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

INTENSE BEAM PRODUCTION FROM RIKEN ECRISS
Y. Higurashi' T. Nakagawa, M. Kidera，M. Kase and Y. Yano RIKEN, Wako, JapanT. Aihara SHI accelerator Service Ltd，Kita-shinagawa-ku，Tokyo 141-0001 Japan
Abstract
We have constructed several high performance
ECRISs in RIKEN and produced intense beam ofheavy ions.( e.g., 2mA of Ar8+, 0.6mA of Kr13+,
0.3mA of Xe20+) During the improvement of theirperformance, we found that the several key parameters
play essential role to increase the beam intensity( plasma electrode position, magnetic field
configuration, property of the chamber wall material,
position of the biased disk and so on).

INTRODUCTION
ECR ion sources have been widely usedas the external ion sources for heavy ion accelerators.
Especially, ECRISs became one of the key devices for
production of radioisotope beam (RI beam). Since
middle of 1990s, RIKEN has undertaken constructionof new accelerator facility so-called RI Beam
Factory.[l] In this project, the production of intense
heavy ion beams is an important task to produce theintense RI beam. For this reason, at RIKEN, several
high performance ECRISs have been constructed( RIKEN 10 GHz ECRIS[2],18 GHz ECRIS [3]andliquid He-free Super conducting ECRIS[4]) in the lastdecade.
Extraction
electrodeouj

Plasma electrodeposition L (cm)

ZLA
j目 ^

Plasma electrode

18GHZ ECRISA detailed description of the RIKEN 18
GHz ECR ion source and performances are described inref. [3]. The mirror ratio has a nominal value of 3.0
with the maximum of mirror magnetic field 〜1.4 T. Thefield strength at the surface of magnets (Br) is 〜1.4 T.
To increase the beam intensity, we used the negatively
biased electrode installed in plasma chamber and thealuminum cylinder to cover the inner wall of plasmachamber [3]. For production of highly charged Ar ions
( Ar11+, Ar12+), we used the A120 3 plating method. [6]Furthermore, the position of plasma electrode andmagnetic field configuration play essential roles toimprove the performance.

ele magnet

RIKENv(ECR zone

using liquid He.[4] After one year of construction, wehave successfully extracted the first beams of highly
charged heavy ions, such as Xe and Kr ions. During
the increase of the beam intensity, we have recognizedthat it is important for increasing the beam intensity
not only to increase the magnetic field strength, butalso to optimize magnetic field conflguration[7].
Applying these methods, the beam intensities fromRIKEN ECRISs have been dramatically increased.
For example, the beam intensity of Ar8+ has been
increasing from several10 ejnA to 2 emA during thepast decade.

Position A (0 cm), B(lcm), C(1.5 cm)
Figure 1:Schematic drawing of the plasma chamber
around the plasma electrode position.
Since the first beam was extracted fromRIKEN 18 GHz ECRIS in 1995, several methods
have been applied to increase the beam intensity ofmghly charged heavy ions( e.g., optimization ofplasma electrode position [5] and A120 3 plating[6]).
The solenoid coils of liquid He-free SC ECRIS iscooled by small refrigerator instead of liquid He. Thisfeature allows us to avoid the complicated operationand bulkiness of ordinary super conducting magnets
higurashi@nken.jp

051015Plasma Electrode Position (mm)Figure 2: Beam intensity of Ar ions as afunction ofplasma electrode position.
As the plasma electrode makes the partof the boundary of ECR plasma, it is natural to thinkthat the position of plasma electrode affects thecondition of plasma. It also affects the beam extraction
conditions. Then, the beam intensity may be increased
finding the most suitable position of the plasma
electrode. Based on this assumption, we tried toThe 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

ECRIS are much higher than those from RIKEN 18
GHz ECRIS. Though both ECRISs have the almost
same chamber diameter (-7.2 cm in diameter) and theRF power was also almost same, there are two essential
differences between two sources, i.e., the plasmachamber length and its magnetic field strength. The ion
confinement time is strongly dependent on the volumeof the plasma chamber and the mirror ratio of mirror
magnetic field. It may conclude that the different
confinement time of ions in plasma in the two ion
sources brings the different results of the beam intensity.

