• H. Aoyagi, T. Bizen, K. Fukami, N. Nariyama
  JASRI/SPring-8, Hyogo-ken
• Y. Asano, T. Itoga, H. Kitamura, T. Tanaka
  RIKEN/SPring-8, Hyogo

An electron beam halo monitor has been developed in order to protect undulator permanent magnets against radiation damage for the X-ray free electron Laser facility at SPring-8 (XFEL/SPring-8). The halo monitor will be installed at the upstream of the undulator and detect the electron beam that might hit the undulator magnets. Diamond detector, which operates in photoconductive mode, is good candidate for electron beam sensor, because diamond has excellent physical properties, such as, high radiation hardness, high insulation resistance and sufficient heat resistance. Pulse-by-pulse measurement suppresses the background noise efficiently, especially in the facilities having extremely high intense beam but low repetition rate, such as XFELs. The linearity of output signal on injected beam has been demonstrated in the range of 10³ to 10⁷ electron/pulse. The feasibility check for this monitor was performed at the SPring-8 compact SASE source (SCSS) test accelerator for XFEL/SPring-8. We observed the unipolar pulse signal with the pulse length of 0.4 nsec FWHM. The beam profiles of the halo can be also measured by scanning the sensor of this monitor.

• X.-M. Maréchal
  JASRI/SPring-8, Hyogo-ken
• Y. Asano, T. Itoga
  RIKEN/SPring-8, Hyogo

Fiber-based beam loss monitors offer the possibility to detect beam losses over long distances, with good position accuracy and sensitivity at a reasonable cost. For the undulator section of the SPring-8 X-FEL, radiation safety considerations set the desirable detection limit at 1 pC (corresponding to a 0.1% beam loss) over more than a hundred meter. While a theoretical approach offers some hints, the selection of the optimum fiber is not straightforward. Glass fibers of different diameter (100 to 600 μm), index profile (graded/stepped) and from three different makers were therefore characterized (signal strength, dispersion, attenuation) at the SPring-8 Compact SASE Source (SCSS), a 1/16th model of the future X-FEL. Beam tests (Fujikura SC400) showed that, at 250 MeV, the detection limit corresponding to a 10 mV signal is below 1 pC over 60 m and 3 pC over 120 m. The position accuracy was found to be better than 30 cm. Finally, the fiber lifetime has been estimated to be over 13000 h from dose measurements at the SCCS.