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

PASJ2014-SUP120

核物質非破壊検知用 220MeV マイクロトロンの空間電荷効果
SPACE CHARGE EFFECTS ON 220-MeV MICROTRON FOR NON-DESTRUCTIVE
NUCLEAR MATERIAL DETECTION SYSTEM*堀 利匡#, A), 全 炳俊 A), 紀井俊輝 A), 大垣 英明 A), 大東 出 A), ネグム ハニ A),
小滝 秀行 B), 神門 正城 B), 羽島 良一 C), 早川 岳人 C), 酒井 文雄 D)
Toshitada Hori#, A), Heishun ZenA), Toshiteru KiiA), Hideaki OhgakiA), Izuru DaitoA), Hani NegmA),
Hideyuki KotakiB), Masaki KandoB), Ryoichi HajimaC), Takehito HayakawaC), Fumio SakaiD)A)
IAE, Kyoto University, Gokasyo, Uji, Kyoto 611-0011
B)
JAEA, Kansai, Kizugawa, Kyoto 619-0215
C)
JAEA, Tokai, Ibaraki 319-1195
D)
Sumitomo Heavy Ind. Ltd., Nishitokyo, Tokyo, 188-8585

Abstract

A nuclear material detection system (NMDS) based on neutron / -ray hybrid approach has been proposed for thecontainer inspection at sea ports [1, 2]. While neutron is to be used for a fast pre-screening, quasi-monochromatic -ray
beam from the laser Compton scattering (LCS) source will be used for an isotope identification on the precise
inspection of the cargoes. Nuclear resonance fluorescence method is going to be employed for the isotope identification
because of its superiority in high selectivity and in high penetration capability through the shielding objects. In thesystem a high energy electron beam of good quality is required for LCS. A 220-MeV racetrack microtron (RTM) is oneof the most promising candidates as an electron source for such the practical use. Suppose a few nC/bunch of electrons
are accelerated, space charge effects (SCEs) on the RTM are investigated using the tracking code Parmela [3].

INTRODUCTION
At present four sets of 150-MeV RTM are in operation
starting from 1990 [4]. While three of them are for theinjector of electron storage ring, the fourth is for various
experiments including LCS at JAEA. On the contrary to
the former three RTMs which have a thermionic gun as
the electron source, the fourth has an RF gun as thesource. Therefore in principle the fourth accelerates asingle bunch at a time. Higher energy RTMs typically 220
MeV for NDMS have been considered on the basis of thiswell-established machine designing [5-7]. Simulations
under no SCE were carried out so far, calculations
including SCEs have been executed as shown below.
We suppose around 1 nC charge in a single bunch isnecessary for this purpose. However, if much the charge
could be accelerated without significant emittance
growth, the situation might be much the better. Thus wehave investigated the effects of space charge on output
beam quality by the simulation code Parmela.
The configuration of RTM was somewhat modified inorder to fit for Parmela calculations (Fig.1). Differences
between the actually designing RTM and this simulation
are described below. The major difference lies on thefocusing method.
 The magnetic field in two 180 bendings (1.4 Tesla) isflat, no n-value is added. It means they cannot generate___________________________________________*Work supported by Funds for integrated promotion of social system
reform and research and development (Grant No. 066)
#tosihori@iae.kyoto-u.ac.jp

vertically focusing force in the bendings.
 There are no reverse field magnets in front of thebendings. Thus, no edge focus effects are expectedwhich are effective on early circulations in low energy
region.
 In compensation, Q-doublets are introduced on every
turn. The field gradients of both QF and QD are equal,
dB/dx=83 G/cm for the common Q-magnets on thelinac axis. The strength of Q-doublets on the backstraight sections is decided proportional to the energyof passing beam. Those Q-doublets keep the focal
length about the same in all orbits.

Figure 1: Components layout used in the Parmela
simulation, keeping the left-right symmetry.
Simulations were executed with 5000 particles startingfrom the entrance of the linac at Einitial=7.20.06 MeV,and initial=9. Accelerated beam qualities wereevaluated at the exit of the linac (Fig.1).

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August 9-11, 2014, Aomori, Japan

PASJ2014-SUP120SPACE CHARGE EFFECTS
The simulation results by the tracking code “mic” [8]excluding SCE were previously reported [6, 7]. Evenwithout SCE, emittance growth was apparent especiallyenhanced in horizontal phase space (Fig. 2). This results
suggest when the smaller initial emittance be assumed,
the smaller final emittance would be obtained.
When initial normalized x,y(rms)=10 mm·mrad, theycome to 12, 15 mm·mrad, respectively, at 223 MeV after
30 times circulation. It should be noted that x(rms)=15mm·mrad at 223 MeV is the value observed on the linac
side. On the contrary, previously reported x(rms)=29 mm·mrad (=0.066 unnormalized) in ref. 5, et al. is the valueobserved on the back straight side (opposite side from thelinac). Because of dispersion free on the linac side, thehorizontal emittance on the linac side is estimated about ahalf of the opposite side’s.
Figure 3: SCEs on emittance growth are evaluated up to 3
nC/bunch by Parmela. The results at 223 MeV after 30times acceleration are shown.
B=x(rms) / C=y(rms) :For the case initial x,y(rms)=0.2 cm·mrad.
D=x(rms) / E=y(rms) :For the case initial x,y(rms)=1.0 cm·mrad.
Judging from both the calculations, Parmela and mic,
with no SCE, the obtained results are about the same,
namely, emittances of the output beam x(rms)=1.8 cm·mrad and y(rms)=1.0 cm·mrad from Parmela, andx(rms)=1.5 cm·mrad, y(rms) =1.2 cm·mrad from mic. It
suggests that the estimation of SCE by Parmela under themodified configuration shown in Fig.1 might be suitable.