(vrlexllsu^UJulsg

111

optimize the plasma electrode position to increase thebeam intensity of highly charged heavy ions. We havechosen different points (see Fig。 1 ) and measured thebeam intensity of Ar, Kr and Xe ions. Figure 2 shows
the beam intensity of Ar ions as a function of theplasma electrode position. The beam intensity gradually
increases when moving the plasma electrode toward theECR zone and then decreases as shown in this figure. It
seems that an optimum electrode position exists tomaximize the beam intensity. The optimum plasma
electrode position moves toward the ECR zone when
the charge state becomes lower. The same tendency canbe seen in case of Kr and Xe ions.
Figure 3 shows the typical beam
intensities produced from gaseous elements at theextraction voltage of 13-17kV and RF power of 700 W.
We obtained 0.3emA of Xe20+, 0.6 mA of Kr13+ and2mA of Ar8+at only 700 W of RF power.

001020Charge state (q)Figure 3: Charge distribution of Ar, Kr and Xe ionsproduced from RIKEN 18GHz ECRIS.

LIQUID HE - FREE SC-ECRIS

35

Figure 4: Beam intensities of Xe ions from SHIVA(closed circles) and CAPRICE (open circles) and Kr ionfrom RAMSES (closed squares) and RIKEN 18GHzECRIS.
It is well known that the magnetic field strengthand shape at the beam extraction side and injection side
influence the charge distribution and beam intensity. [8]Furthermore it is natural to think that Bmin should
influence to the beam intensity of heavy ions from theECRIS[9]. Because the magnetic field configuration ( orgradient of magnetic field ) affects the plasma
confinement and the effectiveness of the electron heating
at resonance zone.

Kriono86o4o2：x-j f \ 20+ofo

110

ss l(vrlsXIJSSSI

The detailed design of the ion source wasdescribed in ref.[4]. The RF injection side coils
generate a maximum field of 3T. The beam extraction
side coils generates a maximum field of 2T. The
solenoid coils are cooled by small refrigerator. The
radial magnetic field strengths on the surface of plasmachamber is 1.2T. The operational microwave frequency
is 18 GHz. It should be stressed that the plasmachamber volume (1.5 L) is much smaller than those ofother SC-ECRISs.
To investigate the effect of magnetic
field strength and plasma chamber volume, we
compared the beam intensities from the RIKEN 18
GHz and liquid He-free SC-ECRIS. Figure 4 shows thebeam intensities of Kr ions. Open and closed squares
are the beam intensities of Kr ions from RIKEN 18
GHz and liquid He-free SC-ECRIS.
The obtained beam intensities of Kr20+,27+
from the SC-ECRIS were 40 and 5 e|iA at 12 kV of theextraction voltage and 600 W of the microwave power.
In this experiment, the typical magnetic filed strengthsof Binj, Bmin and Bext of the SC-ECRIS were 2.0, 0.5 and
1.2T, respectively. The beam intensities of highly
charged Kr ions (25+， 27+) extracted from the SC-

2530
Charge State

0.3

0.4

0.5

0.6

Bmin(T)

Figure 5: Beam intensities of Kr 15+12+ as a function ofBmin.

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

0 Bext::1.15T Binj==1.85To Bext==1.0T bn==1.85Tm o BCxt==0.9T Bi„r 1.85T

吾0.5•&參ロ ロ口 n 真■i_X

O Bext= 1 .18

X Xe Bexl= 1.180Figure 6: Optimum

0.55.5o.

