Proceedings of IBIC2012, Tsukuba, Japan

MOPB81RESIDUAL GAS IONIZATION PROFILE MONITORS IN J-PARC SLOWEXTRACTION BEAM LINE
Y. Sato#, K. Agari, E. Hirose, M. Ieiri, Y. Katoh, M. Minakawa, R. Muto, M. Naruki, S. Sawada,
Y. Shirakabe, Y. Suzuki, H. Takahashi, M. Takasaki, K. H. Tanaka, A. Toyoda, H. Watanabe, andY. Yamanoi, KEK/IPNS, Tsukuba, Ibaraki 305-0801, Japan
H. Noumi, RCNP, Osaka, Japan
Residual gas ionization profile monitors (RGIPMs) for
slowly extracted proton beams at Japan Proton
Accelerator Facility Complex (J-PARC) have beendeveloped to monitor transverse profiles of intense proton
beams up to 50 GeV-15 A. To minimize beam loss andresidual dose on the beam line devices, the RGIPMs
working around 1 Pa vacuum pressure have beendeveloped. The present manuscript reports the working
principles, fabrications, installations, and operations of
the RGIPMs in detail.

INTRODUCTION
Hadron Experimental Facility at J-PARC is a multipurpose facility for particle and nuclear physics, using
intense secondary beams [1]. Primary proton beams
accelerated up to 30 GeV in the Main Ring arecontinuously extracted for 2 seconds in every 6 seconds,
and transported to Hadron Experimental Facility throughthe slow-extraction beam line. The maximum beam power
is designated to be 50 GeV-15 A (750 kW in total
power). Since the beam loss on the transport beam line
must be minimized to keep residual activation of beamline devices as low as possible, non-interceptive type ofbeam monitor is desirable.
Residual gas ionization profile monitors (RGIPMs) are
widely used in many accelerator rings [2][3]. Electrons orions produced by proton beams passing through residual
gas, whose pressure is normally 10-6 Pa or less, are guided
to a micro-channel plate (MCP) with electrostatic and /ormagnetic fields applied between gap electrodes. The
signals amplified in a MCP by typically a factor of 104
are observed as transverse profile distributions. The
conventional RGIPM with MCPs is an acceptable choice,but they have some issues as follows;
1. More than ten monitors required in a transportbeam line enhance purchasing cost due to MCPs.
2. MCPs do not have sufficient hardness against
severe radiation around the production target.
The present RGIPMs have been developed to overcome
above issues [4]. Since the vacuum pressure in the slowextraction beam line is about 1 Pa, the signal issufficiently large enough without MCP. The amount ofcharge measured by the RGIPM can be estimated asfollows,___________________________________________Q  NpdE
P 1.602  10 19
 air air
L,
dx
Pstp
NI

(1)

where Q is measured charge in coulomb per unit length,
dE/dx is an energy loss per unit length when 30 GeV
protons are passing through air, Np is a number of protons,
air is density of residual gas, Pair is pressure of residual
gas in a beam pipe, Pstp is a standard temperature pressure,
NI is mean energy to produce an electron-ion pair innormal air, L is length of electrode in beam direction.
When Np is 1013, dE/dx is 2 MeV/(g/cm2), air is 1.2×10-3
g/cm3, Pair is 1 Pa, NI is 38 eV/pair, and L is 8 cm, theequation (1) gives 8 nC, that is sufficient amount for
charge integration circuits without amplification.
Operation of RGIPM in 1 Pa pressure arises a different
issue. The mean free path of the electrons drifting in 1 Pa
pressure (e) is estimated to be about 30 mm, assumingthat typical elastic cross section of electrons on nitrogenand oxygen molecules is 10-15 cm2 [5]. Since typical gap
length between electrodes in the present RGIPMs is 10
cm or more, observed profile distributions would become
considerably broader than the actual size of beam whenelectrons are guided with a uniform electric field between
the electrodes.
To reduce broadening due to diffusion, a uniform
magnetic field can be applied parallel to the electric fieldin the present RGIPMs. Motion of the electrons drifting ina uniform magnetic field is limited in a Larmor radius (re).
According to a standard transportation theory [6] under amagnetic field, the diffusion coefficient of drifting
electrons with an electrostatic field (D) is given by,

2e
D
,
2

(2)

where  is mean collision time, dependent on mean
velocity of electrons. When a magnetic field is applied
parallel to the electric field, the diffusion coefficient ofelectrons across a magnetic field (D⊥)is given by,2rD  e ,2

(3)

When the applied magnetic field is set to be 420 gauss
and the Larmor radius of electron is 0.25 mm, thediffusion coefficient given by the equation (3) is reducedby factor of (e/re)2= (30/0.25)2=14400. A naive
evaluation of broadning due to diffusion of electrons witha kinematic energy Te is estimated as follows,#yoshinori.sato@kek.jp

ISBN 978-3-95450-119-9
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1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPB81

