Shogo Sakanaka

Suppression of beam instabilities induced by accelerating cavities

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.

Development of an asymmetric power divider for a high-power RF distribution system

We have developed a radio-frequency (RF) power divider that is used for an RF distribution system in high-energy electron storage rings. This power divider has four waveguide ports, and it can split a high RF power by a ratio of 1:2 at an operating frequency of about 508.6 MHz. Since other properties are very similar to those of a magic-T it is very suitable for accelerator applications. By combining this device with a conventional magic-T, one can design an RF distribution system in which three cavities are driven by a single klystron. The design procedure of the power divider, a low-power measurement, and a high-power test result are presented.

Impedance measurements at the Photon Factory storage ring

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.

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.

Production of Short-Pulse Synchrotron Radiation using a Head-Tail Bunch Oscillation in the Electron Storage Ring

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.

Excitation and Detection of a Transverse Quadrupole-Mode Bunch Oscillation in the KEK Photon Factory Storage Ring

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.

Experience with the ultra-high-vacuum protection system for the synchrotron radiation beam lines with high-power wigglers/undulators at the Photon Factory

There are six high-power wiggler/undulator beam lines at the 2.5-GeV synchrotron radiation source at the Photon Factory. In the case of an instantaneous vacuum failure at such a beam line, a normal Fast-Closing-Valve (FCV) system is incapable of functioning to protect the Ultra-High-Vacuum (UHV) of the beam lines and the synchrotron radiation source, since a meltdown of the titanium-alloy FCV is caused by intense photon flux radiation. To avoid such a case, a dedicated UHV protection system has been built where all FCV systems for the high power beam lines have been linked, using high-speed communication links to RF klystrons to allow the FCV systems to initiate blade closure after dumping the positron beam by tuning off the RF power. In this paper, experience with the UHV protection system for the wiggler/undulator beam lines is discussed.

Measurement and Control of Beam Losses Under High Average-current Operation of the Compact ERL at KEK

The compact ERL (cERL) [1, 2] at KEK is a superconducting accelerator aimed at demonstrating ERL technologies for the future light source. In cERL, low-emittance and high-average-current electron beams of up to 10 mA will be recirculated in future. Toward this goal, we studied high-average-current operations where the beam losses should be controlled to very-small fractions. We have so far succeeded in recirculating beams of up to 0.9 mA with very-small beam losses. We report our accelerator tuning method for high-average-current operation, and present measured radiation data showing very-small beam losses. INTRODUCTION In cERL, production and transportation of lowemittance (< 1 mmuf0d7mrad) and high-average-current (uf0b3 10 mA) electron beams are primarily important. In highintensity linacs such as cERL, reduction of beam loss is essential in order to reduce the risk of radiation hazard as well as to avoid damages in accelerator components. Till June of 2015, electron beam having an average current of 80 uf06dA was successfully transported to the beam dump in cERL. Due to careful accelerator tuning and the use of beam collimators, beam losses along a recirculation loop were reduced to small amounts. At this time, we conducted radiation measurements with several methods, and estimated amounts of beam losses [3]. Based on these data, we installed some additional radiation shields, and applied an increase in our authorized beam current, that is, from 100 uf06dA to 1 mA. This application was approved by the government in January, 2016. Until March of 2016, we established high averagecurrent operations of cERL up to a maximum beam current of 1 mA. Typical operational parameters are given in Table 1. We can choose one of two repetition rates of bunches, 1.3 GHz or 162.5 MHz, by selecting one of the laser oscillators of a photocathode DC gun. First, we tuned the machine at a higher bunch-repetition rate (1.3 GHz) with lower bunch charge (0.7 pC/bunch). After this Table 1: Typical Operational Parameters of cERL Beam energy 19.9 MeV Injection energy 2.9 MeV Bunch repetition rate (usual) (for laser-Compton scattering) 1.3 GHz

A high‐speed nuclear‐magnetic‐resonance data‐acquisition system with a personal computer for a polarized target in high‐energy physics

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.