Proceedings of IBIC2012, Tsukuba, JapanTUPB78FLYING WIRE BEAM PROFILE MONITORS AT THE J-PARC MRAbstract
Transverse beam profiles have been measured using
flying wire monitors at the main ring of the Japan ProtonAccelerator Research Complex. We use carbon fibers of 7
µm in diameter and the scan speed of 10 m/s. The wire isattached with an aluminum flame of 140 mm of therotation radius and rotated with a DC servomotor. A
potentiometer is attached to the wire flame and the angle
readout is used for the feedback of the servomotor and thewire position measurement. The secondary particles fromthe beam-wire scattering are measured with a scintillation
counter. Beam profiles are reconstructed by making thescatter plot of the scintillator signal and wire position. We
have successfully measured both horizontal and verticalbeam profiles of up to 1.2×1013 protons per bunch. The
monitors have been proven to be useful for the beamcommissioning.

INTRODUCTION
The main ring (MR) of the Japan Proton Accelerator
Research Complex (J-PARC) is a high intensity proton
accelerator and delivers the protons for the nuclear andelementary particle physics, such as the neutrinooscillation experiment (T2K). The injection energy is 3
GeV and the extraction energy is 30 GeV. For the user run
we have achieved the beam power of 200 kW with 8bunches of 1.3 1013 protons per bunch (ppb). Twobunches of protons are injected in MR from the rapid
cycling synchrotron (RCS) through the beam transportline (3-50BT). Two bunches are injected four times in one
cycle to fill 8 out of 9 rf buckets in 120 ms. The
acceleration starts after the fourth injection.
The beam loss minimization and localization at thecollimator are necessary to minimize residual radiation
activities in MR and required for the maintenance of theaccelerator components. The typical beam loss is 1%during the injection period and 0.7% at the beginning of
the acceleration. It is important to understand the beamloss mechanism for further beam power increase.
The transverse beam profile is one of important beam
parameters to measure for understanding of the beam loss
mechanism. We have installed ion profile monitors (IPM)[1] and flying wire in MR. IPM uses phenomena that theproton beam ionizes the residual gas. Multi-ribbon profile
monitors [2] have been installed at five places in 3-50BT
and one in MR. They are being used to measure theinjection beam profiles.

FLYING WIRE MONITOR
A flying wire monitor uses a carbon fiber of 7 µm indiameter as a target. It is to be scanned across the beam____________________________________________#susumu.igarashi@kek.jp

with the maximum speed of 10 m/s. The wire is attachedwith an aluminum flame of 140 mm of the rotation radiusand rotated with a DC servomotor. A potentiometer isattached to the wire flame and the angle readout is used
for the feedback of the servomotor and the wire position
measurement. The secondary particles from the beamwire scattering are detected with a scintillator (Fig.1). The
beam profile is then reconstructed from the scintillator
response and the wire position measurement. If the wire
scanning disturbs the beam, the profile measurement is
not precise. The minimum target material and fast wire
scanning are necessary. Flying wire monitors have beenused in several proton accelerators. It was also developedin KEK-PS [3] and modified for J-PARC MR [4].
The wire flame is rotated from -150°, which is thestandby position, to +150°	 and comes back to the standby
position. The movement is done in 0.2 s. The wire
scanning takes about 5 ms for a typical size of the beam.
The scintillator signals and the potentiometer output
signals for the wire position are digitized with a digitizer
RTD720A with a 20 ns sampling for a 20 ms time range.
The 1 M sample data are averaged over 10 k sample to
make arrays of 100. The data of the potentiometer signal
is calculated to make wire position data. Scattered plots
are made for the scintillator data as a function of the wire
position to make beam profiles.

Figure 1: The flying wire monitor.
Both horizontal and vertical flying wire monitors wereinstalled in the straight section of the beam injection andcollimation systems in 2008 (Fig.2). Because they wereoriginally at the downstream of the collimator, the beam
background affected the profile measurement. They weremoved to the upstream of the beam injection andcollimation systems in 2010 to avoid the beam
background. The background was then suppressed and theprofile measurement of the high intensity beam became

ISBN 978-3-95450-119-9
Transverse Profiles, Screens & Wires Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○S. Igarashi#, D. Arakawa, Y. Hashimoto, M. Tejima, T. Toyama, KEK, Ibaraki, JapanK. Hanamura, MELCO SC, Ibaraki, JapanTUPB78

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○possible. The betatron amplitude function βx is 15.4 m atthe position of the horizontal flying wire and βy is 18.5 m
at the position of the vertical flying wire. The dispersion
functions ηx and ηy are zero for both monitors.
After the earthquake in 2011, a driver circuit for the DC
servomotor turned out to be damaged. The circuit was
replaced for repair. Otherwise the operation of themonitor has been stable. Wire breakage has neveroccurred in four years since the first installation.

Figure 2: Installation of the horizontal flying wire
monitor.

