Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP012

OPTIMIZATION OF J-PARC LINAC BEAM FOR INJECTION TO RCS
Y. Liu, T. Maruta, KEK/J-PARC, Tokai, Japan
Abstract
An optimized momentum spread is predicted for best
bunching factor and RF capture at RCS injection. A
sufficiently wide range of output momentum spread was
successfully achieved from J-PARC linac for theverification. It is found that not only momentum spread,but also transverse emittance, Twiss parameters anddispersion play import roles. Series of theoretical andexperimental tests were carried out for improving JPARC linac output beam for mitigating beam loss at
injection to RCS.

The optimization of linac for RCS injection requests
the following steps,
1. Achieve dispersion-free at injection foil
2. Find requested center energy for RCS
3. Get optimal momentum spread
4. Check Twiss parameter at injection foil
5. Transverse match at injection foil
A satisfying result for minimized beam loss at RCS
injection was finally achieved, after high-power beamstudies with 30mA in Jan., April and June this year.
This paper mainly focuses on the works with linac
longitudinal tuning, i.e. steps 1~4.

INTRODUCTION
The Japan Proton Accelerator Research Complex (JPARC) is a high-intensity proton accelerator, which
consists of a linac[1], as shown in Fig.1, a 3GeV
synchrotron (rapid cycling synchrotron, RCS), and a main
ring synchrotron (MR).
The J-PARC linac consisted of a 3MeV RFQ, 50MeVDTL (Drift Tube Linac) and 181/190 MeV SDTL
(Separate-type DTL) before 2013.
The J-PARC linac was upgraded from 181 MeV to400MeV in Jan., 2014, with new annular-ring coupledstructure (ACS) installed during the summer shutdown in2013. The present nominal operation current is 15mA.
The designed peak current of 30 mA was applied forbeam studies.
A new RF ion source and a new RFQ (RFQ3) arebeing installed in the summer shutdown this year withdesigned current of 50mA. The commissioning isscheduled in this October.

SCHEME OF LONGITUDINALOPTIMIZATION
An optimized momentum spread is predicted both forbest bunching factor and RF capture at RCS injection,
based on simulations and experiences from previous
beam studies.
A separate-functioned debuncher system [2] is applied
at J-PARC linac, for longitudinal tuning. Fig.2 sketches
the debunchers (DB) in the post-linac beam line L3BT.

Figure 2: Sketch of J-PARC L3BT used for linac
optimization for RCS injection.

Figure 1: Layout of J-PARC linac.
Due to the increase of energy and current and thechange of the setup, the linac should be optimized forminimal beam loss both in linac and RCS injection.
The key points of linac optimization for RCS injection
consists longitudinal tuning for best output momentumspread using the debuncher system, and transversematching. The condition of dispersion-free at theinjection foil should be kept for any tuning.

The main functions of DB1 are to eliminate the energy
jitter due to random RF errors and to control the bunch
length. The main function of DB2 is achieved theoptimized momentum spread for RCS injection.
A beam energy offset is often requested for the optimal
RCS longitudinal painting. It is obtained with thedebunchers, singly or jointly, depending on the amount ofoffset.
The two-debuncher system is designed such that aminimum output momentum spread at the injection could
be achieved with DB2, typically with small amplitude at
the focusing phase. With the amplitude, equivilently thefocusing strength of DB2 decreased or increased, acurtain range of momentum spread could be available forminimizing the RCS injection beam loss. It is
straightforward to believe both sides are equivalent.

- 469 -Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, JapanPASJ2014-SAP012

FIRST EXPERIMENTS
The first beam studies of linac longitudinal tuning
were carried out in Jan. and April this year.
The basic setting is as follows.
RCS requested an energy offset of -0.52MeV fromACS output. It was handled by DB2 solely. Therefore
DB1 served as design and DB2 was responsible both forthe deceleration and focusing/defocusing.
8 points were tested in Jan., as shown as dots in Fig.3.
The simulation was done with 3D PIC code IMPACT.
According to the simulation, the tuning range of theǼp/p| rms was about 0.01~0.15%. An optimal momentum
spread was found with DB2 voltage at 1.9MV at focusingside.

