Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, Japan

PASJ2017 WEP029

PROGRESSES OF J-PARC LINAC COMMISSIONINGY. Liu†, T. Maruta, T. Miyao, M. Otani, K. Futatsukawa, KEK/J-PARC, Japan
A. Miura, JAEA/J-PARC, Japan, S. Fukuoka, University of Tsukuba, Japan
Abstract
In the past year J-PARC linac accomplished many upgrades. From 2016, the J-PARC linac started 40mA operation. Separated amplifiers were ready for the chopper
cavities, and the ringing effects due to previous connection in series were cured. Newly designed bunch shapemonitors for MEBT1 (BSM) (3 MeV) and MEBT2ACS(~200MeV) were tested on-line, which will work forthe longitudinal measurement and 3D matching to DTLand ACS. Lattices in the 200~400MeV part (ACS section)with actually redesigned both in transverse and longitudinal planes were studied and tested. Results of successfully
mitigated beam loss from the intra-beam stripping effectwere obtained.

INTRODUCTION
The Japan Proton Accelerator Research Complex (JPARC)[1] is a high-intensity proton accelerator facility,
which consists of a linac, a 3GeV synchrotron (rapidcycling synchrotron, RCS), and a main ring synchrotron(MR). The J-PARC just accomplished it upgrades withLinac energy from 181 MeV to 400 MeV in Jan. 2014 andLinac peak current to 50mA in Oct. 2014. The upgrades
brought about the condition for RCS output power of1MW.
The J-PARC Linac[2] consists of a 3 MeV RFQ, 50
MeV DTL (Drift Tube Linac), 181/190 MeV SDTL (Separate-type DTL) and 400 MeV ACS (Annular-ring Coupled Structure), as shown in Fig. 1.

In the summer shutdown of 2016, there were two mainhardware upgrades. Separated amplifiers were ready forthe chopper cavities, and the ringing effects due to previous connection in series were cured. Newly designedbunch shape monitors [3][4] for MEBT1 (3 MeV) andMEBT2-ACS(~200MeV) were tested on-line.
Efforts on the simulation studies were made to explore
new lattices with actually redesigned both in transverseand longitudinal planes. Series experimental studies identified the IBSt as the dominant beam loss and successfully
demonstrated beam loss mitigation.

IMPROVEMENTS IN LINAC OPERATION
From 2016, the J-PARC linac started 40mA operation.
The machine protection system (MPS) fires caused bybeam loss monitor (BLM) increased dramatically after40mA operation, as shown in Fig. 2, having become thesecond largest reason of beam trip (just after RFQ). The
phenomena were not easy to understand because it happened transiently and usually accompanying with RFMPS.

Figure 2: Linac trips in 2016.
Two completely different mechanisms were identified
by carefully analyzing the waveforms of RF, BLM andMPS signals and their cause-and-effect relationships.
One of them is the BLM MPS caused by a real RF failure,
which is mainly related with the RF and cavity performance.

Figure 1: Layout of J-PARC Linac.
From 2016, the J-PARC linac started 40mA operation.
Consequently the stepped J-PARC power-up at the neutron target was scheduled, but the schedule was stopped
by a target failure at 500kW. Now the accelerator is operated at conservative power (about 150kW) for stableneutron production. J-PARC power-ramping-up will startagain after target improvement.

Figure 3: Chopper upgrade by amplifier-parallelization.

___________________________________________† yong.liu@kek.jp- 885 -

Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, JapanPASJ2017 WEP029

The other is a little bit complicated but avoidable.
When BLM reaches MPS level and fires, the beam is
stopped in the low energy beam line (LEBT) in a few μs.
And the RF pulse is several hundred μs. Thus RF drops inremained part of the pulse as the beam was suddenly
stopped at upstream. The beam trip due to MPS fires by
BLM increased dramatically as the operation peak current
was increased from 30 mA to 40 mA. But no abnormal
residue radiation was found.
A careful study and countermeasure were carried out tomitigate the latter case [5][6], which is caused by random
BLM events and actually harmless to machine. The BLM
thresholds are kept, but the thresholds of signal width arewidened, so that those slim high peaks are skipped. Thesemisfire were reduced in recent half-year operation.
After beam intensity upgrade in Oct. 2014, the ringingeffects between the two cavities, powered by the same RF
amplifier, in the new chopper system, as shown in the leftpart of Fig. 3, became a source of horizontal emittance
growth. This is mitigated by powering the two cavities
with independent amplifiers, as shown in the right part ofFig. 3.
Newly designed bunch shape monitors, as shown in Fig.
4 and 5, for MEBT1 (3 MeV) and MEBT2ACS(~200MeV) were tested on-line, which will work forthe longitudinal measurement and 3D matching to DTLand ACS.
The many difficulties have been encountered for thelow energy BSM at MEBT1, such as several times ofbroken of the wire by the beam or the high voltage, noise,
nearby magnetic field and etc. A first measurement results
showed the beam bunch shape change vs. the MEBT1
buncher-1 amplitude, which is consistent with the simulation, as shown in Fig. 6.

Figure 4: Layout of BSM in MEBT1.

