Bunch shape measurement of 181 MeV beam in J-PARC linac

Miura, Akihiko ; Feschenko, A. V.*; Mirzojan, A. N.*; Miyao, Tomoaki*; Ouchi, Nobuo ; Maruta, Tomofumi*; Liu, Y.*; Oguri, Hidetomo  ; Ikegami, Masanori*; Hasegawa, Kazuo

In J-PARC Linac, an energy upgrade project has started since 2009 using Annular-ring Coupled Structure (ACS) cavities. We have decided to use the bunch shape monitors (BSM) as the monitors of the longitudinal beam width measurement in order to take the longitudinal matching using two bunchers located in the upstream of ACS cavities, where the RF frequency jumps from 324 to 972 MHz. Three BSMs were completely fabricated and installed in the beam line. The BSMs were commissioned with the beam and their operability was demonstrated. We found the vacuum degradation during the measurement. We once removed all BSMs to have a vacuum conditioning and postponed the longitudinal matching at the establishment of 400-MeV operation of the energy upgraded Linac. In this paper, we introduce the mechanism of the BSM, its operability, the measurement results with the 181 MeV beam and consistency check with the respect cavity amplitude. We also describe the operational vacuum conditions and the outline of the improvement of the vacuum system for the BSMs.