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DEVELOPMENT OF PEPPER-POT EMITTANCE MONITOR FOR
HIGH-INTENSITY ION BEAM ACCELERATED BY
RIKEN AVF CYCLOTRON
Y. Kotaka∗ , K. Kamakura, H.Yamaguchi, N. Imai, Y. Sakemi
Center for Nuclear Study, University of Tokyo, Wako, Japan
J. Ohnishi, Riken Nishina Center, Wako, Japan†K. Hatanaka , Research Center for Nuclear Physics, Osaka University, Ibaraki, Japan
Abstract
At the Center for Nuclear Study of the University of Tokyo,
the measurement of Electric Dipole Moment of Francium(Fr) is underway with the world highest precision. Fr isgenerated by nuclear fusion reaction by irradiating gold withoxygen ion beam accelerated by RIKEN AVF Cyclotron.
The required beam intensity is 18 eµA or more. However,
the average beam transport efficiency drops to be around
66% as the beam intensity exceeds 10 eµA. To solve theproblem, a pepper-pot emittance monitor (PEM) for highintensity beams has been developed. Referencing the PEM
used for the injection beams of AVF Cyclotron, we havedeveloped three additional items. The first is reducing theradiation damage to a camera, which is placed away fromthe beamline. The distance between the camera and PEM is
2.2 m, and the average image position accuracy of 0.15 mm
is achieved. The second is the angular accuracy suitable
for the accelerated beam. The required angular accuracy isestimated to be less than 0.3 mrad. A beam test for the firstand second items is planned. The third is a beam shutter
system to prevent PEM from heating due to beam. The
measurement time by the system reaches 0.27 seconds now.

Figure 1: AVF Cyclotron and beamlines to the experimentdevices. EM_C01 is shown in the right.
sured by 2D emittance monitor (EM_C01) shown in Fig. 1
tend to expand as beam intensities increase. We think beamtransport efficiencies decrease as beam emittances expand as
a function of beam intensities. However, we cannot measurea high intensity beam emittance because the cooling abilityof EM_C01 is suitable for less than 130 W beam power [3].

INTRODUCTION
At the Center for Nuclear Study of the University of Tokyo(CNS), the measurement of Electric Dipole Moment (EDM)
of Francium (Fr) is underway with the world highest precision [1]. Fr is generated by nuclear fusion reaction byirradiating the gold target of EDM experiment device (EDM
target) with 18 O6+ ion beam of 7 MeV/u accelerated by
RIKEN AVF Cyclotron. Our goal is transporting 18 O6+ ion
beam of more than 18 eµA (400 W), without loss and converging beam to be 1 mm of standard deviation (SD) of thebeam distribution on the EDM target shown in Fig. 1.
CNS performs experiments in the three courses of C12
for EDM, CRIB [2], and E7B for students of the Universityof Tokyo in the E7 experiment room of RIKEN Nishina
Center shown in Fig. 1. Figure 2 shows the beam transport
efficiencies from AVF Cyclotron to C12, CRIB, and E7B
as a function of beam intensities. As beam intensities arelarger than 10 eµA, the average beam transport efficiency is
0.66. The stronger beam intensity tends to lower the beamtransport efficiency. Figure 3 shows beam emittances mea∗ kotaka@cns.s.u-tokyo.ac.jp† Deceased.

Figure 2: The relationship between beam intensity and transport efficiency to C12(red), E7B(green), and CRIB(blue)

Figure 3: The relationship between beam intensity and 95%
horizontal (blue) and vertical (red) beam emittance.
To solve the problem, a pepper-pot emittance monitor(PEM) [4, 5] for high-intensity beams has been developed.
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Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 0 6

Referencing the PEM for the injection beams of AVF Cyclotron [6], we have developed three additional items which
are reducing the radiation damage to a camera, improving
angular accuracy suitable for the accelerated beam, and abeam shutter system to avoid heating PEM due to beam.

REDUCING RADIATION DAMAGE
Figure 4 shows our developed PEM. The mask is 1 mm
thick copper plate having holes set at the same interval according to a grid system. The beam image passing througheach hole and glowing on the fluorescent plate, which istilted 45 degrees against beam axis, is taken with digital
camera through the window. We get the relationship between the beam intensity passing through each hole and thebeam position on the fluorescent plate from the image.

Figure 5: The images taken by camera with tele lens from
2.2 m away. The left view is without intermediate lens andthe right view is with intermediate lens.

Figure 6: The relationship between the displacement oftransformed positions with respect to designed positions.