In early simulation studies there were also emittance
growth in the defocusing side due to mistake of remained
dispersion at the injection. After correction it was foundthat there is almost no emittance growth in thelongitudinal defocusing side.
Questions No.1 and 2 pointed out it is possible to findbetter optimal with bigger Ǽp/p, and there were twoways. The first is to carefully try higher amplitude at
focusing side. The second is to try lower amplitude. DB2
was conditioned for about 0.6~2.2MV. Below 0.6MVneeds long-time conditioning to be stable. ACS groupagreed to try up to 2.5MV.
Therefore in the high-power beam study in April, twomore points, as shown in red circle in Fig.3, were tried.
But they were worse than the previous optimal.
There was only one direction left, i.e. the lower
amplitude.

FINAL EXPERIMENTAprilÅ
CavityunstableregionÏ

It is not easy to touch the unstable region of DB2
without hardware preparations. So there are two simpleways. One is to try the lowest stable amplitude. The otheris to set DB2 to 0, using DB1 deceleration. Simulation
results for this new setting are shown in Fig.5 and Fig.6.

0.08%ÏFigure 3: Simulation for DB2 amplitude tuning withsimultaneous deceleration by 0.52MeV, together withmeasurement results in Jan. and April for the focusing
side.
There were some questions left open. First, the optimal
setting was the highest DB2 amplitude tested, and thepredicted optimal Ǽp/p (0.08%) was still higher; Second,the defocusing side offered higher Ǽp/p, but no benefit
was found. Third, compared with previous 181MeV case,the measure beam profiles showed as if there was noadiabatic damping for the 400MeV. Fourth, the measured
momentum at RCS with tomographic method was larger
than the simulation.

ÏÏFigure 5: Simulation for DB2 amplitude tuning with DB1
deceleration, together with measurement results in June.

Figure 6: Simulated emittance for DB2 amplitude
tuning with DB1 deceleration.
Figure 4: Simulated emittance for DB2 amplitude tuningwith simultaneous deceleration by 0.52MeV.
The simulated emittance, as shown in Fig.4, helped tounderstand the third question. Transverse emittance
growth was found at the focusing side.

In the high-power beam study in June this scheme wasrealized and final optimal is found to be with DB2 set to0. For this setting the momentum spread is less but
comparable with the optimal for the setting of DB2
deceleration. Almost no transverse emittance growth was
found both in simulation and experiment.

- 470 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP012

Together with transverse matching and optimization ofRCS tune, the RCS injection beam loss was reduced tothe ideal level, which only includes loss due to stripping
foil. Linac optimization for RCS was satisfyingly solved.

TRANSVERSE EMITTANCE

the wave number representing the total focusing strengthfrom applied and space charge field, Ȝis the emittance(unnormalized). The “setting temperature” T stands forthe free energy in each plane. In the longitudinal focusingside, the ݇߳௭ becomes larger than ݇߳௬ , so that theemittance exchange from plane z to y happens.

A very interesting phenomenon found in thelongitudinal tuning is the different behavior of transverse
emittance growth for focusing side and defocusing side.

(a)

(a)

(b)

(b)

(c)

(c)

Figure 7: Simulation for experimental tested points forDB2 amplitude tuning with simultaneous deceleration by
0.52MeV. (a) vertical emittance; (b) vertical “settingtemperature”; (c) longtudinal “setting temperature”.
Fig.7 and Fig.8 show some simulated evolution ofemittance and etc. for the longitudinal tuning with thesetting 1: DB2 amplitude tuning with simultaneousdeceleration by 0.52MeV, and the setting 2: DB2
amplitude tuning with DB1 deceleration.
In Fig.7a and Fig.8a it is clear to see the verticalemittance growth in the longitudinal focusing side (thesolid lines), but not in the defocusing side.
This phenomenon could be understood by looking at
the “setting temperature”, defined as ܶ ൌ ݇߳, where k is

Figure 8: Simulation for experimental tested points forDB2 amplitude tuning with DB1 deceleration. (a) verticalemittance; (b) vertical “setting temperature”;(c)longtudinal “setting temperature”.

ACKNOWLEDGEMENT
The satisfying result of linac optimization for RCS wasbased on the efficient and pleasant teamwork and kindhelp by the members in the J-PARC linac group and RCSgroup, especially Drs. Hideaki Hotchi, Akihiko Miura,
Kota Okabe, Pranab Saha and et al.

REFERENCES[1] Hidetomo Oguri, et al, “Present Status of J-PARC
Linac”, this procedings.
[2] Tomohiro Ohkawa, Masanori Ikegami, NIM A 576(2007) 274–286- 471 -