JAERI-Conf 95-021

PAlO
DEVELOPMENT OF JAERI 18-GHz ECR ION SOURCE
Watalu YOKOTA, Tak:ayukiNARA, YuichiSAITOH, YasuyukiiSHIIandKazuoARAKAWA
Advanced Radiation Technology Center, Japan Atomic Energy Research Institute1233 Watanuki, Takasaki, Gunma 370-12, Japan
YouyangWU
Depertment of Engineering, Nagoya University
Furo, Chikusa, Nagoya 464-01, Japan

Abstractsolenoid coil

An 18-GHz ECR ion source for multiply charged ions
was constructed and is now in operation for development. A
new distribution of the mirror field was adopted, of which
minimum strength is varied by a solenoid coil installed
between the mirror coils. Measured mirror field distribution
is close to designed one and its maximum strength exceeds
1.4 T. The source is now being tuned by use of Ar ion
generation. Relatively high base pressure in the plasmachamber has been improved by installing an additional
vacuum pump. The source performance has been growing up
gradually with the vacuum and Ar ions with charge states up
to 13+ has been observed so far.

mirror coil 1l.

~Introduction

1500-l/s TMP

The JAERI research programs on materials science andbiotechnology using cyclotron beams require various heavy
ion species including metallic ions in a wide energy range 1>.
According to the requirement, a new ECR ion source wasconstructed to generate ions with M/Q (ratio of mass tocharge numbers) of less than 6.5, highest limit of the JAERIAVF cyclotron acceptance, for all the stable atomicelements.
The basic design of the source design was reported atlast symposium 2>. The detailed designs of the source,
electric current supplies, a microwave power supply and abeam analyzing system were completed in October in 1993.
After being manufactured, they were installed in the ion
source room cited on the basement of the cyclotron building
in Feburuary, 1994, and the first plasma was fired at the endof June. The source was named ECR-18 after the microwave
frequency.

2. Source Description
We adopted a solenoid coil between a pair of themirror magnets to vary the shape and the size of the ECR
shell as shown in Fig. 1. The relative position of the ECR
shell to the extraction hole is adjustable by moving themagnet assembly in the axial direction. This structure alsoallows easy replacement of the plasma chamber. The
specification of the source is summarized in Table 1 .
Magnets were designed to obtain sufficiently strong fields
so that a closed shell with the strength equal to twice theECR field was produced (2c.oce shell) 3>. The maximum mirror
field strength over 1.4 T was obtained in the calculation.
Two mirror magnets have the same structure with soft ion
yoke of 8 em in thickness. They are magnetically separated
and the mirror ratio reaches 14. Figure 2 shows thecalculated and the measured mirror fields and they are ingood agreement.
In order to obtain a strong sextupole field, we
examined various configurations and combinations of theFig. 1 Schematic drawing of the ECR-18.
Table 1 Specification and measured magnetic field of theECR-18.
mirror ma~net (including yoke)
2
number104 emouter diameter
8.0 em (smaller)
inner diameter
18.5 em (larger)
27 em
length224
number of tum
1.4 T (on axis at 700A)
maximum fieldsolenoid coilnumber82 em
outer diameter
18.0 em
inner diameter
206
number of tum
0.9 T (on axis at 700A)
maximum fieldsextupole ma~net
NdFeB
material
4.5 emthickness
30 em
length8.0 embore diameter1.05 T (at 0.5 em distancemaximum fieldfrom inner surface)plasma chamber
100 emlength7.5 emouter diameter
7.0 eminner diameter
klystron amplifier
18 GHzfrequency
2.5 kWmaximum powerturbo-molecular pump1500 1/s (extraction chamber)evacuation speed
340 1/s (plasma chamber end)

-49-JAERI-Conf 95-021

calculated {1.5==

measured--

0The outer surface of the sexupole is cooled by air toavoid conduction of heat from the coils of which temperature
0rise up to 45 C, because the temperature leads to 1.5 %reduction of the permanent magnet field and the heat cycle
may deteriorate the magnet. The air is fed from an opening
between a mirror coil and the solenoid, passes throughnarrow gaps about 3 mm between the coils and thesextupole, and goes outside from an opening between themirror coil and the plasma chamber. Air flow rate of 10.
0Vmin keeps the sextuple temperature lower than 30 C.
Ion beams extracted from the source are focused by an0einzel lens in the extraction chamber and analyzed by a 90
bending magnet with the trajectory radius of 40 em.