OUTPUT BEAM QUALITY
Figure 2: In comparison with Parmela simulations, “mic”
results w/o SCE are shown in three cases, where initialnormalized x,y(rms)= 2, 5, 10 mm·mrad, respectively.
Shown are all on the linac side.
SCEs on emittance growth are evaluated up to 3nC/bunch by the particle tracking code Parmela. The
results at 223 MeV after 30 times acceleration are shownin Fig.3 for two cases, one evaluated for the initialnormalized x,y(rms)=0.2 cm·mrad (B, C) , and the otherfor x,y(rms)=1.0 cm·mrad (D, E). When starting fromx,y(rms) =1.0, it seems no significant influence appearedon the emittance of the output beam until 1 nC/bunch.
Over 1 nC/bunch, however, the effects increase linearly
up to 3 nC/bunch. When starting from a smaller
emittance like x,y(rms)=0.2 cm·mrad, SCEs appear
strongly even below 1 nC/bunch. It suggests that when we
expect 1nC/bunch for the intensity of output beam, thenthe initial x,y(rms)=1.0 cm·mrad might be adequate, andsmaller than this value would not be meaningful.

Output beam qualities acquired by the Parmela
calculations are shown (Fig. 4). Distributions of 5000
particles in three phase spaces are plotted on the typicalcase of 1nC/bunch charge, in summary x(rms)=1.9 cm·mrad, y(rms)=1.0 cm·mrad, and (E, )=(0.07MeV,3.5).
Up to 1nC/bunch, there is no beam loss while 30 times
acceleration. All 5000 particles survived. However, forthe case 3 nC/bunch acceleration, transmission efficiency
decreases down to 90%. Major beam loss occurs in thebending (right) at low energy circulation. SCEs makevertical beam size significantly large in the bending,resulted in the beam loss eventually. In general, whenbeam loss occurs, calculated emittance of the output beam
tends to be small. Therefore, to compare the emittancearound ~100% accelerated beam, the gap of bending isenlarged from 1.0 to 2.0 cm for such the case.
Assuming straight accelerations 30 times to 223 MeV
like a long linac with 1nC/bunch charge, there are noemittance growths for the initial x,y(rms)=1.0 cm·mrad.

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August 9-11, 2014, Aomori, JapanPASJ2014-SUP120

While some increasing found for the initial x,y(rms) =0.2
 0.6 cm·mrad.

Above results suggest us when considering the 223
MeV acceleration by RTM, the expected beam intensity~1nC/bunch seems appropriate. We do not need to worry
about the bad influence of SCE. However, the influenceof SCE might be not negligible when considering theacceleration of the charge 2nC/bunch or more.

REFERENCES[1] H. Ohgaki et al., “Conceptual Design of a NMDS Based on
the Neutron/Gamma-ray Hybrid Approach”, Proc. 2010
IEEE Int. Conf. on Technologies for Homeland Security,Waltham, MA, USA, Nov. 2010, p.525.
[2] H. Ohgaki et al., “Non-Destructive Inspection System forSpecial Nuclear Material Using Inertial Electrostatic
Confinement Fusion Neutrons and Laser Compton
Scattering Gamma-rays”, Proc. 2012 IEEE Int. Conf. on
Technologies for Homeland Security, Waltham, MA, USA,Nov. 2012, p.666.
[3] L. Young et al., “The Particle Tracking Code Parmela”,
Proc. PAC’03, Portland, Oregon, May 2003, p.3523.
[4] T. Hori et al., “Improvement of 150 MeV Racetrack
Microtron”, Proc. PAC’91, San Francisco, May 1991,p.2877.
[5] R. Hajima et al., “Compact Gamma-ray Source for Nondestructive Detection of Nuclear Material in Cargo”, Proc.
IPAC11, San Sebastian, Sept. 2011, p.3663.
[6] T. Hori et al., “Concepts of 220-MeV Racetrack Microtron
for Non-destructive Nuclear Material Detection System”,
Proc. IPAC13, Shanghai, May 2013, p.4127.
[7] T. Hori et al., “Concepts of 220-MeV Racetrack Microtron
for Non-destructive Nuclear Material Detection System”,
Proc 10th PASJ Annual Mtgs, Nagoya, Aug. 2013, p.787[8] T. Hori, “R&D of Injector Microtron for Compact SRRing”, SOKENDAI Doctoral Thesis (in Japanese), March,
2002.

Figure 4: Distributions of 5000 particles at 223 MeV after
30 times acceleration for the case 1 nC/bunch of spacecharge (Parmela calc.).

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