Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

RECENT PROGRESS IN BEAM COMMISSIONING OF J-PARC LINAC
Takatoshi Morishita1,A), J-PARC linac beam commissioning teamA), B)
A)
Japan Atomic Energy Agency (JAEA)
2-4 Shirakata-Shirane, Tokai, Naka, Ibaraki, 319-1195
B)High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki, 305-0801
Abstract
The beam commissioning of J-PARC linac has been started since November 2006. After the scheduled shutdown insummer 2007, the beam is successfully delivered from the linac to the RCS. Since then, nine beam commissioning runs
were conducted in total, in which a stable beam provision to RCS was emphasized. During these commissioning runs,
the stability of the ion source, the RF system, and cavities has steadily been improved. Commissioning cycle of the JPARC linac is summarized in this proceeding together with the operating experiences, and the demonstrations for ahigher power operation and the residual radiation level of the accelerator are discussed.

J-PARC リニアックビームコミッショニングの進展SDTL cavities are moved to L3BT (Linac-3GeV BeamTransport line) to utilize them as debunchers.
With 181 MeV operation of the linac, the goal beam
power is 36 kW. Table 1 summarizes the design
parameters of the linac and those achieved to date.
Table 1: Summary of achieved beam parameters

INTRODUCTION
The J-PARC (Japan Proton Accelerator ResearchComplex) is a multi-purpose accelerator facility aiming at
achieving 1 MW class proton beam power[1]. The JPARC comprises a linac, an RCS (3GeV rapid-cyclingsynchrotron), an MR (50 GeV main ring synchrotron) andexperimental facilities. The front-end system of the linac
consists of a volume-type negative hydrogen ion sourcewith 30 mA peak current and an RFQ (Radio-FrequencyQuadrupole linac) with 3 MeV output energy. The 3 MeVH- beam from RFQ is chopped by an RF-chopper system
in an MEBT1(Medium Energy Beam Transport 1). For
matching to a DTL (Drift Tube Linac) with 50 MeV
output energy, we have the quadrupole magnets and twobunchers in the MEBT1. From 50 MeV to 400 MeV, thelinac accelerator consists of an SDTL (Separated-typeDTL) with 191 MeV output energy and an ACS (AnnularCoupled Structure linac). At the initial commissioning
stage, we started the beam operation with 181 MeV due tothat the installation of ACS is postponed until an energy
upgrade in the near future. The R&Ds for the production
version of the ACS module are ongoing and a satisfiable
performance is obtained from the high power test.
Presently, we are optimizing the procedure of the mass
production of the ACS cavity for the energy upgrade to
400 MeV [2]. In this initial commissioning, the last twoParameter
Output energy[MeV]
Peak current[mA]
Average current
after chopping[μA]
Linac beam power[kW]
Pulse length /Repetition /Chopper beam-on duty
[ms / Hz / %]30

Achieved to date181
30 (RFQ exit)*

27 (RCS inj.)
200

6.6 (w/o chop)19.5 (w/ chop)**36

1.2 (w/o chop)
3.5 (w/ chop)**
5mA :
0.5 / 2.5 / 100
0.05 / 25 / 6025mA:
0.05 / 2.5 / 100
0.12 / 25 / 26**
>0.5 / 25

0.5 / 25 / 53

RF flat-top width>0.5 / 25/ Repetition[ms / Hz]* Pulse length : 0.25 ms** Sustained for four minutes by the limitation of dump capacity

J-PARC linac has finished its installation in summer
2006. The beam commissioning of the linac started inNov. 2006. Before the beam commissioning, we
conducted a high power conditioning of RF cavities forone month. From Nov. 2006 to Jun. 2007, the beamcommissioning was addressed for aiming at realizing thebeam quality necessary for a coming RCS beamcommissioning. The basic beam commissioning wascompleted in the series of commissioning runs [3]. Afterthe scheduled shutdown in summer 2007, the beam wassuccessfully delivered into the RCS in Oct. 2007. Since
then, we conducted nine commissioning runs until theFigure 1: Block diagram of the J-PARC linac.
___________________________________________