Proceedings of IBIC2012, Tsukuba, Japanσ e  2D  t , t  G

me
,
eVg

(4)

where e denotes broadening width at the electrode, t istravelling time from the center to the electrode in the gap,
G is width of the gap, me is electron mass, and Vg is
voltage applied between the gap. When G is 10 cm and Vg
is 100 V, initial kinetic energy of an electron is 10 eV, e
is 0.27 mm, which is sufficiently good spatial resolution
for profile measurement.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○FABRICATIONS AND INSTALLATION
Figure 1 and 2 are photos of the present RGIPM. The
gap width of inside electrodes is 10 cm. The gap
electrodes are made of ceramic or G10 boards plated withnickel-gold. The segmented pattern (typically 32
channels) is made on the surface plane, and the other areasurrounding the patterned area is connected to signal
ground. The signal cables are mounted on the backplanewith through holes. By omitting MCP, the structure ofelectrodes is quite simplified and robust against radiation
damage.
Two permanent magnets made from strontium-ferrite
material for horizontal and vertical monitors are mounted
outside the chamber. Field strength at the centre of the
gap is 420 gauss in the case of 10 cm gap length. It is
known that some type of permanent material, especiallyneodymium-iron-boron (NdFeB), shows significant
demagnetization by radiation [7][8][9]. Since the present
RGIPM is installed near the production target, the lifetimeof monitor due to demagnetization of strontium-ferrite
must be evaluated. A radiation damage test of permanent
magnet materials of NdFeB, Sr-ferrite, Sm1Co5, andSm2Co17 was carried out up to irradiation of 1017
protons/cm2 with 60 MeV proton beam at Tohoku
University Radioisotope Center [10]. No demagnetization
was observed in Sr-ferrite material chosen for the present
RGIPM up to 110 MGy (1017 protons/cm2). According to
the MARS15 simulation code [11], the accumulated dosenear the production target, on which beams of 30 GeV1014 protons per second at maximum were irradiated for2500 hours per year, it takes 7.4 year to reach 1017
hadrons/cm2. Therefore, the present RGIPM is considered
to have sufficient lifetime under severe radiation
environment.
For the slow-extraction beam line at J-PARC, several
types of RGIPMs with different gap width are fabricated,
as shown in Table 1. In total 14 RGIPMs are installed in
the beam line. Two RGIPMs installed near the beamdump, no magnetic field is applied in the monitor since
the profile width near the beam dump is wide enough.
Vacuum pressure in the beam duct is controlled by MKS
250E mass flow controller. Total amount of charge isadjustable in a desired dynamic range for charge
integration circuit.

Figure 1: A side view of RGIPM.

Figure 2: A front view of RGIPM.
Table 1: Specifications of RGIPMs installed in the slowextraction beam line at J-PARC
Gap
width

Pitch

Mag.
field atcenter(mm)# ofchannels

10

1

64

420

1

10

2

32

420

1

10

3

32

420

6

30

6

32

220

4

40

10

32

0*

2

(cm)

Amount(gauss)* Collecting positive ions

READOUT ELECTRONICS
The signals of RGIPMs are transferred to charge
integration circuits outside shielding enclosure. The 32
channels of charge integration circuit with a dynamic
range of 4nC/5V are fabricated on a standard VME board[12]. Output voltage signals are scanned by analog-todigital converters with 12 bit resolution (ADVANETISBN 978-3-95450-119-92Transverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, Japan

advme2607 [13]). Charge signals from the monitors areintegrated twice in the acceleration and the extraction
period. Subtracting the background distributions
measured in the acceleration period (no beam in the beamline) from the distributions measured in the extraction
period improves signal-to-noise ratio significantly. Gatetiming for the integrators are precisely controlled via JPARC timing distribution system [14]. Digitized data of
the profile distribution of each monitor are processed by a
VME on-board computer (VMIVME7807 [15]) andstored to EPICS [16] waveform records. Users candisplay and print out all the profile monitors with a GUI
program based on WxPython [17]. When the amount ofsignals is increased as the beam intensity increases, amountain plot of the profile distribution during extraction
period would be possible by multiple trigger during asingle extraction.

MOPB81(30 GeV-9 A). The difference of the measured
emittances from the design values may be attributed tosmallness of space-charge effect for the current beam
intensity.
I

Figure 3 shows typical profile distributions measured
by the present RGIPM near the production target. The
horizontal and vertical widths of the profiles are 1.2 mm
and 1.9 mm in RMS, which is consistent with the design
values. Clear profile distributions are obtained bysubtracting background profiles during beam-off period
from those measured in beam-on period. Figure 4 show
summed signals measured by the RGIPMs with 10 cm
and 30 cm gap, changing applied voltage. Flat regions
have been observed in the RGIPMs with both 10 cm and30 cm gap. As applied voltage increases, amount ofcharge increases due to multiple-ionization process inresidual gas. The operating voltage of the RGIPMs withthe 10 cm and 30 cm gaps is set to be 100 V. The amountof charge measured with the RGIPM in the plateau region
is consistent with the one evaluated by the equation (1).
The main difference comes from ambiguity ofmeasurement of vacuum pressure with Pirani gauges.
Currently, the maximum beam intensity extracted to theslow-extraction beam line is 1.7×1013 PPP (14 kW in
power), and all the RGIPMs are working properly. No
demagnetization due to radiation dose is observed.
Figure 5 shows a comparison of the vertical profile
distributions measured by the RGIPM with a 10 cm gap
and aluminium wire scatter installed close to the RGIPM.
The aluminium wire with a 300 m diameter was scannedin the vertical direction and the number of the scatteredparticles was counted by coincident signals of threephotomultipliers. The observed profile distributions wellagree in each other.
Figure 6 shows the measurements of the horizontal andvertical beam emittance with the parabola method. The
RGIPM with a 30 cm gap was used to measure horizontal
and vertical profile widths by changing the focusing
power at the monitor position with a quadrupole magnet
located upstream of the monitor. The measured horizontal
and vertical beam emittances are 2.35 π and 5.60 π mm
mrad, respectively, compared to the designed value of 4.4
π and 10.4 π mm mrad for the maximum beam intensity

