Operating Experience of the J-PARC Linac

The beam commissioning of the J-PARC linac started in November 2006 and 181 MeV acceleration was successfully achieved in January 2007. The linac has delivered beams to the 3 GeV Rapid Cycling Synchrotron for its commissioning, and then, the subsequent Main Ring Synchrotron and the neutron target commissioning. The linac uses a Cs-free LaB6-driven ion source and 20 units of 324 MHz klystrons. As of June 2008, the operation times are about 3,000 and 6,000 hours for the ion source and the RF source, respectively. The operating experience of the linac is described. INTRODUCTION The J-PARC (Japan Proton Accelerator Research Complex) is a multipurpose facility with 1 MW class proton beam[1]. The facility is under construction at the JAEA/Tokai site as a joint project between the Japan Atomic Energy Agency (JAEA) and the High Energy Accelerator Research Organization (KEK). The J-PARC accelerator consists of a linac, a 3 GeV rapid cycling synchrotron (RCS), and a 50 GeV main ring synchrotron (MR). At the initial stage of the project, the linac accelerates a negative hydrogen beam up to 181 MeV with a current of 30 mA, a pulse width of 0.5 msec and repletion of 25 Hz. The linac consists of an RFQ, a Drift Tube Linac (DTL) and a Separated-type Drift Tube Linac (SDTL). The features of the linac are described in [2]. High power conditioning of the cavities and beam test started in October and November 2006, respectively, then 181 MeV energy acceleration was achieved in January 2007. Since then, the linac beams have been used for the linac commissioning for its own, for the RCS commissioning, for neutron production, and for RF capture experiment at the MR. We have adopted a three to four weeks commissioning cycle, which consists of a two or three week beam commissioning run and a one or two week interval. Adjusting the terms of intervals to accommodate appropriate maintenance periods, 17 beam commissioning cycles have been experienced from November 2006 to June 2008. Detailed beam commissioning results are described in [3]. FRONT END The J-PARC linac uses a Cs-free, LaB6-driven, multicusp H ion source[4]. The diameter and the length of the plasma chamber are 100 and 120 mm, respectively. The source plasma is confined by the multicusp magnetic field by 18 rows at the sidewall and four rows at the upper flange. The filament is a cylindrical spiral structure LaB6 and the size is 29.5 mm and 49 mm in diameter and height, respectively. Figure 1 shows the history of the ion source operation. During intervals between commissioning cycles, we have performed maintenance or ion source study. The pink and green bars denote the typical H beam current in the commissioning days and the ion source study days, respectively. The maximum beam current from the ion source is 38 mA at the arc duty factor of 0.8 % (0.32 msec in pulse width and 25 Hz). Since the commissioning tasks doesn't always require a high peak H beam current, the ion source has been operated either at the low current mode of 5 mA or at the high current mode of 30 mA. To keep the condition of the plasma chamber surface good, the ion source keeps running of the filament or beam extraction during the nighttime in the commissioning days. Operation time of the source has been reached to 3,360 hours as of June 2008. Figure 1: Operation history of the ion source. On October 6 and December 18, 2006, filament had troubles of short-circuit to the neighboring spirals. The troubles were caused by filament deformation due to the arc power heating. Then the filament shape was modified to have a wider gap between spirals from 0.3 to 0.6 mm. We have not had similar troubles since then. On August 21, 2007, we had a cutting off of the filament. We consider that the filament was broken by some strong mechanical stress because the filament showed no signs of remarkable consumption. The filament was replaced with a studious care. That filament lasted by June 28, 2008. The beam time was 2,030 hours, ___________________________________________ [email protected] Ion source operation history (as of 30-Jun-2008) 0 5 10 15 20 25 30 35 40