The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan 1993

Effects of Gas Mixing and Electron Injection
IllProduction of Multiply-Charged Ion with The HyperECR
Y. Yamashita, Y. Isoya, T. Ishida
Research Center for Advanced Technologies, the Japan Steel Works,Ltd.
1-3 Takanodai, Yotsukaido-shi, Chiba 284, JapanandM. Sekiguchi
Institute for Nuclear Study, University of Tokyo
3-2-1 Midori-cho, Tanashi-shi, Tokyo, 188, Japan

Abstract
In an experiment by using the gas mixing techniqueand electron beam injection technique at a multiplycharged heavy ion source ( HyperECR) with 14.25 GHzmicrowave frequency, an enhancement of the intensity forhighly charged ions has been observed. It was confirmedthat these methods work well also in this source.

1

Introduction

charge state in the plasma chamber is higher than that of
the extracted beam. It is, therefore, important for the ion
source development to find out not only how to trap ionsbut also how to extract raw ions in the plasma.
An experiment to show clearly the effects of gas mixing
and electron injection has been carried out.

2Experimental Setups

2.1

Gas Mixing

To increase the intensity of the highly charged ions,Two variable leak valves (ANELVA,951-7170) are used
gas mixing is one of the common techniques in which one for the main gas and the assist gas. Their minimum conuses an assist gas together with a main gas for ioniza- trollable leakage rate is 5 x 10- 8 TorrL/sec ( 0.004 seem )tion. For the assist gas, He or 0 2 are usually used. In according to the specification. This corresponds to thethese days, this ·effect is realized to come from the fact beam current of 286 ep.A for Ar and 572 ep.A for 02 if allthat the lighter ions cool the heavier ions - ion cooling. the atoms are singly ionized,
The heavy ions ( atoms) in the plasma chamber get some
After passing the valves, two gases are admixed andenergy from the electrons that are accelerated with the fed into the plasma chamber ( Fig.1 ). Since it was foundmicrowave power. But after the heavy ions collide many that there is not much difference in performance between
times with the light ions (atoms), they lose the kinetic en- the cases when the gas is fed through a quartz pipe on theergy (cooled), so that the heavy ions are trapped better in axis or fed directly into the plasma chamber without using
the negative potential well to be ionized into more highly the tube, usually a quartz tube is not used in this source.
charged states.
Instead, this space is used for other purposes.
Electron injection to the plasma chamber is another_.,.,-··· SOLENOID COILS
technique for that purpose. It is commonly realized that
supplying electrons into the plasma chamber from outside
increases the electron density in the plasma and that the\ionization efficiency becomes better. Actually in several
sources, this phenomenon is confirmed experimentally [1].
In an ECR ion source, once an electron enters the mirror magnetic field, the electron gains energy at the ECR. G>S llll.ET
region in the perpendicular direction to the magnetic flux
( B J. direction). So the electron will be confined morestrongly inside the mirror field. The electron will be lost
1-'0LECLA..AR Pllf.1P
160 L/secby the electron capture process of ions, by losing the momentum in the B J. direction in collisions with ions (escape01020itm)
IIITI.<ISO HOLECIA.AR PIH'from the mirror field ) or by bombarding the inside wall of500 L/sec
the plasma chamber.
Fig.l. Drawing of the HyperECR.
An assumption that a negative potential is made byelectrons inside a plasma chamber is reasonable [2]. This
Since a lighter gas should be more effective for ion coolpotential traps ions, and the higher an ion is charged, the ing if the model on the gas mixing mentioned before is true,stronger it is trapped. This suggests that the mean ion H2 gas is also tried as the assist gas in this experiment.
l~BOI-104-1

_.__, ___ ,___ ;. __ . ____i__-t--

Electron Injection

2.2

A commercially available filament made of lanthanum
hexaboride ( LaB 6 , DENKA W-2 ) is used for an electronemitter. It is attached at on the electrodes at the top ofa pipe inserted axially ( Fig.l ). The filament current wasdecided to be about 23 A from the catalog data and theelectron emission current. An optimum position of thefilament was found to be outside the one of the peaks ofthe mirror field in a search to get a good performance. The
magnetic field at that position is about 4 kG.

3:---'---~-----:.___:__:

Fig.2. Thned to ArB+, Ar + 02 gas, filament on.