0401020

Charge state

1.2Ar Bexl-1.15T B inj=1.72T▲ Kr Bex, = 1.0 T Binj=1.9T

A Kr Bext= 1.15T 6 ^ = 1 .9 T
A Kr Bext=1.25T Binj=1.9T- 4

RAMSES 〜
0.48T★A' 善 -------------------------- n~K"4- Ar Bexl= 1.18_□0.65

o 0 Bext=:1.I5T Binf

RAMSES. This is likely one of the reasons why weobtained only 1.5mA of ひ • For production of heavier
ions, such as Xe20+, the Bmin is slightly higher than theoptimum value. It means that if we can set the optimum
value for Bmin without changing Bext, we may increase
the beam.

p J)

For studying the effect of Bmin on the beam
intensity produced from the SC-ECRIS, we changed Bmin
from 0.25T to 0.6 T without changing Bext and Binj.[7]Figure 5 shows the beam intensity of Kr20+,15+ as afunction ofIn this experiment, Bext and Binj werefixed to 1.15 and 1.9 T, respectively. The injected
microwave power was 600 W. The extraction voltage
was 12 kV. The gas pressure, the position o f electrodeand its negative bias voltage were changed to maximize
the beam intensity. The typical gas pressure at Bminニ0.5
T was 3xl0'7 Torr for Kr15+ ions. We did not use the gas
mixing method for simplifying the experiment. The
beam intensity gradually increased with increasing
up to -0.49T and then gradually decreased. Above 0.6 Tof Bmin, the extracted beam became unstable. The valueof Bmin (optimum Bmin) for maximizing the beam
intensity of Kr15+ was almost same as that for Kr20+ ions.
Figure 6 shows the summary of the experiment(optimum Bmin for multi charged 0 , Ar, Kr and Xe ions).
We measured the optimum Bmin at several conditions( Bext = 0.9〜1.25T, Binj =1.72 〜1.9T ) for various heavy
ions.

SHIVA 〜
0.39T

T B iirj=1.85T
T B jnj=1.85T
T Binj=1.85T

I 1 1 _ I_ I__ L
1020Charge Statefor various charge state of heavyions.

In these experiments, we observed that theexistence of optimum value for Bmjn to maximize thebeam intensity and it is not dependent on the charge stateof heavy ions. The optimum value for Bmin was
0.47-0.49T for production of various multi-charged
heavy ions as shown in Fig.6.
RIKEN 18 GHz ECR ion source has only
two sets of solenoid coils for producing the mirror
magnetic. It means that we cannot control Bmin and Bext
independently. Figure 7 shows the magnetic fieldstrength of Bext and Bmin for maximizing the beam
intensity of various heavy ions. For most of theexperiments, we kept the Binj 〜1.40T，which is themaximum magnetic field strength of RIKEN 18 GHzECRIS. The dashed line is the optimum Bmin obtained
from the experiment using RAMSES. For production of0 5+, the Bmin is significantly lower than the optimum
Bmin , which is expected from the experiment with

® 0.8

0.6
010Charge state

20Figure 7: Bmin and Bext of RIKEN 18GHz ECRIS for
production of various charge state of heavy ions.

REFERENCES[ 1 ] Y. Yano, Proceedings of the 15th International
Conference on Cyclotron and Their Application(IOP, Bristol, 1999)p696..
[2] T. Nakagawa, Jpn. J. Appl. Physics 30(1991)L930.
[3] T. Nakagawa and Y. Yano, Rev. Sci. Instrum.
71(2000)637 and references therein[4] T. Nakagawa et al, Rev. Sci. Instrum. 73(2002)513
and references therein.
[5] Y. Higurashi et al” Jpn. J. Appl. Phys.
42(2003)3656[6] M. Kidera et al, Rev. Sci. Instrum. 73(2002)611•[7] H. Arai, et al, Nucl. Instrum. Methods A491(2002)[8] S. Gammino et al, Rev. Sci. Instrum. 70(1999)3577.
[9] D. Hitz et al., Rev. Sci. Instrum. 71(2000)839.