1

Design181

takatoshi.morishita@j-parc.jp-81-Proceedings of the 5th Annual Meeting of Particle Accelerator Society of Japanand the 33rd Linear Accelerator Meeting in Japan (August 6-8, 2008, Higashihiroshima, Japan)

scheduled shutdown in summer 2008. In thesecommissioning runs, we focused on providing a stablebeam to RCS, and also continued the beam tuning anddemonstrations for a coming higher power operation. In
this proceeding, we report our operation schedule and theimprovement of the accelerator stability, the beam tuning
results, and the residual radiation level in the accelerator
tunnel.

Advancing the RF aging since Oct. 2006, the stability of
the RF cavities has steadily been improved. The recovery
period after the intermission has been shortened, and it
takes about 24 hours for the beam acceleration. Afterchecking the reproducibility of the beam in the linac, wecan deliver the beam to RCS in the third day of thecommissioning run.
The short unscheduled downtime is mainly due to an
RF reflection from a cavity detected by an LLRF (lowlevel RF) fast interlock system. The trend of this fault isshown in Fig. 3. The fault rate is improved in later runs.
However, the fault rate increased in run 7, presumably,
because the linac was commissioned with the peakcurrent of 25 mA for the first time. Since the start of thelinac commissioning, the operation time of a klystronexceeds 5900 hours without any serious trouble.

OPERATION CYCLE AND STABILITYISSUES
Operation scheduleNumber of faults / hour

A typical run cycle for the beam commissioning
consists of a beam operation of two or three weeks with
one-week interval. The run history since the start of thelinac operation is summarized in Fig. 2. In this chart, eachvertical rectangular block (labelled with the name of themonth on top of it) shows the operation history of amonth, and each row (labelled with its date) shows theoperation status of a day. A day is divided into threeeight-hour shifts, and each shift is represented with asquare. A blank square denotes scheduled downtime, anda yellow square denotes the operation for RF conditioning
without beam. A blue and a green squares are respectively
show the beam-on time for linac beam study and those fordownstream facilities. Finally, a red square denotes
unscheduled downtime. It should be noted here that abeam fault shorter than four hours is not shown in thischart. The beam commissioning is interrupted in themidnight in our operation schedule, although, the RF
conditioning is continued 24 hours during a run. The
unscheduled downtime by faults longer than one minuteis analyzed from the operation log [5]. In run 17, theunscheduled downtime is 6.4 % of the total beam time(259 hours).

160
12080
400RUN5

RUN6RUN7RUN8RUN9RUN10 RUN11 RUN12 RUN13 RUN14 RUN15 RUN16 RUN17

Figure 3: The rate of RF faults detected by LLRF fastinterlock system

Maintenance cycle for ion source
The lifetime of the filament for the ion source should be
carefully estimated for scheduling the maintenance cycle.
To maintain the discharge condition, the ion source isoverhauled (for the cleaning of the filament and thedischarge chamber) during every interval. The ion source
has been operated continuously for 24 hours since run 16.
In addition, this 24-hours operation was kept for three
weeks without any maintenance. The beam current
decreasing rate at the 30 mA operation is less than 0.015
mA/h without operator’s assistance [6]. The total

Stability of RF source and cavity
The stable operation of the cavities and the RF powersources is essential for a stable beam operation.

Figure 2: Linac operation history since Oct. 2006.
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operation time of the ion source so far is 3360 hours,
during which the filament was exchanged three times due
to short-circuit and cut-off.

