacuum failure at the experimental hall, the intense photon-flux radiation from the wiggler could cause a meltdown of the titanium-alloy fast-closing valve. The authors have developed a UHV protection system. Upon a vacuum failure, the protection system can dump the positron beam by turning off the RF power in the four RF klystrons (150 kW maximum), and then initiate a blade closure of the fast-closing valve. In this paper, the operational performance of the vacuum protection system for the intense-photon-flux wiggler beamlines is evaluated and discussed regarding the actual vacuum deterioration downstream of a beamline at the experimental hall.

The PF‐AR is a full‐time single‐bunch light source for SR experiments in the X‐ray region. Electrons are injected at 3.0 GeV and ramped up to the final energy of 6.5 GeV for normal user operation or 5.0 GeV for medical application. The typical initial beam current is 55 mA at 6.5 GeV and the beam lifetime is 15 h at 50 mA. The initial single‐bunch impurity better than 10−8 was achieved, however, degradation of the impurity during the users’ run has to be solved. Since the present emittance of 290nm⋅rad is not satisfactory, we are making an effort to realize the minimum attainable emittance of 160nm⋅rad. The high quality of pulse X‐ray has brought us abundant occasions to do time‐domain and/or time‐resolved measurements.

A three‐month operation with a beam is scheduled for the autumn of 1995 at the TRISTAN MR for the sake of light‐source development and research programs using it. The lattice will be modified so as to enable the installation of an undulator 5.4 m long and achieve a very low‐emittance beam of 5 nm at 10 GeV. The emittance damping wigglers will enhance the radiation‐damping rate, which will stabilize the coherent beam instabilities as well as reduce the emittance more. A considerable number of accelerating cavities will be removed from the ring in order to minimize the impedance of the higher‐order modes of the cavities that may cause coupled‐bunch instabilities and limit the intensity of the stored beam. For the stability of the light beam, a local feedback system will be applied to the electron orbit in the MR. The beam emittance will be measured by detecting the angular distribution of the Compton scattering of laser photons from the beam electrons. To measure the beam emittance in an independent way, visible synchrotron light from a bending magnet and 100 keV x rays from an undulator will be utilized.

A new `higher-order-mode (HOM)-free' accelerating cavity has been developed which can provide an accelerating voltage of more than 400 kV per cavity at a frequency of 714 MHz. The harmful HOMs in the cavities, which can induce beam instabilities at high beam currents, were heavily damped by using four special waveguide ports and broadband microwave loads. Two cavities of this design were installed in the 1.54 GeV accelerator test facility (ATF) damping ring at KEK, and successfully stored beams. This cavity will also be very useful for synchrotron light sources. The basic design, characteristics of HOMs and construction of this cavity are reported.

A means of producing short pulses of synchrotron radiation in the electron storage ring is proposed. By applying successive transverse kicks on a head and tail of an electron bunch in opposite directions, a head-tail oscillation can be excited. At certain moments with a regular period, the vertical slopes of the particles in the bunch are linearly correlated with their longitudinal positions. This correlation can be used to cut unwanted synchrotron radiation from the head and tail of the bunch, thereby providing short-pulse X-ray radiation, similarly to that proposed in previous papers.

A high‐speed nuclear‐magnetic‐resonance (NMR) data‐acquisition system, incorporating a personal computer, has been developed to determine the polarization of the polarized target for high‐energy physics experiments. A dedicated analog‐to‐digital converter (ADC) board is used for automatic ADC sampling synchronizing with an internal clock. Within about 1.5 s, 128 NMR signals are obtained and the averaged signal is derived.

Ion trapping was studied at the Photon Factory storage ring at KEK with bremsstrahlung from electron-ion collisions. Differences in the bremsstrahlung yields were found in the operations between the partial and uniform filling modes. Observed count rates for the uniform bunch filling were always higher than those for the partial filling, indicating that a small number of ions were captured in the latter case. The results clearly show the reason why ion-trapping problems can be alleviated in the partial filling.

A requirement for electron storage rings for the synchrotron radiation experiments is that circulating beam is very stable. In particular, for small emittance machines, obtaining a high‐quality beam is the most important subject. Though the rf‐accelerating cavities are essential for the storage rings, sometimes they cause serious beam instabilities, and lead to a current limit and an emittance growth. The Photon Factory storage ring (KEK) is a dedicated synchrotron radiation source. There are four single‐cell cavities in the ring. Beam instabilities arising from the cavities have been investigated. The resonant frequencies of these cavities were trimmed by choosing the proper length of blank‐flanges attached to the cavity ports, to avoid the conditions unstable for all existing coupled‐bunch instabilities. Reduction of the instabilities was remarkable.

It has been reported that the beam lifetime in electron storage rings can be improved by applying a small phase modulation on the rf accelerating voltage at a frequency of two-times the synchrotron oscillation frequency. This effect can be explained as being due to the improved Touschek lifetime that is caused by an excited bunch-shape oscillation in the stored bunches. In order to understand this phenomenon further, we investigated the induced bunch-shape oscillation in the 2.5-GeV Photon Factory storage ring at High Energy Accelerator Research Organization (KEK). Using both a streak camera and a spectrum analyzer, we observed the quadrupole-mode oscillations of a stored single bunch very clearly. It showed a splitting of a single bunch into three or two bunchlets, which depended on the modulation frequency. We report on these experimental results, and discuss them using computer simulations and an analytical formalism.

A 714-MHz higher-order-mode (HOM) damped cavity was developed at KEK, and two cavities of this design are under operation in the Accelerator Test Facility (ATF) damping ring. Each cavity is equipped with four waveguide ports to extract HOM power. In order to achieve the very low HOM impedance required for the damping ring, each waveguide port should be terminated by a broadband, low-reflection load. We have developed a broadband HOM load for this purpose. We describe the design, construction and RF characteristics of the HOM load.

A 714-MHz RF system for the Accelerator Test Facility (ATF) damping ring has been constructed and commissioned. The RF system comprises two higher-order-mode (HOM) damped cavities, a 50-kW klystron, a WR-1150 waveguide network and low-level RF electronics. By using HOM-damped cavities, longitudinal and transverse coupled-bunch instabilities can be avoided even at a maximum beam current of 600 mA. A low-level control scheme, which can manage heavy transient beam loading, was designed, and a subset of the system was implemented. We report in this paper the construction and commissioning of this RF system.

In the 2.5-GeV Photon Factory storage ring at KEK, we have found that the beam lifetime can be improved by modulating the phase of an rf accelerating voltage at a frequency of 2 times the synchrotron oscillation frequency. By applying this phase modulation with a peak-to-peak amplitude of 3.2\ifmmode^\circ\else\textdegree\fi{}, the beam lifetime could be improved, typically, from 22 to 36 h under a beam current of about 360 mA. At the same time, the longitudinal coupled-bunch instability could be considerably suppressed. The improvement in the beam lifetime can be explained as an improved Touschek lifetime, which was caused by a quadrupole-mode longitudinal oscillation of the stored bunches.