ESTIMATING REQUIRED ACCURACY

Figure 4: The image of the developed PEM.
The radiation damage of camera due to beam is unavoidable. To reduce the radiation damage, we think setting acamera away from the beam line and surrounding it withshielding material. Moreover, we select an inexpensive camera outputting NTSC signals (720×480 pixels) in view of
frequent replacing a damaged camera. This time, we explain
the camera optic system. The diameter of the window to see
the fluorescent plate is 48 mm to install PEM on the flange of
ICF152. The effective area (50×50 mm2 ) of the fluorescentplate must be viewed. With only tele lens, we cannot viewthe entire area of the fluorescent plate as shown on the left
view in Fig. 5. By inserting an intermediate lens between
the tele lens and the fluorescent plate, we can view the entire
area [3]. We use an achromatic lens with a focal length of
200 mm as an intermediate lens to suppress image distortion.
Under these conditions, the image of the fluorescent plate isshown on the right view in Fig. 5. The distance between thecamera and PEM can be extended to 2.2 m.
We estimate the position accuracy of this image. We
marked the lines to make grid points on the fluorescentplate. After the projective transform coefficients between
the bitmap coordinates of the grid points and the designed
plane coordinates of correspondent grid points are determined, all the bitmap points of the image are transformed todesigned plane coordinates. The relationships between thedisplacement of the transformed grid positions with respect
to the correspondent designed positions on plane coordinates and the designed positions on plane coordinates areshown in Fig. 6 . The SDs of the displacements was 0.16
and 0.14 mm in the horizontal (left view) and vertical (rightview) direction, respectively.

The required measurement accuracies for PEM will be
estimaed to calculate beam trajectories by applying the measured beam emittances as initial values because they dependon the beam emittances of AVF Cyclotron.
EM_C01 is located 3550 mm from AVF Cyclotron shownin Fig. 7. The measuring way of EM_C01 is moving the slit
with the width of 0.1 mm set at intervals of 1/√2 mm andmeasuring the beam position distribution passing throughthe slit by the beam profile monitor (BPM) set 320 mm
behind the slit. The BPM measures the beam position distribution in one dimension by sweeping wire with the diameterof 0.1 mm perpendicular to the beam direction. The slit andBPM of EM_C01 are named SL_EM and PF_EM, respectively. The beam angle is calculated by using the displacement of the beam positions measured by BPM with respect
to the position of slit. However, we cannot measure true
beam emittances because of unsolved measurement errors.
Therefore, we estimate the true emittance in another way.

Figure 7: The schematic view of beam transport line to C03.
We can solve the 𝜎 matrix with three measurements of
BPMs theoretically. We have five BPMs which are PF_C01a,
PF_C01b, PF_EM, PF_C02, and PF_C03 shown in Fig. 7.
However, we must note the beam distribution of the AVF
Cyclotron beam is not a normal distribution and is left-rightasymmetric. As the typical SD of the beam distributionof PF_C03 is around 1 mm, the bounds of solutions of theequation can be limited. Therefore, we select the measured
beam distributions of PF_C03, PF_C02, and PF_EM irradiated with 19 F7+ beam of energy 6.68 MeV/u and intensity
5 eµA shown in Fig. 8. Horizontal and vertical directions

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03 Transverse Profile and Emittance Monitors

12th Int. Beam Instrum. Conf.
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perpendicular to beam direction are denoted as x and y (mm),respectively. The angles of x and y are denoted as x’ andy’ (mrad), respectively. The SD of x, x’, y, and y’ distributions are denoted as SDx, SDx’, SDy, and SDy’, respectively. The (SDx, SDy) of PF_C03, PF_C02, and PF_EMare (1.6, 1.3), (5.6, 4.8), and (7.1, 2.8), respectively. The
equations of 𝜎 matrix are shown in Eq. (1) and Eq. (2).

Figure 8: The three measured beam distributions. The top
is PF_EM. The middle is PF_C02. The bottom is PF_C03.

𝜎11 (𝐶02)
𝑚11
⎛
⎞
⎛
⎜
⎟
⎜
⎜
⎟
⎜
𝜎
(𝐶02)
⎜ 21
⎟ = ⎜𝑚21
𝜎
(𝐶02)
⎝ 22
⎠ ⎝𝑚31

𝑚12
𝑚22
𝑚32

𝑚13 𝜎11 (𝐸𝑀)⎞⎛⎜𝜎 (𝐸𝑀)⎞
⎟
⎟
⎟
𝑚23 ⎟
⎟⎜
⎜ 21
⎟
𝑚33 ⎠ ⎝𝜎22 (𝐸𝑀)⎠(1)

𝜎11 (𝐶03)
1
⎛
⎞
⎛
⎜
⎟
⎜
⎜
⎟
⎜
𝜎
(𝐶03)
=
⎜ 21
⎟ ⎜0
𝜎
(𝐶03)
⎝ 22
⎠ ⎝0

2𝐿
1
0

𝐿 2 𝜎11 (𝐶02)⎞⎛⎜𝜎 (𝐶02)⎞
⎟
⎟
⎟
𝐿⎟
⎟⎜
⎜ 21
⎟
1 ⎠ ⎝𝜎22 (𝐶02)⎠(2)