-------- - -------- ---,---- -.-.- - · - - - :- - - ------ --------- -'------' --------- --The experiment is carried out in the following
The currents through the two mirror
procedure.
coils ( 600A/550A ) and the microwave power ( 650W) were
fixed. The source and the charge state analyzer were tuned
to optimize the beam current of ArB+ after the analyzer
with oxygen gas admixed. The potential of the filament
relative to the plasma chamber has been searched with aconstant voltage power supply. It was found that a valueof -600 V is the best although this is limited by the capacity of this particular power supply. This bias voltage isput on and off with the filament power supply.
The charge state distributio n observed after the analyzer at this step is shown in Fig.2. Hereafter, the source
parameters were not varied except the flow rates of themain and assist gas. The ion spectra in each step areshown as follows:

2.
3.
4.

5.

1\med to ArB+, Ar+02 gas, filament on
02 gas closed, some Ar gas added
He gas added
He gas closed, H2 gas added
Filament off:- - - :-: -T --~-;--<-:_~~;--::-~_,_;~~A:'a;~~1~ -,~ 4~~~~~~

Results

1.

!

______ ; __ :__; ____ "i... _: _____ Ar 8+ 210 ep.A _:·-------------·------;_.;.~ _ _ ...; _______ ! _ _ ;--;.~Fig.2
Fig.3
Fig.4
Fig.5
Fig.6

From these spectra, the beam current of ArB+ and thetotal Ar ion current were deduced together with the mean
charge state. They are summarized in Table 1.
.
magne t .
T able 1 A r b earn aft er an altyzmg
Ar 8 + beammean charge total Ar b earncurrent( eJJ.A) current( eJJ.A)
state
255
693
6.54
Fig. 2
210
595
6.20
Fig. 3315
784
6.76
Fig. 4315
896
6.61
Fig. 5190609
6.03
Fig. 6
It can be seen from these results, firstly, that any ofthe mixing of H2, He or 0 2 gas increases the total Ar beam
current. Secondly, this gas mixing makes the mean charge
state higher. Thirdly, it is clear that the electron injection
from the filament works well.
According to the current model of ion cooling, thepresent experimen tal results are consistent with the factthat the gas mixing and the electron injection help to trap
the highly charged ions in the negative potential.

-105-:.= _1--,--------··-:----- 3o~j,A. fdiv---- ·--------------

11-.,..--~---'----

-----

___-_
-:-: 11_-~-.,---------,~----~--__-___'-'·..

-------------~~-~~-------~-----------'

Fig. 5. He gas closed, H2 gas added.

- ·- -"A~·+ i90e,tr ----- ·--·-- ·----- · ·· · ___________ _30 epA fdiv ':-"--·-· :. .

~1:==--=- -·-~:.
___j _____ ;.._~:

Fig. 6. Filament off-~--------.:-------.

4Discussions

Experiences on the operation of the HyperECR showthat performance for the Ar beam becomes always better
after oxygen beam production although oxygen beam is notdetected so much in the spectra. After running the source
for Ar for a while, however, this effect disappears. It isreported from several ECR sources that coating the innersurface of the plasma chamber with Si0 2 , MgO, Al 2 0 3 etc.
works to increase beam current [1, 3].
Since oxygen is not so light as H2 or He, it is difficultto expect that oxygen is contributing to the ion cooling.
Instead, it is more natural to think oxygen plays a role ofan electron emitter. The inner surface may be coated withoxide during oxygen production.
Higher energy electrons in the plasma have larger Larmor radius and they will eventually hit the inner surfaceof the plasma chamber and produce secondary electrons.
If the number of the secondary electrons are enough, raising the temperature of the inner surface of the chamberand making it with a material whose work function is low
will result in higher electron density of the plasma. In thiscase, the electron injection technique from outside of thechamber may not be necessary.
In this experiment the RF power was fixed at the level
and the source was not necessarily in a best condition.
When more RF power is fed and the source is tuned more
carefully, it has been proved that this source can produce,for examples, 480 ep.A of 0 6 +, 60 ep.A for 07+ and 480
e11A for ArB+ by using the techniques as described in thispaper.

5Conclusions

On the HyperECR ion source, an experiment has been
carried out to show the following points:• In an Ar beam production test, the gas mixing technique with H 2 , He or 0 2 and the electron injection
technique increase the beam current and the meancharge state.
• Some other methods may produce more current thanthose currently employed.
References
1) C. M. Lyneis et al., Proc. lOth Int. Workshop on ECRIon Sources, ORNL, 1991, p.47.
2) G. D. Shirkov, Preprint JINR E9-92-33, Dubna, 1992.
3) T. Nakagawa et al., Proc. 8th Symp. Ace. Sci. Tech.,
1991, p.55.

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