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• W2559055364 abstract "In J-PARC linac, a 181-MeV negative hydrogen beam is supported to a succeeding 3-GeV synchrotron with normal operation power at 100-300 kW. During operation, a beam loss in the straight section of the beam transport line immediately after the linac exit is found. The residual radiation level reaches 0.3 mSv/h on the surface of the vacuum chamber several hours after the beam shutdown with the linac beam power of 12 kW. We suppose that the residual gas scattering of negative hydrogen ions generates neutral hydrogen atoms and they give rise to the beam loss by hitting the vacuum chamber wall. To confirm this speculation, the vacuum pressure level in the linac had been changed in order to find the dependence of the beam loss on it. After data analysis, we found the relationship between beam loss amplitude, which was attained from beam loss signal, and vacuum pressure was linear. In this paper, we present the experimental result and some analyses in this study. INTRODUCTION J-PARC linac nowadays mainly consists of a 50-keV negative hydrogen ion source, a 3-MeV RFQ (Radio Frequency Quadrupole linac), a 50-MeV DTL (Drift Tube Linac), and a 181-MeV SDTL (Separate-type DTL). We plan to upgrade the linac energy to 400 MeV by adding ACS (Annular Coupled Structure linac) in the straight section after the SDTL exit [1]. The beam power of J-PARC linac for user operation has been gradually increased and reached 13.3 kW just before the earthquake in March 2011. The mitigation of beam loss is a main issue in the J-PARC linac because it is indenting to have tenfold increase of the beam power by the energy and intensity upgrade. In the user operation of J-PARC linac, we have been experiencing a beam loss widely distributed in the future ACS section as shown in Figure 1. A preliminary experiment was conducted to see the response of the beam loss level to the vacuum pressure level intentionally increasing the vacuum pressure level by turning off some of the vacuum pumps. The result for this experiment indicated that the mechanism for the beam loss is the H0 component generated by residual gas scattering of Hbeam. We extend the study in this paper to further confirm the conclusion. Figure 1: Beam loss of J-PARC linac in normal operation. Beam loss in section A mainly caused by X-ray leaked from DTL and SDTL tanks. Pink: 12 kW beam power from linac & 200 kW from the RCS; Black: 7.2 kW from linac & 120 kW from the RCS. BEAM LOSS REDUCTION BY VACCUM IMPROVEMENT We have added several NEG pumps (Non-evaporable Getter Pump) in the downstream part of SDTL and the upstream to middle part of the future ACS section. More specifically, we have added four NEG pumps in the SDTL section after the 26th, 27th, 28th, and 29th tanks and four NEG pumps in the future ACS section at the addresses of ACS07, 11, 13, and 15 shown in Figure 2. Figure 2: Beam loss profile in the beam transport line after the SDTL exit before and after the vacuum improvement. The red arrows show the locations of NEG pumps in the future ACS section. Figure 2 also shows the beam loss signal before and after the vacuum improvement. In this figure, it is readily seen that the beam loss is significantly reduced in the middle to downstream part of the future ACS section. Then, it has led us to the next step where we demonstrate reduction of the beam loss by improving the vacuum pressure in the SDTL section and the future ACS section with additional vacuum pumps. ___________________________________________ *Work supported by JAEA weigh@post.j-parc.jp WEPS048 Proceedings of IPAC2011, San Sebastian, Spain 2598 C op yr ig ht c ○ 20 11 by IP A C ’1 1/ E PS -A G — cc C re at iv e C om m on sA tt ri bu tio n 3. 0 (C C B Y 3. 0) 04 Hadron Accelerators A08 Linear Accelerators EXPERIMENT OF RESIDUAL GAS STUDY IN SDTL 8-15 SECTION Experiment Conditions and Method An online experiment had been done to find the relationship between beam loss in the future ACS section and residual gas in last part of SDTL section. The depiction is shown in Figure 3. Firstly we turned off the ion pumps in SDTL 8-15 section in order to increase the vacuum pressure in SDTL 8-15 section. Then the measured data was record as a curve due to the time for ion pump vacuum meter and BLM. Here the online beam condition is that FRQ tank level 95 %, no-chopped beam with frequency 1.0 Hz, pulse length 100 microseconds, energy 181 MeV at the end of linac. we surveyed for beam with peak current both 15 mA case and 20 mA case. Figure 3: Simply depiction of online experiment study of beam loss due to vacuum pressure control by ion pump. Experiment Results After data taking, we made the relation between those two data curves from Vacuum meter and BLM. Here firstly we should also need to transfer data from Beam Loss Monitor signal to ‘so called’ corrected beam loss amplitude [2], which is supposed to be linear to the number of lost particles. The corrected beam loss (vc) amplitude is obtained by fitting the raw BLM signal (vr) with the following seventh-order polynomial as shown in equation (1)." @default.
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• W2559055364 date "2011-01-01" @default.
• W2559055364 modified "2023-09-24" @default.
• W2559055364 title "Dependence of beam loss on vacuum pressure level in j-parc linac" @default.
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