Where 𝜎11 , 𝜎22 , and 𝜎21 are SDx2 , SDx’2 , and the covariance of x and x’. The m𝑖𝑗 in the Eq. (1) is matrix elementfrom PF_EM to PF_C02 including doublet quadrupole magnet. Equation (2) is the matrix for drift length of L from
PF_C02 to PF_C03. The unknown six values which are
𝜎21 and 𝜎22 of three BPMs can be solved as the number of
equation is six. For the 𝜎 matrix of y, Eq. (1) and Eq. (2)can be solved by changing the subscript 2 to 4 and 1 to 3.
As 𝜎22 of PF_C02 and PF_C03 obtained by solving
Eq. (1) and Eq. (2) were negative, they are not solutions.
Presuming the SDx of PF_EM to be 8.0 mm changing from
7.1 mm, the solutions shown in Table 1 are obtained. Equation (3) is the 𝜎 matrix at the EM_C01 calculated with Table 1. We consider the effect of replacing 7.1 by 8.0 mm
of SDx of PF_EM is small because the beam distributionis left-right asymmetric, and the full width is about 40 mm.
Besides, a coupling between x and y is not considered.
Table 1: Solved 𝜎 Matrix from Three BPMs Presuming SDx
of PF_EM to Be 8.0 mm
𝜎11
PF_EM 64.0
PF_C02 31.36
PF_C03 2.56

𝜎21
67.46
-8.45
-0.33

𝜎22 𝜎33
71.20 7.84
2.48 23.04
2.48 1.69

𝜎43
1.47
-5.87
-0.65

𝜎44
0.56
1.59
1.59

𝜎11
⎛
⎜
𝜎21
⎜
⎜
⎜
⎜
⎜𝜎31
⎝𝜎41

𝜎12
𝜎22
𝜎32
𝜎42

𝜎13
𝜎23
𝜎33
𝜎43

𝜎14
28.1 44.6
⎞ ⎛
⎜
𝜎24 ⎟
44.6 72.0
⎟
⎜
⎟=⎜
⎜
⎟
⎜
𝜎34 ⎟
0
⎟ ⎜ 0
𝜎44 ⎠ ⎝ 0
0

0
0
7.9
1.3

0
⎞
0⎟
⎟
⎟
⎟
1.3⎟
⎟
0.6⎠
(3)
Using Eq. (3) presumed to be true, we estimate the error in the beam distribution at the EDM target due to the
EM_C01 measurement error. Our purpose is to make a beamtransport system having the beam distribution with SD of
1 mm on the EDM target. Defining the beam direction asz axis (mm), Fig. 9 is the envelope from EM_C01 (z = 0)
to EDM target (z = 8120) calculated by TRANSPRT [7]applying Eq. (3) as initial values. The views of lower andupper half are envelopes of x and y, respectively. The red box
indicates the quadrupole magnet, and the blue box indicatesthe bending magnet. Though the last quadrupole magnet iscurrently doublet, a triplet quadrupole magnet is virtuallyused to converge beam at the EDM target. The momentum
dispersion (dp/p) is 0.006 as an average value. The dp/p isvariable by the beam condition of AVF Cyclotron.

Figure 9: The beam envelopes applying Eq. (3) as initialvalues by TRANSPORT for 𝜎11 and 𝜎33 to be around 1 mm
at the EDM Target.
Next, we will explain the process flow to see the beam sizeat EDM target as a function of angular error as following. We
generated 500000 particles as random numbers following 4variables normal distribution having the element of Eq. (3).
The 4-variables correspond to (x, x’, y, y’) of a particle. We
select the particles passing slits at z = 0 with a width of
0.1 mm set at intervals of 1/√2 mm in the (x, y) plane andtransport them to z = 320 behind the slits. At z = 320, we
generate the random numbers following normal distribution
having SD as a position error, and add the random numbers
as an error to the (x, y) of the transported particles. The
angles of particles are recomputed by using the displacementof the (x, y) of transported particles given error at z = 320
with respect to the (x, y) of the setting position of slits at
z = 0. For every particle at z = 0, we replace the original(x, y) by the designed (x, y) of the setting position of the slits
and replace the original (x’, y’) by the recomputed (x’, y’).
Using the replaced (x, x’, y, y’) of particles, we calculate
𝜎 matrix including angular error. Inputting the 𝜎 matrix
to TRANSPORT, we calculate SDx and SDy at the EDM
target keeping magnet parameters. Figure 10 shows therelationship between the SDx and SDy at the EDM target
and the angular error. We found angular accuracy of PEM
needs to be less than 0.3 mrad.
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ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 0 6

