from the shutdown of most of the nuclear power plants in Japan after the disastrous earthquake on March 11, 2011 and the successive occurrence of accidents in the Fukushima Dai-ichi nuclear power plant. We expect that the situation of electricity supply will get better and we will have the full benefit of the improvement of the RF cavity cooling system in the near future. Research and Development of Beam Position Monitors for Second-Order Moment Measurement We are installing new BPMs at the SPring-8 linac, which measure the transverse second-order moments of electron beams, as an enhancement of non- destructive beam diagnosis during top-up injection. To design the system, we first constructed the comprehensive theory for BPMs [16]. This theory presents an analysis and design method for a stripline- type BPM that detects the multipole moments of a charged particle beam. A numerical analysis based on the finite difference method was also carried out to calculate the electric fields in a BPM. According to this design method, we have developed six-electrode BPMs with circular (Fig. 9) and quasi- elliptical cross sections for non-dispersive and dispersive sections, respectively. The results of the numerical calculations show that the second-order moment can be detected for beam sizes > 0.42 mm (circular) and >0.55 mm (quasi-elliptical). The actual beam sizes are >0.5 mm in the non-dispersive sections and > 0.75 mm in the dispersive sections, that is, they are sufficiently large to enable accurate measurements of the second- order moments using these BPMs. The measurement of a second-order moment requires a good measurement accuracy of about 10 -4 . Therefore, we developed a low-noise signal processor (Fig. 10) with six channels. The previous signal processor with four channels contained sample-and- hold circuits that generate large sample-and-hold noises. The new signal processor, however, does not employ any sample-and-hold circuit; thus, fast and low- noise analog-to-digital converters (LTC2393, LINEAR TECHNOLOGY) directly acquire signals from the peak- hold circuits. The dynamic range of the signal processor is finally increased to 80 dB, and the maximum signal- to-noise (S/N) ratio is 80 dB. Beam Performance 146 References [1] G. Guignard: Phys. Rev. E 51 (1995) 6104. [2] M. Takao: Proc. of EPAC’06, Edinburgh, UK, p.1975. [3] M. Takao: Phys. Rev. ST Accel. Beams 9 (2006) 084002. [4] M. Masaki et al .: Proc. of EPAC’08, Genoa, Italy, p.3035. [5] M. Masaki et al .: Phys. Rev. ST Accel. Beams 12 (2009) 024002. [6] SPring-8 Research Frontiers 2009, p.153. [7] A. Baron: SPring-8 Information 15 (2010) p.14. [8] K. Soutome et al .: Proc. of IPAC’10, Kyoto, Japan, p.4497. [9] T. Tanaka and H. Kitamura: SPECRA code ver. 9.02 (2012). [10] C. Mitsuda et al .: Proc. of SRI’09, Melbourne, Australia, AIP Conference Proc. 1234, p.193. [11] C. Mitsuda et al .: Proc. of PAC’09, Vancouver, Canada, p.1171. [12] W. Guo et al .: Phys. Rev. ST Accel. Beams 10 (2007) 020701. [13] K. Kobayashi et al .: Proc. of ICALEPCS 2009, Kobe, Japan, p.659. [14] T. Nakamura et al .: Proc. of ICALEPCS 2005, Geneva, Switzerland, PO2.022-2. [15] M. S. Zolotorev and G. V. Stupakov: SLAC-PUB-7132, March 1996. [16] K. Yanagida et al .: Phys. Rev. ST Accel. Beams 15 (2012) 012801. Developments and Upgrades of Linac Fig. 9. Six-electrode BPM with circular cross section. Beam Position Monitor Band-Pass Filter CF:2856 MHz BW:10 MHz Logarithmic Detector ADL5513 ANALOG DEVICES Peak-Hold Circut Droop:0.4 V/ms 16-bit Analog-to-Digital Converter LTC2393 LINEAR TECHNOLOGY Data Acquisition System Fig. 10. Block diagram of signal processor (one of six channels). Haruo Ohkuma, Shigeki Sasaki and Hirofumi Hanaki SPring-8/JASRI E-mail: ohkuma@spring8.or.jp
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