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 B), N. Hayashi B), S. Hiroki B), R. Toyokawa B), Y. Teruyama B)
A) KEK, Tsukuba, Ibaraki, 305-0801, JapanB) JAERI, Tokai, Naka, Ibaraki, 319-1195, Japan(e-mail:seishu.lee@kek.jp)

Abstract

ESTIM ATION OF IONIZED PARTICLE

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 cfynamics on timescale of arevolution period The profile is determined by detecting theionized electrons, guided in a strong magnetic field The
external field assures precision in the (^termination of theabsolute beam profile and prevents the detection of
background electrons originated outside the monitor’s active
volume. In this paper, a design and construction ofnondestructive residual gas ionization profile monitor are

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 a material ofatomic number Z, density N is given bydE

e4z 2dx

4 K p 2m 0c 2e l

NZ\ In2m0c 2ず
1一がand number of ionized particle n isn :IdEINT R O D U C T IO NI dx

Nondestructive
profilemonitor(NDPM)
forcirculating ion beams in synchrotron is important for bothtuning and experiments of instabilities. The NDPM is also
used to observe 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]. The monitor detects
ions to obtain a cross section of the beam in the KEK-PS[4]. In storage ring CRYRING, ionized electrons are detectedwith the magnetic field of a bending 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 to collected electrons are diffused byintense space charge effect. Thus, additional magnetic fields
are required to guide ionized electrons. The distortion ofbeam trajectory by external magnetic field has to be canceled
In this paper, we present the design and construction of anondestructive diagnostic system to measure beam density
profile and halos, based on the residual gas ionization andexternal magnetic field.

where p=v/c, I is the ionization constant and m 0 is mass ofelectron. Although the composition of the residual gas inbeam pipe is not known precisely, rough estimation of /~90
eV is enough to calculate the expected signals. The
calculation result of ionized particles is shown in F i g . 1.
Constant signal level is expected in energy range of 400
MeV~3 GeV. The RCS is a rapid cycle proton synchrotron.
Its parameters are shown in Table 1.
Table 1 : Assumed RCS parameters used in calculation ofCircumference

348 mParticle type

ProtonInjection energy

400 MeV/181 MeVExtraction energy

3 GeV

Number of particles

8 .3 x l0 13

Acceletation cycle

40 msec

Beta function

7 m (at NDPM)

Vacuum pressurelxlO '6 Torr

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

f3cl^

52.

2^L

54ol
112(uopntoAal/UIo/} ^al^,
5<JQqmtl^) 5 0 01000

1500

200025(3000Beam Energy [MeV]Calculated result of number of ionized particles in aFigure 1: 丨revolution period.

Figure 3: Schematic of MCP detectors. Center detector of 32
channel observes the core density profile. Others of each 8
channel are prepared to investigate the beam halos.

EXPER IM ENTA L APPARATUS

Difficulty of NDPM in the J-PARC RCS is originatedin the strong space charge effect by intense proton beam.

Schematic layout o f NDPM system is shown in Fig. 2.
The system consists of electrodes for electrostatic field, awiggler type magnet to obtain the absolute beam profile andthree multichannel plate (MCP) assembly of 30mmx80mm
active area. A central MCP detector is equipped to measure
beam core region, and others are used to investigate thebehaviour of beam halos (Fig. 3). The bias voltage for eachdetectors and gain of preamplifier can be controlled
separately.

Figure 2: Schematic layout of the beam profile monitor. The
proton beam ionises the residual gas atoms. The products are
driven electrostatically towards the MCP detectors withgyromotion around the external magnetic field lines.
The applied electrostatic field has to exceeds the field
induced by space charge (~lkV/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 the time
resolution is limited by average drift time to MCP plate.

r[cm]Figure 4 (a): Calculated field distribution in X-Y plane.
External magnetic field is designed to suppress the diffusionof ionized particles. A two-dimensional simulation of model
magnet is performed Figure 4 (a) shows magnetic field

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

lines in X-Y plane. Sufficient homogeneity of magnetic
field less than ±0.2% for active area is obtained.

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-tum is required for each coil to obtainthe magnetic field of 500 G at beam position.

SUM M ARY
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, and investigates
the behaviour of beam halos. However, it is indispensable
to evaluate the performance of NDPM system practically.
The ionized particle motion should be confirmed in the areaof intense electrostatic and magnetic fields are overlapped
Experimental studies using a prototype in the KEK-PS orother accelerators are essential to develop the NDPM for theJ-PARC RCS.

REFER EN C ESuu

p1J

4060

Z[cm]
pu

Figure 4 (b): Calculated field distribution along the beam line.
[4!f2uFigure 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.

]

I H. DeLuca, IEEE Trans. Ncl. Sci. Ns-16(1969).
H. Weisberg et al., IEEE Trans. Ncl. Sci. Ns30(1983)2179.
B. Vosicki and K. Zankel, IEEE 91 Part. Accel. Conf.,
eds. L. Lizama and J. Chew, v o l .5 (IEEE, New York,1991)2814.
T. Kawakubo, T. Ishida, E. kadokura, Y. Ajima, and T.
Adachi, Nucl. Inst, and Meth. A 302(1991)397.
T. Quinteros, D. R. Dewitt, A. Paal,R. Schuch, Nucl.
Inst, and Meth. A 3 7 8(1996)35.