Figure 3: Typical horizontal and vertical distributions
measured by the present RGIPM with 64 channel
segmented electrodes of 1 mm pitch. The beam intensity
is 30 GeV-7.1×1012 (6 kW) protons per pulse (PPP) andthe vacuum pressure is 0.3 Pa.

Figure 4: Plateau curve measurements with 10 cm and 30
cm gap RGIPMs.

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 ○COMMISSIONING AND OPERATIONMOPB81

Proceedings of IBIC2012, Tsukuba, Japanare working properly up to 30 GeV-1.7×1013 proton
beams. No significant demagnetization due to radiation
has been observed. The present RGIPMs areindispensable devices for the beam commissioning of theslow-extraction beam line at J-PARC.

ACKNOWLEDGMENT

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 5: Comparison of vertical profile distributions
measured by the RGIPM with a 10 cm gap andaluminium wire scatter. The horizontal axis is relative
position of the scatter and the vertical axis is normalizedcounts.

Figure 6: Horizontal and vertical emittance measurements
by the parabola method. Measured horizontal and verticalemittances are 2.35 π and 5.60 π mm mrad, respectively.

The present work was supported in part by Grants-in
Aid for Scientific Research (A) No.17204019, (C)No.23450357, and for Encouragement of Young
Scientists (B) No.17740715 of the Japan Ministry of
Education, Culture, Sports, Science and Technology.

REFERENCES[1] http://j-parc.jp/[2] T. Kawakubo, et al., Nucl. Instr. Meth. A302, p.397-495(1991).
[3] P. Cameron, et al., Proc. of the 1999 Particle Accelerator
Conference, p.2114-2116 (1999).
[4] Y. Sato et al., “Development of residual gas ionization
profile monitor for high intensity proton beams”, Nuclear
Science Symposium Conference record, 2005, Vol. 2, p.
1043-1046 (2005).
[5] L. G. H. Huxley and R. W. Crompton, The diffusion anddrift of electrons in gases, (New York: Wiley, 1974).
[6] F. F. Chen, Introduction to PLASMA Physics, (Maruzen,
1977).
[7] E. W. Blackmore, IEEE Trans. on Nucl. Sci., Vol. NS-32,
No.5, October 1985, p.3669-3671.
[8] R. F. Holsinger, Proc. of the 1995 Particle Accelerator
Conference, 1995, Vol.2, p.1305-1309.
[9] F. Conincks, et al., “Radiation effects on rare-earth cobalt
permanent magnets”, CERN/SPS/TIS-RP/IR/83-07.
[10] Y. Sato et al., Proc. of the 4th annual meeting of Particle
Accelerator Society of Japan, 2007, p. 294-296.
[11] N.V. Mokhov, "The MARS Code System User's Guide",
Fermilab-FN-628 (1995); N.V. Mokhov, O.E. Krivosheev,"MARS Code Status", Proc. Monte Carlo 2000 Conf., p.
943, Lisbon, October 23-26, 2000; Fermilab-Conf-00/181(2000); N.V. Mokhov, "Status of MARS Code", FermilabConf-03/053 (2003); N.V. Mokhov, K.K. Gudima, C.C.
James et al, "Recent Enhancements to the MARS15 Code",
Fermilab-Conf-04/053(2004);
http://wwwap.fnal.gov/MARS/. MARS was developed inpart with Government funding provided under DOE
Contract DE - AC02 -76CH03000.
[12] http://www.gn-d.com/guest/vme.htm[13] http://www.advanet.co.jp/products/vme2607/index.html[14] T. Katoh, et al., Proc. of 2005 Particle Accelerator
Conference, 2005, p.302-304.
[15] http://www.lhs.co.jp/ge/index.html[16] http://www.aps.anl.gov/epics/[17] http://wxpython.org/

SUMMARY
Residual gas ionization profile monitors working in 1
Pa pressure have been developed and installed in theslow-extraction beam line at J-PARC. Applying amagnetic field parallel to the electric field in the gap
electrodes reduces diffusion of electrons, and the spatial
resolution less than 1 mm is achieved. All the monitors
ISBN 978-3-95450-119-9
4Transverse Profiles, Screens & Wires Monitors