solenoid current
zero
70%

zero

1.0

0.5-30-20-10010Z (em)Fig. 2 Mirror distribution on the axis.
magnetization direction of permanent magnets in thecalculation. Since the field strength of permanent magnets
increases as a function of a permeance, the strongest field
can be produced, in principle, by a configuration in whichmagnetization directions gradually change from one pole tothe next. The best result was obtained by the configurationin which the pieces magnetized in the radidal and the0azimuthal directins have the angular size of 10 and the0intermediate pieces 20 , and the maximum field was over 1. 4
T on the magnet surface. The configuration is similar to that
proposed by Halbach 4J and realized by Schiemenz et al. 5 \0
The angular size of all the pieces were finally fixed at 10 , ofwhich magnetization direction rotates by 30° from one pieceto the next, as shown in Fig. 3, for smoother change of thedirection and the convenience of manufacture. The measured
field is about 95 % of the calculated value on the innersurface of the plasma chamber.
The calculated field without the solenoid current
has a closed ECR shell of 5.6 em in diameter and 22 em inlength at full excitation of mirror coils, and the length andthe diameter decrease down to 17 em and zero, respectively,
with increasing solenoid current. The ECR shell disappears
at the solenoid currents over 70 % of the maximum because
the field strength exceeds the resonance value everywhere
inside both peaks of the mirror field distribution. The shellof 20lCe appears around the sextupole extremities andexpands with the increase of the solenoid field. However,the shell is not closed in the plasma chamber even at thehighest solenoid current.
The microwave is fed through wave guides from therear end of the plasma chamber in the axial direction. It isgenerally considered from many experiences that a
microwave power of a few hundreds watts is enough for asmall ECR plasma6l In our design, the plasma size isrelatively large and the higher frequency is apt to result inlarger power loss in the waveguides.
Therefore themaximum power was fixed at 2.5 kW.

Fig. 3 Configration of the sextupole magnet. Arrows show
magnetization direction.

3. Operational Experience
We have been tuning the source by generating Ar ions.
In the initial operations, the ions with higher charge statethan 11 + were not observed. This was considered to be due
to relatively high base pressure which was 1.8x1o-5 Pa in
the extraction chamber and was assumed to be higher by tentimes or more in the plasma chamber. For improvement ofthe vacuum, an additional evacuation port equipped with a340 Vs TMP was installed at the rear end of the plasmachamber. Consequentrly the base pressure went down below
5x10-6 Pain the extraction chamber, and ions with charge
states up to 13+ were observed.
Oxygen gas is fed into the plasma chamber as ansupport gas for Ar ion generation, however, it does not work
effectively. This suggests that the plasma chamber is stillfilled with too much outgas and the oxygen pressure can not
be optimized. The charge state distribution of Ar ion,shown in Fig. 4, has the highest intensity for AfB+ and thesimilar results have been observed for the conventional ECR
ions sources. However, the intensity decreases more rapidly
at higher charge states. The rapid decrease may be also
attributed to the relatively high pressure in the plasmachamber.
The optimum solenoid currents for highly charged Ar
ions is about55% of the maximum and show a tendency toincrease slightly with charge state, for example, thedifference between the optimized currents for Ar8+ and-50-JAERI-Conf 95-021

100.

-~E~

----4-,...,<::t10....

=""'""'
u=Q,le~=Q,l1~·.

")

0.1 :

0.01:''\-.._,Q,l.
6~\I

810Charge

State1214Fig. 4 Charge state distribution of Ar ions.

Ar12+ is about 4 %. The mirror ratio at the optimum current isestimated at 2.4, which is close to that of the Caprice-typesources.
4. Conclusion
The 18-GHz ECR ion source was constructed at JAERI
and is now in test operation by generating Ar ions sinceJune, 1994. The source performance has been improving
with vacuum in the plasma chamber, and the charge states up
to Arl3+ have been observed so far. The optimum currentsof the solenoid coil, mounted between the mirror coils tovary the mirror ratio, show a tendency to increase withcharge states. Further investigation is necessary for an
appropriate explanation on a rule of the solenoid. The
source performance will still grow by further optimization of
the magnetic field, the position of the extraction hole andthe improvement of the vacuum in the plasma chamber.
After goal performance is attained by using Ar ions, themetallic ions will be generated in the next stage.

Acknowledgment
The authors are grateful to Mr. S. Kimura and Mr. T.
Ushiku of IDX Co. for their helpful advice in detailed design
and manufacture of the source.

References
1) R.Tanaka, K.Arakawa, W.Yokota, Y.Nakamura,T.Kamiya, M.Fukuda, T.Agematsu, H.Watanabe,N.Akiyama, S.Tanaka, T.Nara, M.Hagiwara,S.Okada andM.Maruyama, Proc of Int. Conference on Cyclotron andTheir Applications, May 8-12, 1989, Berlin, Germany,p. 566.
2) W.Yokota, Y.Saitoh, Y.Ishii, T.Nara and K.Arakawa,
Proceedings of the 9th Symposium on Accelerator
Science and Technology, Aug. 25-27, 1993, Saitama,Japan, pp. 92
3) B.Jaquot, M.Pontonnier, Nucl. Instrum. Meth.,
A287(1990)341, and B.Jaquot, M.Pontonnier, Nucl.
Instrum. Meth., A295(1990)5-51-4) K.Halbach, Nulc. Instrum. Methods 187 (1981) 1095) P.Schiemenz, A.Ross and G.Graw, Nucl. Instrum
Methods, A305(1991)156) see papers, for example, in the proceedings of thelOth International Work Shop on ECR Ion Sources,Nov. 1-2, 1990, Knoxville, Tennessee, CONF9011136.