LINAC BEAM TUNING
Taking time out from beam delivery to RCS, occasional
beam studies have been performed for linac. Details on
the beam studies are left for separated papers [7]. We justtouch upon the demonstration studies for future beam
power ramp-up.
The design power for 181 MeV operation of the linac is36 kW, in which the pulse length, the peak current, therepetition, and the chopper beam-on duty factor are 0.5ms, 30 mA, 25 Hz, and 53 %, respectively. As a step
toward the beam power ramp-up, we demonstrated 3.5
kW beam power (0.12 ms, 25 mA, 25 Hz, 26 %) in run 14.
This beam was accelerated to 3 GeV at RCS and extracted
to a beam dump successfully. This operation wassustained only for four minutes due to the limited beam
dump capacity. For a longer pulse length operation, wedemonstrated 0.5 ms pulse length in run 15. In thisdemonstration, the unchopped beam was transported tothe linac beam dump, in which the peak current and therepetition of the beam were 5.7 mA and 2.5 Hz
respectively. During these high power demonstration andthe long pulse demonstration, we could not observe any
problems such as an instability, an increase in RF fault, aworse vacuum, an unexpected beam loss, and so on.

Figure 4: Residual radiation level after run 17.

SUMMARY AND FUTURE PLAN
Since Sept. 2007, the J-PARC linac has conducted ninecommissioning runs in total without any serious troubles,
where the 181 MeV beam was delivered to RCS with anacceptable beam quality and stability. Meanwhile, thebeam tuning was continued to enhance the beam qualityand demonstrated the performance of the linac for ahigher power operation. Currently, the residual radiationlevel is still low. A higher duty operation with more than 1
kW output power of the linac is planned from Sept. 2008,
in which a significant radiation level might be detected.
And then, the production run will start in Dec. 2008.

RESIDUAL RADIATION LEVELREFERENCES

A residual radiation level of the accelerator components
was surveyed after the end of each run. Figure 4 shows
the survey results after run 17, in which the value wasmeasured with contact on vacuum chamber. With the
exception of the beam window and the dump section, thehighest value of 10 μSv/h was detected at the entrance ofthe debuncher 2. In Fig. 4, the survey results at thedebunchers 1 and 2 after run 5 are also noted and thoseare more than one order of magnitude higher than thoseafter run 17. The beam aperture is narrowed at theentrance of the debunchers 1 and 2, because wetemporally utilize SDTL cavities as debunchers. Then, therelatively higher radiation level was detected at theentrance of those debunchers. We have experienced thatthe relatively higher residual radiation was observed after
a run, in which the beam tuning such as phase scan andmatching are conducted. The residual radiation inducedby the beam losses during the beam tuning processes
seems to have the majority and should be carefullycontrolled.
The residual radiation level is nearly negligible in thepresent stage. However, we are aiming at realizing thebeam power more than one order of magnitude larger thanthe current value. Therefore, we need to carefully monitor
the trend of the residual radiation level as we increase thebeam power, and take a counter measure if necessary inan appropriate timing.

[1] Y. Yamazaki ed., “J-PARC Technical Design Report”,
KEKReport 2002-13, JAERI-Tech 2003-44.
[2] H. Ao, et al., “Status of the ACS cavity development
for the J-PARC linac energy upgrade”, in theseproceedings; T. Hori et al., “Present status of the 972
MHz RF test stand for J-PARC 2008”, ibid.; K.
Hirano et al., “High power test of the first ACS
module for the J-PARC linac”, ibid.
[3] M. Ikegami, et al., “Beam commissioning of J-PARClinac”, Proceedings of the 4th Annual Meeting ofParticle Accelerator Society of Japan, Wako,
Aug. 1-3, 2007.
[4] F. Naito, “Present status od J-PARC”, Procs. of
APAC07, Indore, INDIA, Jan. 29 - Feb. 2, p.1 (2007).
[5] M. Ikegami, et al., Procs. of HB 2008, to be published.
[6] H. Oguri et al., “Development and operation of a Csfree J-PARC H− ion source”, Rev. Sci. Instrum, 79,02A506 (2008).
[7] H. Sako et al., “Transverse beam tuning and injection
to RCS at J-PARC linac”, in these proceedings;
S. Sato et al., “Beam energy measurement at J-PARClinac”, ibid.; M. Yamazaki et al., “Present status of RF
source operation at J-PARC linac”, ibid.; T. Kobayashi
et al., “Pulse-by-pulse switching of beam loading
compensation in J-PARC linac LLRF”, ibid.

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