The 11th Symposium on Accelerator Science and Technology, Harirnq,Science Garden City, 1997 ,·Study of the Extraction Configuration ofNIRS-HEC Ion Source
Mitsugu YAMAMOTO, Takanori OKADA, Yusei KAGEYAMA, Satoshi OGAWA, Takeo NAKAJIMA,Mitsuru HANAGASAKI, Atsushi KITAGAWA*, Masayuki MURAMATSU*, Satoru YAMADA*,
Kouichi UNo··, and Janos vAMost••
Accelerator Engineering Corporation (AEC),
2-13-1 Konakadai, Inage, Chiba 263, Japan·National Institute of Radiological Sciences (NIRS),
4-9-1 Anagawa, Inage, Chiba 263, Japan··sumitomo Heavy industries, Ltd. (SHI),
5-2 Soubiraki, Niihama, Ehime 792, Japan•••Institute of Nuclear Research (ATOMKI),
H-4001 Debrecen, P.O.Box 51, HungaryAbstract
In order to increase the beam intensity ofNIRS-HEC,
we have studied the extraction configuration with He+ions. It was observed that the high voltage extraction iseffective to increase the beam intensity when the extraction
current density is high. The results of computer simulationagreed excellently with the experimental data.
1 Introduction

A 10GHz ECR ion source (NIRS-ECR) and a PIG ion
source supply heavy ions for cancer treatment and fundamental experiments, ·in Heavy Ion Medical Accelerator
in Chiba (HIMAC), National Institute of Radiological
Sciences (NIRS) [1], [2]. A typical intensity of C4+ ion isabout three hundred ejJA with both ·sources and satisfies
medical requirement. For heavier ions than At+, however,higher beam intensities are desired especially by physicists,biologists etc. So an 18GHz ECR ion source with Highvoltage Extraction Configuration (NIRS-HEC) was
installed in March, 1996 [3], [4].
Features of NIRS-HEC are as follows. One of the mostimportant features of NIRS-HEC is the high extraction
voltage to reduce space charge effects in the beam extraction region. The insulation between the plasma chamber
and the extraction electrode was carefully designed. Major
components of the source including the plasma chamber,mirror magnet, permanent magnet, and other devices, areput on a high voltage platform. The maximum extraction
voltage reaches to 60kV. The other feature is a strong
magnetic field to confine the source plasma in a smallregion. The maximum axial field strength reaches about
12kG. A radial size of the plasma chamber is made as small
as 40mm, so that a sextupole field strength of 11kG is easily
obtained.

configuration, we m1:1asured the extracted beam ~nts atthe different extraction voltages. The distance between theextraction slit and the extraction· electrode, the extraction
current density and the mirror magnetic field of theextraction side are also varied in the measurements
to search OJ)timum configuration of the extraction system..·
The experimental data were compared with the simulation·results.
Figure 1 shows a schematic layout of the extraction
system of NIRS-HEC. An analyzed beam. current I...c ismeasured at the front of the analyZing magnet as a current
from the platform to the ground. An unextracted ~nt !ext
which flows into the. extraction electrode is also measured.
The total extracted current ltotat from the extraction slit ·isestimated as a sum of 18cc and !ext. Although there is an
ambiguity in Loxt due to the secondary electron emission
from the electrode, it does not give serious effects on thediscussions described here. The extraction voltage VeXt. isapplied to extraction electrode. A symbol d..ct denotes adistance between the extraction slit and the extraction
electrode. Helium gas was used in this experiment. Because,
it has the simplest charge state distnbution to compare the.
experimental Clata with the simulation.

~ Extr-...'I e~'II
I

PlasmaChamber-+- l...,.orow.t
Al:ce~ClllEinzel_LCIII

GapI----x[?r-~ [TExtraction

slitVext(-10 to -SOkV)(<40kV).Ailllyzillg
Map·VIIJC(=ntV fiDel)-rltotal = lext + lace

2 Experimental method

In order to improve the beam intensity, we must
optimize the extraction configuration [5]. The extraction
electric field is affected by the extracted beam current anddistribution due to the space charge effects. An optimum extraction field can be searched by moving theextraction electrode or by changing the extraction
voltage. In order to investigate optimum extraction

Platform!!extFig. 1 A layout of the extraction systcBJ. ofNIRS-.HEC ion source
3 Experimental results3-ll)ependence on extraction voltage-171-.

Dependencies of I;..., on extraction voltages m sbowli. _The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

in Figs. 2a) to 2c). In these figures, lace is normalizedwith Itotai and horizontal axis is given in form of electricfield strength instead of Yext· The experimental conditions
are as follows. The pulse length was 5ms, the repetition rate
was 2Hz. The mirror magnet current of extraction side isfixed at 600A, V~xt was varied from 10 to 50kV, d.xt wasvaried 20, 40, and 60mm. ItotaJ was tuned to 6, 9 and 12mA.
It is clear from these figures that higher extraction
fields are needed at higher extraction currents to overcome
the strong space charge defocusing effects. Low valuesof IacJltotai for d.x.~=20mm suggest that the distortion in theaxial extraction field may affect on the focusing character-j0.8~ 0.6

I' ~<;;

0 0.6

lfj.;::::;/(j ...

