M. Tobiyama

KEKB beam instrumentation systems

For the stable high-luminosity operation and luminosity increase, the electron and positron storage rings of the KEK B-Factory (KEKB) is equipped with various beam instrumentations, which have been working well since the start of the commissioning in December, 1998. Details and performance of the beam-position monitor system based on the spectrum analysis using DSPs, the turn-by-turn BPM with four-dimensional function available for measurements of the individual bunch position, phase and intensity, the parametric beam-DCCTs designed so as to avoid the magnetic-core-selection problems for the parametric flux modulation, the bunch-by-bunch feedback system indispensable to suppress the strong multibunch instabilities in KEKB, the various optical beam diagnostic systems, such as synchrotron radiation interferometers for precise beam-size measurement, the tune meters, the bunch length monitors and the beam-loss monitors are described. Delicate machine tuning of KEKB is strongly supported by these instrumentations.

Measurement of the longitudinal bunch structure in the Photon Factory positron storage ring with a photon counting method

Abstract A photon counting system for measuring the time structure of bunches, such as the single-bunch impurity and the longitudinal distribution of electrons, was installed and has been improving in the KEK PF positron storage ring. Agreement between the experimental results concerning the rate of impurity growth and a calculation taking account of the relativistic effects of electron-electron scattering, which is the origin of the impurity growth, was fairly good. A deformation of the distribution at a high beam current was observed, and was analyzed phenomenologically.

Study of transverse bunch-by-bunch feedback systems based on the two tap FIR filter

Abstract A signal-processing system based on two-tap digital filters has been systematically studied considering its application to bunch feedback systems. Through studies we have found that the two-tap-filter-based system is a hopeful candidate for the signal processor used in high-speed, large-scale bunch feedback systems.

Initial test of a bunch feedback system with a two-tap FIR filter board

Initial beam test of the KEKB bunch-by-bunch feedback system prototypes has been performed in the TRISTAN-AR on the longitudinal plane. A simple two-tap finite impulse response (FIR) filter system consisting of hardware logic realizes the function of the phase shift by 90°, the suppression of static components, and the delay of up to a hundred turns. With the prototype filter board and a longitudinal kicker, the feedback loop has been closed successfully. The shunt impedance of the kicker was estimated from the excitation amplitude and measured damping. The damping time of the system has been measured by using the single-bunch Robinson instability. The feedback system stabilized the coupled-bunch instability completely under 8-bunch operation.

Accelerator plan for a light‐source study at the TRISTAN MR

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, vi...

Status of R&D efforts toward the ERL-based future light source in Japan

The energy recovery linac is a very promising synchrotron light source in future. We are contemplating to realize a ERL_based next generation light source in Japan, under a collaboration between KEK, JAEA, ISSP, and other SR institutes. To this end, we started R&D efforts on its key technologies, including a low-emittance photocathode gun and superconducting cavities. We also plan to assemble these technologies into a small test ERL, and to demonstrate their operations. We report our R&D status.

Longitudinal feedback system for the Photon Factory

In the KEK Photon Factory, longitudinal coupled- bunch instabilities are suppressed using the RF phase- modulation technique during the users operation. This technique is very effective not only to suppress the instabilities but also to enlarge the beam lifetime. Together with the study for top-up operation, longitudinal bunch-by-bunch feedback system has been developed. A two-port longitudinal kicker based on DAPhiNE-type cavity were designed and installed in the storage ring in the summer of 2006. FPGA-based signal processing part is under development based on the KEKB design. A mode feedback system which suppresses a specific coupled-bunch mode was tested successfully as a preliminary test of the kicker. By using a commercial FPGA evaluation board, a simple bunch-by-bunch feedback system was implemented and succeeded to suppress the coupled-bunch instability up to 52 bunches in the ring.

Recent Progress at KEKB

We summarize the machine operation of KEKB during past one year focusing on progress for this period.

Beam feedback systems

Outlines of bunch-by-bunch feedback systems for suppressing multibunch instabilities in electron/positron storage rings are shown. The design principles and functions of the feedback components are reviewed. The application of the feedback system as a tool to analyze instabilities using transient-domain techniques is also shown.

Achievements and problems in the first commissioning of superKEKB vacuum system

The first (phase-1) commissioning of SuperKEKB, an asymmetric-energy electron–positron collider at High Energy Accelerator Research Organization, known as KEK, Japan, began in February 2016 and successfully ended in June 2016. Three major tasks of the phase-1 commissioning were as follows: (1) vacuum scrubbing of the new beam pipes in anticipation of a sufficiently long beam lifetime in the next commissioning, (2) checking the stabilities of various new vacuum components at high beam currents of approximately 1 A, and (3) beam tuning to study stability and achieve low emittance. The coefficient of photon stimulated desorption rate, η (molecules photon−1), decreased steadily with increasing photon dose, as expected. The temperature increases of the bellows chambers, connection flanges, etc., were less than 5 °C at 1 A. The effectiveness of an antechamber, a TiN coating, and a clearing electrode to suppress the electron cloud effect (ECE) was confirmed. However, the ECE in the Al-alloy bellows chambers with...