We have been developing an electron beam halo monitor as an interlock device for protection of undulator permanent magnets against radiation damage. When permanent magnets are irradiated with a halo part of 8GeV electron beam, magnetic fields undulators are degraded permanently and laser oscillation is weakened. Therefore, the interlock device is required during machine operation. Diamond detector, which operates in photoconductive mode, is good candidate for electron beam sensor. The excellent feature of this monitor is pulse mode operation to suppress the background noise efficiently. The beam test of the beam halo monitor using the diamond detector heads has been carried out at the beam dump of the SPring-8 booster synchrotron. Minimum number of injected electron was about 10~3/pulse, and we observed the pulse length of 0.33nsec FWHM without a preamplifier. The signal noise level is 0.5mV rms. The linearity of output signal on injected beam was also checked. The feasibility of the beam halo monitor equipped with the diamond detectors is described.
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