---8 0.4..::l0.200246810 12 14 160

Vext/dext (kV/cm)

0a)

3-2 Relation ofmagnetic field, ltotaZ, and lace

Figure 3 shows a ratio of lace to Itotai for different valuesof mirror magnet field in the extraction side, M2, at 400,600, and 700A. In these measurements, Itotai was fixed at9mA. Other parameters are the same as those described in
3-2.
Effects of the magnetic field distribution on theextracted current will be discussed in 4-2.

L-)h-~

0.8.;::::;

istics of the extraction electrodes.

vJ246

8010 12 14 16

Vext/dext (kV/cm)

0b)

1//_t

0.6~.;::::;---0 0.4]-~,t'//004'2J('4~810 12 14 16!J .r / v00a)..-------

2---8 0.4..::l0.2

46810 12 14 16

Vext/dext (kV/cm)b)

Fig. 3 V.x.~ld.xt- IacJltotai (M2=400,600,700A, Itow=9mA)a) M2=400A, b) M2=600A, c) M2=700A
4 Simulation Results

468JlO 12 14 16

Vext/dext (kV/cm)

J.--"'~fti

0.8!(r·i~.;::::;(/62]0 0.6/j/

Vext/dext (kV/cm)

fdext--20mm-+-40mm--60mmc)

v'h- ..

0.80.2 -······· ...

:L't--'11~-Fig. 2 V.xtld.xt- IacJltotai (d.x.~=20,40,60mm, M2=600A)a) Itow=6mA, b) Itow=9mA, c) Itow=l2mA

0.8/_

0.800v/~·12468dext--20mm-+-40mm--60mm

10 12 14 16

Vext/dext (kV/cm)c)

4-2 Flux density distribution in extraction region

4-1 Effectiveness of high extraction voltage

Figure 4 and 5 show the typical results of simulations. In these calculations, we use the FUGUN code [5].
This is a modified Herrmansferd's EGUN code by adding a
1-D sheath theory to solve the particle motion at theextraction region in plasma. The axial position of theplasma meniscus is determined with the Child-Langmuirlaw on the space charge limited emission. In Fig. 5,
calculated values of lace are given for different Itotab
comparing with experimental results. In these calculations,values of M2 and d.xt are fixed at 600A and 40mm,respectively. The simulation results agreed well with theexperimental data.

Extracted current intensity depends on the initial ion
density distribution at the extraction slit. In the TrapCAD
code, the cross sectional density distribution of the ions isestimated with three different assumptions; the density
distribution is homogeneous, the density distribution is
proportional to the magnetic flux density at the slit and thedensity distribution is proportional to the magnetic flux
density multiplied by the plasma volume along the magnetic flux line [6]. (See Fig. 6)
The calculated results are shown in Fig. 7 together withexperimental values.

-172-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997i• V"')

0.8'{-a:g 0.6.._Qt)

<II;

0.4v,

0.2~.,. .,.

r-~~.,.

• 6mA9mAIIBI" 12mA-6mA--'- ·9mA-- ·12mA

I/JlOOmm

1/

10Fig. 4 Typical trajectory simulated by FUGUN

02

4 6 8 10 12 14 16
Vext /dext (kV/cm)

Fig. 5 Calculated dependence of lace on Vext for different
values of !total together with the experimental dataa) Experiment b) Calculation

Plasma Chamber
1.00,.....

::i 0.80~0.60.~~ 0.40~-homogeneous--n......._n*VB

0.20
0.00
0.00

0.20

0.400.60

r (em)

Fig. 6 Flux density distribution by TrapCAD
1210 -·-·· --··-·-· 8---~.v.

v.'-7/6/

4~- - ·homogeneous-n·····n*UB• Exp.

~----/a)Db)

2001020 30 40
Vext(kV)

5060

Fig. 7 Experimental and calculation results of
Vext ·lace (M2=700A, dext•=40mm, ltotal=9mA)a) Calculation b) Experiment

5 References[1] A. Kitagawa et al., Rev. Sci. Instrum., 67 (3) (1996) 962[2] E. Takada et al., In this proceedings.
[3] A. Kitagawa et al., Proc. of 12th lntnl. Workshop on
ECRIS, Wako (1995), p,[4] T. Okada et al., Proc. of 7th Symp. on Beam Engineer-ing of Advanced Material Syntheses., Tokyo (1996),
p.135[4] S. Fu et al., Rev. Sci. lnstrum., 65 (4) (1994) 1435
[5] J. Vamosi and S. Biri, Computer Physics Communications 98 (l996) 215-173-