BEAM BACKGROUND
Beam profiles were measured during the injection
period of 120 ms for the intensity of 0.95 1013 ppb. Twobunches were injected at the injection timing, k1. The
measurements were performed with the trigger timing ofk1, k1+20 ms and k1+120 ms. The data were acquired in
the different shots but in the same condition. Figure 3
shows the beam profiles at the injection timing k1 inblack circles. It is well fitted with the Gaussian function
as shown in the solid line. Profiles at 20 ms after theinjection and at 120 ms after the injection are shown in
red circles and blue circles respectively. The beam
background was observed at the level of about 2 % withrespect to the peak of the profile. If the profile is in theGaussian function, the profile signal of 2.8 σ should be at
the background level. A part of 99.5 % is then observableand the rest of 0.5 % is not observable by the background.
If the beam profile is not in the Gaussian function but has
more tail distribution, the ratio of the unobservable part
increases.
For the high intensity beam operation we would like tomaintain small beam losses, typically about 1 % or less.
The beam halo of much less than 1 % level should thenbe measured in order to understand the beam loss
mechanism. More detail studies of the beam halo and loss
mechanism would be possible if we could improve thesignal to noise ratio. Ideas to improve the signal to noise
ratio such as a use of scintillator telescope, an

optimization of wire thickness and wire movement should
be explored.

Figure 3: Horizontal beam profiles of the two-bunch
beam for the intensity of 0.95 1013 ppb. The beamprofile at the injection timing k1 is shown in black circles
and the fitted Gaussian function is shown in the solid line.
Profiles at 20 ms after the injection and at 120 ms after
the injection are shown in red circles and blue circles
respectively.

PROFILE CHANGE DURING THE
INJECTION PERIOD
Both horizontal and vertical profiles have beenacquired for 1.19 1013 ppb. Trigger timing was set to k1,k1+20 ms and k1+120 ms to observe the profiles during
the injection period.
Figure 4 shows the horizontal profiles during theinjection period. The profile at the k1 timing wasobserved to be in a Gaussian shape. The 2σ emittancewas 20.1π mmmrad. A small change of the profile has
been observed at the k1+20 ms timing from the Gaussian
shape. The profile change was clear at the k1+120 mstiming. The profile was in a parabola shape. The 100%emittance was 36π mmmrad.
The vertical profiles in the same condition have been
observed to have the same change as shown in figure 5.
They were in a Gaussian shape at the k1 timing and in aparabola shape at the k1+120 ms. The 2σ emittance was
24.7π mmmrad and at the k1 timing. The 100% emittancewas 26π mmmrad at the k1+120 ms.
To understand the mechanism of the profile change, wehave performed multi-particle tracking simulations withthe space charge effect. The study revealed that the spacecharge tune shift and the linear and 3rd order coupling
resonances are the causes of the profile change [5].

ISBN 978-3-95450-119-92Transverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, JapanTUPB78BEAM PROFILES IMMEDIATELY AFTER
THE INJECTION

Figure 4: Horizontal beam profiles at the trigger of k1 (a),k1+20 ms (b), k1+120ms (c).

Figure 6: Horizontal beam profiles at the injection for thebeam intensity of 0.6 1013 ppb.

SUMMARY
Flying wire monitors have been used for the horizontal
and vertical beam profiles in the J-PARC MR. The
operation has been stable and the profile data have beenacquired up to the beam intensity of 1.2×1013 ppb. Wire
breakage has never occurred in four years since theinstallation. We have successfully reconstructed beamprofiles such as the profile immediately after the injection
and during the injection period. The monitors have been
proven to be useful for the beam commissioning.
Figure 5: Vertical beam profiles at the trigger of k1 (a),k1+20 ms (b), k1+120ms (c).

ISBN 978-3-95450-119-9
Transverse Profiles, Screens & Wires Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The profile data have been acquired with changing thetrigger by 1 ms step with respect to k1 (the injection
timing of the first bunch), because we would like toobserve the profiles immediately after the injection.
Figure 6 shows the horizontal beam profiles for the beam
intensity of 0.6 1013 ppb. In order to reconstruct thebeam profile immediately after the injection, we shouldtake the profile data with smaller trigger steps such as 0.2
ms and rearrange the data as a function of the timing withrespect to the injection timing. The analysis has not been
done for this sequence of profiles. We have, however,observed a change at the top of the profile in 10 ms afterthe injection. The profile peak was observed to be sharper
immediately after the injection.

TUPB78

Proceedings of IBIC2012, Tsukuba, JapanREFERENCES

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○[1] K. Satou et al., “IPM Systems for J-PARC RCS andMR”, Proceedings of HB2010, Morschach,Switzerland, Sep. 27 ~ Oct. 1 2010.
[2] Y. Hashimoto et al., “Multi-Ribbon Profile Monitor
Using Carbon Graphite Foil for J-PARC”,
Proceedings of HB2010, Morschach, Switzerland,
Sep. 27 ~ Oct. 1 2010.

[3] S. Igarashi et al., “Flying Wire Beam Profile
Monitors at the KEK PS Main Ring”, Nucl. Instrum.
Meth. A482;32-41, 2002.
[4] S. Igarashi et al., “Flying Wire Beam Profile Monitor
at the J-PARC MR”, Proceedings of the 5th Annual
Meeting of Particle Accelerator Society of Japan,
Higashihiroshima, Japan, Aug. 6~8 2008.
[5] S. Igarashi et al. “Beam Profile Measurement using
Flying Wire Monitors at the J-PARC Main Ring”,
Proceedings of IPAC2011, San Sebastian, Spain,
Sep. 4~9 2010.

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4Transverse Profiles, Screens & Wires Monitors