Figure 5: Layout of of BSMs in ACS.

Figure 6: First measurement of BSM in MEBT1.

IBST MITIGATION
After J-PARC linac energy upgrade, beam loss causedby the intra-beam stripping effect (IBSt) in the200~400MeV ACS section was predicted as the dominantsource of residue radiation.
An early testing experiment in 2015 illustrated the dependency of the beam loss on the lattice setting, as shownin Fig.7. Two weaker quadrupole settings in the ACS
section, marked with T=0.7 and T=0.5, resulted in considerable reduction of beam loss, where T stands for the“temperature ratio” between transverse and longitudinalplanes. This beam loss-lattice dependency not only identified the dominancy of IBSt in ACS section (as well as in
the whole linac), but also explored the possibility of mitigation. The stability of the lattices, compared with thebaseline design used in the normal operation, need to bestudied both theoretically and experimentally. For instance, the red line, for T=0.5 and the more away fromdesign, has lower beam loss at the first half of the section,but ends with a peak due to instability. These are consistent with the simulation study, as shown in Fig. 8 andFig. 9.
It is not enough to look at Fig. 8 only, which predicted
the beam loss rate according to lattice settings. A finallyoptimization should be a overall compromise with Fig. 9,i.e. the “Hofmann Chart”, which describes the intensity ofcoupling resonances and the consequent stability. A
transverse “redesign”, exactly same as in the testing experiment in 2015 with results shown in Fig. 7, moves theACS tune down left, i.e. to the weaker transverse focusing
and towards the resonant region. The IBSt were mitigatedfor T=0.5 (partly) and 0.7, but instability occurs for theT=0.5 case, too.

Figure 7: Testing results for beam-loss correlation withlattice setting.

- 886 -Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, JapanPASJ2017 WEP029

Figure 8: Theoretic prediction of beam loss by IBSt, fortransverse and longitudinal lattice setting combinations.

Series of new experiments were carried out in years
2016 and 2017, with a longitudinal redesign by systematically reduce longitudinal focusing and pull back the tune
from the resonant region, as shown the Fig. 9. Both further mitigation of beam loss by IBSt and beam stability
were obtained.
The experiment results are shown in Fig. 10. In the firststep, the cases of T=0.3, 0.5, 0.7 as well as nominal caseand a comparison case of T=1.3 were reproduced. The
cases of T=0.3, 0.5 are identified with beam loss reduction at the first half of the section, but also encountering
instability at the end. The T=0.7 case was the optimized
one according in previous experiment.
For each case longitudinal “redesign” was done in thenext steps, reducing the tank level and shifting the synchronous phase, thus reducing the longitudinal focusing
and pull the tune back to the stable region. It was observed that near the setting of 95% of ACS tank level the“T=0.3” (according to design tank level) case was stabilized. Thus a new candidate of ACS lattice was found.
According to simulation, shown in Fig.11, 60% of IBSt
beam loss will be gone. When the ACS tank level wasreduced down to 90%, the ACS longitudinal acceptancebecame too small, and part of the longitudinal halo will belost in the ACS section. The experiment results showed
good consistency with the simulation, as shown in thecomparison of Fig. 10 and 11.

Figure 11: ACS lattice candidates predicted by simulation.
The plots are the simulated emittance (blue: x, green: y,red: z) along whole ACS section, for cases with different
transverse and longitudinal focusing.
Based on simulation and experiment studies on thecandidate lattices shown in Fig. 11, further optimizationand verification will be carried out to find the new working point with less beam loss as well as stability and good
beam quality.

Figure 9: Stability chart for J-PARC ACS.

CONCLUSION AND DISCUSSIONS

Figure 10: Beam loss measured in the experiments in2016.

J-PARC linac increased the peak current to 40mA in
2016, and new BLM MPS setting was carried out to mitigate the consequently increased misfire. Separated RF
supplies for chopper cavities were successfully tested andthe ringing effects of the new chopper cavities were cured.
New BSMs were in stalled at MEBT1 and ACS and thebeam test were in progress.
Lattice studies of beam loss by IBSt were done. Morecandidate settings were found with additional longitudinalredesign.

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Proceedings of the 14th Annual Meeting of Particle Accelerator Society of Japan
August 1-3, 2017, Sapporo, Japan

PASJ2017 WEP029

REFERENCES[1] Kazuo Hasegawa et al., “Status of J-PARC Accelerators”, in these proceedings.
[2] Hidetomo Oguri et al., “Present Status of J-PARC Linac”,
Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan, 2014.
[3] Shota Fukuoka, Kenta Futatsukawa et al., “Developmentof Bunch Shape Monitor and Study of Longitudinal
Beam Matching at J-PARC Linac”, in these proceed-[4] Kenta Futatsukawa et al., “Development of Bunch
Shape Monitor at MEBT2 of J-PARC Linac”, in theseproceedings.
[5] N. Hayashi et al., “Beam-Loss Monitoring Signals of Interlocked Events at the J-PARC Linac”, IBIC 2016 proceedings.
[6] Naoki Hayashi, “Interlocked events of beam loss monitors
at the J-PARC linac”, in these proceedings.

ings.

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