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

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ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

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ISSN: 2 6 7 3 - 5 3 5 0
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By the above-mentioned error simulation method, we
have possibility to divide error components from the beamemittance presumed to be true. Therefore, we plan to identify the measurement errors of PEM by beam test. As we
require low beam emittance, no correlation between x andy, and three BPMs in a beam line, we decided to test in
Micro Analysis Laboratory, Tandem accelerator (MALT),The University of Tokyo [8]. The PEM installation with telelens camera optics was complete. This beamline has two
BPMs at 735 mm upstream and 2070 mm downstream fromthe PEM. Moreover, as the PEM becomes BPM with theaverage position error of 0.15 mm, we have three BPMs.

Figure 10: The relationship between angular errors (mrad)of an emittance monitor and the calculated SD of beam
position distribution at the EDM Target.

BEAM SHUTTER SYSTEM
The beam shutter system is developed to shorten the measurement time to avoid heating PEM due to beam irradiationof 400 W or more. The beam stops by applying a highvoltage to the electrode set in the beam line just behind an
ion source and bending the beam sideways. High voltage
power supply (Hamamatsu Photonics C9619) with a voltageof -2 kV and a rise time of 0.15 seconds is used. Figure 11
shows the schematic diagram of beam shutter system.

Figure 12: The measured chopping time by oscilloscope.

CONCLUSION
Improving camera optical system to cover the entire areaof the fluorescent plate extends the distance between thecamera and the PEM up to 2.2 m while the average position
error of the fluorescent plate is 0.15 mm. The angular accuracy of the PEM required to calculate the beam trajectory
to the EDM target is 0.3 mrad. To investigate the measurement errors, we installed PEM in MALT. We achieved themeasuring time of 0.27 seconds by a beam shutter system toavoid heating PEM due to beam irradiation.

ACKNOWLEDGEMENTS
We would like to thank S. Shimoura, M. Kase, and A. Gotofor great support.
This work was supported by JSPS KAKENHI Grant Number JP20K12481.

REFERENCES[1] Y. Sakemi et al., “Fundamental physics with cold radioactive
atoms”, AIP Conf. Proc., vol. 2319, no. 1, pp. 080020, 2021.
doi:10.1063/5.0037134[2] Y. Yanagisawa et al., “Low-energy radioisotope beam separator
CRIB”, Nucl. Instrum. Methods Phys. Res., Sect. A, vol. 539,
no. 1, pp. 74–83, 2005.
doi:10.1016/j.nima.2004.09.041[3] Y. Kotaka et al., “Current status of developing 4D emittancemonitor for ion beams accelerated by RIKEN AVF Cyclotron”,
in Proc. 18th Annual Meeting of PASJ, pp. 873-877, 2021.
[4] T. Hoffmann et al., “Emittance measurements of high current
heavy ion beams using a single shot pepperpot system”, AIP
Conf. Proc., vol. 546, pp. 432–439, 2000.
doi:10.1063/1.1342615

Figure 11: The schematic diagram of beam shutter system.
The measurement flow consists of (1) beam stop (HV ON),
(2) PEM insertion, (3) beam irradiation (HV OFF), (4) beam
image capture (5) beam stop (HV ON), and (6) PEM evacuation. Time reduction is required between (3) and (5). We
measure the time change of this flow by seeing the output ofhigh voltage power supply through an oscilloscope. Though
the output high voltage is -2 kV, it is dropped to -2 V by a highvoltage probe (FLUKE 80K-40) to input it to oscilloscope.
Figure 12 shows the oscilloscope view while measuring andwe achieved 0.27 seconds from (3) to (5).

[5] L. E. Collins and P. T. Stroud, “Extraction of high current ion
beams with low divergence”, Nucl. Instrum. Methods, vol. 26,
pp. 157-166, 1964. doi:10.1016/0029-554X(64)90067-9[6] Y. Kotaka et al., “Development of the Calculation Methodof Injection Beam Trajectory of RIKEN AVF Cyclotron with
4D Emittance Measured by the Developed Pepper-Pot Emittance Monitor”, in Proc. IBIC’19, Malmö, Sweden, Sep. 2019,
pp. 351–354. doi:10.18429/JACoW-IBIC2019-TUPP022[7] D. C. Carey, K. L. Brown and F. Rothacker, “ThirdOrder TRANSPORT with MAD Input A Computer Programfor Designing Charged Particle Beam Transport Systems”,
FERMILAB-Pub-98/310, 1998.
[8] Micro Analysis Laboratory, Tandem accelerator website, The
University of Tokyo, https://malt.um.u-tokyo.ac.jp/.

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