The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

DESIGN STUDY OF A NONDESTRUCTIVE BEAM PROFILE AND HALOSMONITOR BASED ON RESIDUAL GAS IONIZATION FOR THE J-PARC RCS
S.Lee A), T.Toyama A), D. Arakawa A), T.Miura A),
J. Kishiro , N. Hayashi B), S. Hiroki B), R. Toyokawa B), Y. Teruyama B)
B)A)

B)

KEK, Tsukuba, Ibaraki, 305-0801, JapanJAERI, Tokai, Naka, Ibaraki, 319-1195, Japan(e-mail:seishu.lee@kek.jp)

Abstract
A nondestructive beam profile monitor has been
designed to measure density profiles and halos in J-PARC
rapid cycle synchrotoron (RCS). The detector collects andamplifies the signal from the residual vacuum gas
electrons/ions created by the beam passage. It can be
expected to observe fast beam dynamics on timescale of arevolution period. The profile is determined by detecting
the ionized electrons, guided in a strong magnetic field.
The external field assures precision in the determinationof the absolute beam profile and prevents the detection of
background electrons originated outside the monitor’s
active volume. In this paper, a design and construction of
nondestructive residual gas ionization profile monitor aredescribed.

ESTIMATION OF IONIZED PARTICLE
The residual gas electrons/ions are created by thecollision with the beam passing through the vacuum duct.
The number of ionized particles is directly proportional tothe both beam density and vacuum pressure. The energy
loss rate of passing particle of atomic number z in amaterial of atomic number Z, density N is given by− 2m0 c 2  β 2 dEe4 z22
β=−NZln1  1 − β 2 dx 4πβ 2 m0 c 2 ε 02and number of ionized particle n isINTRODUCTION
Nondestructive profile monitor (NDPM) for circulating
ion beams in synchrotron is important for both tuning andexperiments of instabilities. The NDPM is also used toobserve emittance growth and has been developed on
several accelerators [1,2]. In these devices the residual gas
electrons/ions produced by the beam are collected.
Electrons produced in the ionization of atoms from a gas
jet crossing the beam are detected in the LEAR [3]. Themonitor detects ions to obtain a cross section of the beamin the KEK-PS [4]. In storage ring CRYRING, ionizedelectrons are detected with the magnetic field of abending magnet to lower the background and increase
position resolution [5]. However, it is difficult to obtain
the absolute profile of high intensity proton beam due tocollected electrons are diffused by intense space charge
effect. Thus, additional magnetic fields are required toguide ionized electrons. The distortion of beam trajectoryby external magnetic field has to be canceled. In thispaper, we present the design and construction of anondestructive diagnostic system to measure beam
density profile and halos, based on the residual gas
ionization and external magnetic field.

n=1 dEI dx

where β=v/c, Iis the ionization constant and m0is mass ofelectron. Although the composition of the residual gas inbeam pipe is not known precisely, rough estimation of
I~90 eV is enough to calculate the expected signals. The
calculation result of ionized particles is shown in Fig. 1.
Constant signal level is expected in energy range of 400
MeV~3 GeV. The RCS is a rapid cycle protonsynchrotron. Its parameters are shown in Table 1.
Table 1: Assumed RCS parameters used in calculationof expected signals.

Circumference

348 mParticle type

ProtonInjection energy

400 MeV/181 MeVExtraction energy

3 GeV

Number of particles

8.3 ×1013

Acceletation cycle

40 msec

Beta function

7 m (at NDPM)

Vacuum pressure

1×10-6 Torr

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

halos
2.5 10 4
2 104

~200

Beam core

1.5 10 4

MCP 32ch

80

Number of ion pairs (/cm/revolution)

3 104

1 104

MCP 8ch5000

30005001000

1500

2000

2500

3000

Beam Energy [MeV]

Figure 1: Calculated result of number of ionized particles
in a revolution period.

Figure 3: Schematic of MCP detectors. Center detector of
32 channel observes the core density profile. Others ofeach 8 channel are prepared to investigate the beam halos.
Difficulty of NDPM in the J-PARC RCS is originatedin the strong space charge effect by intense proton beam.

EXPERIMENTAL APPARATUS
Schematic layout of NDPM system is shown in Fig. 2.
The system consists of electrodes for electrostatic field, awiggler type magnet to obtain the absolute beam profileand three multichannel plate (MCP) assembly of
30mm×80mm active area. A central MCP detector isequipped to measure beam core region, and others areused to investigate the behaviour of beam halos (Fig. 3).
The bias voltage for each detectors and gain of
preamplifier can be controlled separately.

160

1090
1000

290
257

730

Ø257150

Ø290

330

200

690
600

400200

Beam+1%1200504

By [G]

502500498

Figure 2: Schematic layout of the beam profile monitor. Theproton beam ionises the residual gas atoms. The products are
driven electrostatically towards the MCP detectors withgyromotion around the external magnetic field lines.

496-1%
024681012r [cm]Figure 4 (a): Calculated field distribution in X-Y plane.
The applied electrostatic field has to exceeds the field
induced by space charge (~1kV/cm). In order to collectionized particles, power supply of 40 kV is prepared for
RCS large vacuum pipe (~300 mm). The high voltage isalso preferred to examine the ion collection because thetime resolution is limited by average drift time to MCPplate.

External magnetic field is designed to suppress thediffusion of ionized particles. A two-dimensional
simulation of model magnet is performed. Figure 4 (a)shows magnetic field lines in X-Y plane. Sufficienthomogeneity of magnetic field less than ±0.2% for active
area is obtained.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Coil winding forms of both only center pole and contain
three poles are examined. It is clear that the stray field of
three-coil system is smaller than the stray field by single
coil. In this estimation, it was also found that magnetic
force of about 6000 A-turn is required for each coil toobtain the magnetic field of 500 G at beam position.

SUMMARY
A nondestructive beam profile monitor was designed tomeasure high intensity proton beam in the J-PARC RCS.
The NDPM consists of three MCP detectors, 40 kVelectrostatic field for electrons/ions collection and 500 G
wiggler type magnetic field to determine the absoluteprofiles. The NDPM system contributes to survey theinstabilities or other intense beam physics, andinvestigates the behaviour of beam halos. However, it isindispensable to evaluate the performance of NDPM
system practically. The ionized particle motion should be
confirmed in the area of intense electrostatic and magnetic
fields are overlapped. Experimental studies using a
prototype in the KEK-PS or other accelerators are
essential to develop the NDPM for the J-PARC RCS.

600400By [G]

20001coil-200-400

3coil
-600
020406080100

Z [cm]

Figure 4 (b): Calculated field distribution along the beamline.
Figure 4 (b) also shows the side view of a designed modelmagnet. Divided three poles in longitudinal direction areneed to cancel the distortion of beam trajectory.

REFERENCES[1] W. H. DeLuca, IEEE Trans. Ncl. Sci. Ns-16(1969).
[2] H. Weisberg et al., IEEE Trans. Ncl. Sci.
Ns-30(1983)2179.
[3] B. Vosicki and K. Zankel, IEEE 91 Part. Accel. Conf.,
eds. L. Lizama and J. Chew, vol. 5 (IEEE, New York,1991)2814.
[4] T. Kawakubo, T. Ishida, E. kadokura, Y. Ajima, andT. Adachi, Nucl. Inst. and Meth. A302(1991)397.
[5] T. Quinteros, D. R. Dewitt, A. Paal, R. Schuch, Nucl.
Inst. and Meth. A378(1996)35.