The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Calculation of Field Configurations of the Electrostatic Septum and Massless Septum Magnet
for the Slow Extraction System of the JHF Main Ring
Tamaki WATANABE, Noboru TOKUDA*, Masahito TOMIZAWA, Yoshitsugu ARAKAKI, Shinji SHIBUYA**,Shilllji MACHIDA, Yoshiharu MORIHigh Energy Accelerator Research Organization
3-2-1, Midori-cho, Tanashi-shi, Tokyo 188, JAPAN*1-I Oho, Tsukuba, Ibaraki 305, JAPAN**Sumitomo Heavy Industries, Ltd., 5-2 Soubiraki, Niihama, Ehime 792, JAPANAbstract
Since accelerating beam intensity is enormous in theJHF synchrotron, even small beam losses during the slow
We
extraction leads to unacceptable level of radiation.
set a criterion such that tolerable beam loss in the slow
extraction process should be less than I% of the averaged
beam current of 10 f..LA. We have examined the field
configurations of the electrostatic septum and lthe massless
septum magnet, respectively.
The calculations ofelectrostatic and magnetic fields were carried out by thecomputer code POISSON.

between two plates at fixed potentials. In order toexamine the field distortions for different wire thickness,the electrostatic fields have been calculated by changing
the diameters but keeping the wire-wire distance constant.
The geometrical configuration and the dimension of thisFixed potential of

140.0
130.0

100/cV

110.0

1. Introduction
A low beam loss extraction is necessary for the highintensity proton synchrotron. In slow extraction, thesystem is composed of the electrostatic septums andmagnetic septum magnets. The electrostatic septum has
advantage because the configuration of the electrostatic
field edge at the wires can be made sharply. But it may
have the radiation problem because the beam hits thewires.
On the other hand, the massless-septum magnet forslow extraction is very attractive because it does not need
the wires or septum coils. This new type of massless
septum magnet has been proposed by LANL group.[I][2]The massless septum magnet has a configuration that is
consisting of two pairs of magnetic poles and the magnetic
field produced by each poles is opposite. Therefore theextracted beam and the circulating one are separated eachother.
2. Computer Calculation of the Electrostatic Field
around the Wires for the Electrostatic Septum
The electrostatic fields in the septum are calculated
for an electrostatic ·problem consisting of two wires

I.\

10.00.0llash s i ze of .dx=5 Ullx (1111)

Fig. I The geometrical configuration and the dimension ofthis model.

model are shown in Fig. I. The mesh size of L1x is 5 f..Lmand L1y1 is I mm except for the region around the wires(109.5 mm ~ y2 ~ 1I0.5 mm); the mesh size of L1y2 is 5f..Lm in this region. The distance between two wires is I
mm and the wire diameter is changed from 50 f..Lm to 300f..Lm. A fixed potential of 100 kV is applied to the 10 mmthickness plate (y = 130 - I40 mm). The electrostatic
field between the plate and wires (II 0 mm ~ y ~ I30mm) is 50 kV/cm. Those calculated equipotential lines
are shown in Fig. 2 and three-dimensional plots are also
shown in Fig. 3. Fig. 4 shows the field strengths averaged
over the x-distance for the following condition: (a)changing the diameter of the wire D and keeping the wire
spacingS constant; (b) changing Sand D constant.

-374-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

(distance = 1 mm}

Diameter of Wire(a)~ .-~~~~--.--.--.---.--.--,--,

50

E"0 40>I ' :.><~30±:5!
Q)
U:::200"I:tS 10Q)iii··-··~!;~ !

1I1!
i-10 ..__.__._~..._--'-_ _._,_..........._,..,......~._...,.,.....,--~-:-:'
109.5
109.7
109.9
110.1
110.3
110.5

(a)D=50jUD!

Distance Y (mm)(c)D=200Jilll

Distance of Wire(diameter= 100 J.lm}

(b)

50

E"_g40:s::::;;-30a;
U:::20(b)D= lOOIUD(d)D=300j1mFig. 2 The calculated equipotential lines. The
distance between two wires is I mm and thewire diameters are changed from 50 Jlm to300 Jlm.

3000

3000

2500

2500

2000
1500

500

2000
1500
1000
500

00HiOO

3000

3000

2soo

2500
2000
1500
1000

2000
1500
1000
500
. 0
10.6

0.40.2(b) D= 100 IJ.Ill•tS10Q)
iiiFig. 4 The electrostatic field strength average overthe x-distance for the following condition:(a) D = 50- 300 Jlm, S = 1 mm;(b) D = 100 Jlm, S =0.5- 3.0 mm

3. Computer Calculation of the Massless Septum
Magnet5000·o.a

0'1:

10.8
0.6

0.4
0.2(d)D'= 300 lllll

Fig. 3 The three-dimensional plots of the calculated
potential. The wire parameters are same as inFig. 2. The axis are x (0 - 1 mm),y (-90.4- -89.6 mm) and z (0- 3000 V).

We are examining a possibility to use the massless
septum magnet. The upper half of the designed magnet isshown in Fig. 5. The calculated field lines are alsopresented in this figure.
The gap height from the medianplane is 40 mm in the circulating-beam region (left region)and 20 mm in the extracted beam region (right region).
The magnetic flux density at the median plane is plotted asa function of the horizontal distance in Fig. 6. When thecoil currents are 1300 A for the left coil and 2800 A for theright coil, the magnetic flux density (By) in the extracted
region is 0.175 T. The tables presented in Fig. 6
summarize the coefficients in a series of multiples in theregion from x = -40 mm to +25mm. The kick angle Llx'
given by the magnetic flux density is-375-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

where Bp is the magnetic rigidity, 1 the effective
length of the magnet and EfkJ the coefficients in aseries of multiples of the magnetic field,respectively. The problem is that the gradient of
By is not so sharp. The effects of such a field on
circulating and extracted beam is now being
investigated.

to the wire spacing is rather small. We .also haveexamined the possibility of using the massless septum
magnet. A simulation based on the calculated magnetic
field distribution is in progress.

Acknowledgement
The authors would like to thank Prof. S. Ohnuma
for his valuable suggestions and discussions about thedesign of massless septum magnet.
References[I]

H. S. Butler et al., Proc. ofthe Advanced Hadron
Facility Accelerator Design Workshop, 1988.
[2] J. Liu et al., Proc. of the Advanced Hadron Facility
Accelerator Design Workshop, 1988.

Fig. 5 The upper half of the designed magnet.
The calculated field lines are also presentedin this figure.
2000"jj) 1500
fh:::1tils 1000"0Q)

u::0:;:::500(])

c:Oltil~0-500·1204060Distance (mm)Fig. 6 The magnetic flux density at the median plane
plotted as a function of the horizontal distance.
The presented tables summarize the coefficientsin a series of multiples in the region from x = -40
mm to +25mm.

5. Concluding Remarks
The electrostatic field around the wires ofelectrostatic septum has been calculated by a computercode POISSON.
We found that the field distortion due-376-