JASRI/SPring-8, Hyogo-ken
NewSUBARU/SPring-8, Laboratory of Advanced Science and Technology for Industry (LASTI), Hyogo
RIKEN Nishina Center, Wako
Transverse beam instabilities were suppressed with chromaticity modulation (CM)* in the electron storage ring, New SUBARU. The horizontal and vertical betatron tune spread inside a bunch were introduced by CM with synchrotron oscillation frequency driven by an AC sextuple magnet**, to obtain Landau damping of the coherent bunch motion. The tune spread in a bunch is usually introduced by octupole field, however, its high nonlinearity reduces the dynamic aperture. And usual feedback against instabilities work only on m=0 mode and it is not easy to be applied to hadron synchrotrons because of their varying revolution period. The CM scheme has not such disadvantages. The damping time of coherent motion excited by external kick was measured and was found as less than 1ms, one order faster than that without CM. To observe the effect on instabilities, we intentionally tuned an HOM in a cavity to excite a horizontal multi-bunch instability. The instability peak in the spectrum of the beam motion was vanished with CM turned on and the instability was suppressed. We also observed the increase of the threshold current of the vertical single-bunch mode-coupling instability by factor 3 with CM.
* T. Nakamura, Proc. of PAC'95, p.3100 (1995).
** T. Nakamura, et al., Appl. Superconduct., IEEE Trans. Vol. 18, p.326 (2008).
RIKEN/SPring-8, Hyogo
JASRI/SPring-8, Hyogo-ken
We report the design, performance, and installation of the beam diagnostic system of XFEL/SPring-8. The electron beam bunches of an XFEL accelerator are compressed from 1 ns to 30 fs by bunch compressors without emittance growth and peak-current fluctuation which directly cause SASE fluctuation. To maintain the stable bunch compression process, the accelerator requires rf caivty beam position monitors (BPM) with 100 nm resolution, OTR screen monitors (SCM) with a few micro-meter resolution, fast beam current monitors (CT) and temporal structure measurement systems with resolution under picosecond. The performance of the developed monitor instruments, such as the BPM, the SCM, and the CT, was tested at the SCSS test accelerator and satisfied with the requirements. To measure the temporal structure of the electron bunch, three type measurement systems, which are a streak camera, an EO sampling measurement, and a transverse deflecting cavity with a resolution of few-tens femtosecond, are being prepared. The streak camera and EO sampling shows the resolution of sub-picosecond. The installation of these beam diagnostic systems is going on smoothly.
RIKEN/SPring-8, Hyogo
The pulse width of an X-ray laser at XFEL/SPring-8 is several tens femto-seconds, which requires reference rf signals to have the same time-stability. The reference signals with a low phase-noise oscillator are sent to instruments in 19" racks developed along an accelerator by an optical fiber system. The temperature drift of the fiber makes phase shifts of the reference signals. Therefore, the fiber is put in a thermal-insulated duct. By feeding temperature-controlled water (26.1 ± 0.1 deg. C) in a pipe attached to the duct, the fiber temperature was kept to be 26.2 ± 0.08 deg. C at the ambient temperature change of 29.1 ± 1.7 deg. C. From this temperature controllability, the phase shifts of the signals through a 400 m fiber of a thermal coefficient of 5 ps/km/K are 160 fs. Further reduction of the shifts is required and will be achieved by a fiber-length feedback control in a future plan. Vibration of the fiber also degrades the quality of the signals. The fiber is embedded on a vibration buffer material. A test to evaluate the effect of the vibration to the transmitted signal phase was carried out. The test result will be also shown in this paper.
RIKEN/SPring-8, Hyogo
University of electro-communications, Tokyo
The intensity of SASE generated by undulators is sensitive to the peak intensity fluctuation of an electron bunch. The bunch is formed by velocity bunching in an injector and magnetic bunching in bunch compressors (BC). The peak intensity is sensitive to rf phase and amplitude of off-crest acceleration at injector cavities and 5712 MHz cavities before the BCs. Thus, demanded stabilities of the rf phase and amplitude for stable SASE generation are very tight. These are 0.6 degree (p-p) and 0.06 % (p-p) at the 5712 MHz cavities, respectively. We are constructing a low-level rf (LLRF) system comprising a master oscillator, an optical rf signal transmission system, and a digital rf control system using IQ modulator/demodulator to drive klystrons. To realize the demands, much attention was paid to temperature stabilization for the system. A water-cooled 19-inch rack and a water-cooled cable ducts are employed for almost all part of the system. Temperature stability of the rack was 0.4 K (p-p) even though outside was 4 K (p-p). The phase and amplitude stabilities of the LLRF modules were measured to be 0.30 degree (p-p) and 0.56 % (p-p). These stabilities are sufficient for our demands.
JASRI/SPring-8, Hyogo-ken
SES, Hyogo-pref.
RIKEN/SPring-8, Hyogo
In top-up operation at SPring-8 the horizontal beam oscillation had been excited because the injection bump orbit is not closed perfectly. For this problem, we had made an effort to reduce the residual beam oscillation by the improvement of bump magnet design, reducing the effect due to the nonlinearity of sextupole magnet and introducing pulsed corrector magnet, etc. By these improvements the average amplitude of residual oscillation has now been suppressed to the level of less than 0.1 mm. Still remaining relatively large residual oscillation comes from a non-similarity of a temporal shape of magnetic field of four bump magnets. We then started development fast kicker magnet system to give a counter kick to this part of residual beam oscillation. A key technology in this development is how to generate a large pulsed current in a short period to meet the oscillation characteristic. A newly developed fast pulsed power supply can generate a current of about 300 A, or corresponding magnetic field of 4.61 mT, with a pulse width of 1.2 us. Recently, we succeeded in the reduction of the horizontal beam oscillation at the timing of firing bump magnets by using this kicker system.