1 30 cid242 1 9 2006 c 9 cid27 p U cid212 n cid134 cid216 cid212 n HIGH ENERGY PHYSICS AND NUCLEAR PHYSICS Vol 30 No 9 Sep 2006 Study of Extraction Property of RF Ion Source SUN ZhenWu1 HUO YuPing LI YuXiao LI Tao School of Physics Engineering Zhengzhou University Zhengzhou 450052 China Abstract Ecid11ects of geometry parameters of extraction electrode extraction voltage and oscillator power on the extraction property of RF ion source are measured and the experimental results are analyzed It t......
30
2006
9
9
HIGH
ENERGY
PHYSICS
AND
NUCLEAR
PHYSICS
Vol. 30, No. 9
Sep., 2006
Study of Extraction Property of RF Ion Source
*
SUN Zhen-Wu
1)
HUO Yu-Ping LI Yu-Xiao LI Tao
(School of Physics Engineering, Zhengzhou University, Zhengzhou 450052, China)
Abstract
Effects of geometry parameters of extraction electrode, extraction voltage and oscillator power on
the extraction property of RF ion source are measured and the experimental results are analyzed. It turns
out that the RF ion source has a good extraction property. There is the best
D/d
on condition that other
parameters don’t change. The
D/d
rising is beneficial to the ion beam focusing from over focusing. Focusing
degree increases and extraction beam intensity drops with
b/D
increasing. Focusing degree rises with
B/d
decreasing and the rising tendency slows down when
B/d
is smaller than 4. The suitable ranges of
D/d, b/D
and
B/d
are respectively 1.6—2.1, 0.7—1.1 and 4—7 when considering comprehensively the focusing degree,
the extraction beam intensity and the pressure. Change of extraction voltage and oscillator power has opposite
effect on the ion source performance and they have respectively best operation values.
Key words
ion source, ion beam, focusing degree
1
Introduction
Ion source is a crucial part of electrostatic
low and extraction electrode is quickly damaged for
ion bombardment. Electrostatic accelerator can’t be
ensured to work steadily. So the factors influencing
the extraction property of RF ion source are studied
in this article so that the technological parameters of
RF ion source can be selected rationally to make RF
ion source work in the best state.
The physical quantities expressing the extraction
property of ion source are focusing degree
α
and ex-
traction beam intensity
I
M
·
α
is defined to be the
ratio of the current intensity
I
M
of the ions going
through extraction electrode to the current intensity
I
K
of the ions striking extraction electrode.
I
M
and
α
are related to the geometry parameters of extrac-
tion electrode extraction voltage, pressure and os-
cillator power
[2, 4]
accelerator
[1, 2]
. Its property affects directly the per-
formance of electrostatic accelerator. So the work-
ing state of RF ion source used was studied in the
process of developing electrostatic accelerator. The
former article studied the factors influencing the RF
ion source discharging and the effects of plate volt-
age of oscillator, gas pressure and extraction voltage
on the intensity and the proton ratio of ion beam .
The work ensures that the property of ion beam can
satisfy the requirements of electrostatic accelerator.
But the article didn’t consider whether the extrac-
tion state of RF ion source was the best. The ex-
traction property of RF ion source wasn’t touched
on. Ion source in the best working state has not only
a strong intensity and a high proton ratio but also
a fine focusing degree. If the extraction property of
ion source is bad, the efficiency of extracting ions is
[3]
. The extraction voltage, the pres-
sure and the oscillator power can be adjusted when
ion source works, but the geometry parameters of ex-
traction electrode cannot be changed generally after
ion source is designed and machined. So it is obvious
Received 24 January 2006, Revised 27 March 2006
* Supported by Special Foundation of He’nan Educational Committee
1) E-mail: sunzw@zzu.edu.cn
902 — 906
9
RF
903
that the geometry parameters of extraction electrode
affect
I
M
and
α
most greatly. Moreover, the geom-
etry parameters of extraction electrode influence the
pressure and the extraction voltage to some extent.
In studying the geometry parameters of extraction
electrode, most engaged in computing and simulating
by theories of ion optics and plasma
[5—8]
voltage) and extraction voltage
V
b
on
α
are mea-
sured with the extraction electrode of inner diameter
1.5mm. Fig. 2 shows that
D/d
and
b/D
greatly in-
fluence
α
so that
α
has the greatest value with
D/d
rising, ion beam begins to focus when
b
is equal to
0.5D and that
α
quickly rises with
b/D
increasing.
At the same time, it is observed in experiments that
I
M
promptly decreases and the best extraction volt-
age rises when
b/D
increases. Fig. 3 indicates that
α
rises in a line with
B/d
decreasing and that the
tendency of
α
rising slows down when
B/d
is smaller
than 4. This is consistent with the experimental re-
sults of
. Fig. 4 and Fig. 5 make clear that
there are the best extraction voltage and the best
plate voltage for
α.
The best extraction voltage in-
creases and the best plate voltage decreases with
D/d
rising. It is found that pressure has similar property
to oscillator plate voltage.
. This arti-
cle studies the effects of the geometry parameters of
extraction electrode, extraction voltage and oscillator
power on focusing degree
α
and extraction beam in-
tensity
I
M
by experiment to provide direct data for
designing extraction electrode of RF ion source.
2
Experiment
The extraction electrodes used in experiments
(Fig. 1) are made of aluminum. Their inner diam-
eters are respectively 1mm, 1.5mm and 2mm. Ex-
periments are carried out on the table experimental
system . Faraday box 1 is far from focusing elec-
trode in the system, so
I
M
can’t be measured accu-
rately for ions scattering and annihilating when focus-
ing voltage doesn’t exist. However, it isn’t convenient
to measure
I
K
if focusing voltage exists. So another
moveable Faraday box is installed at the base of fo-
cusing electrode to be able to measure precisely
I
M
without focusing voltage.
[3]
Fig. 2. Effect of
D/d
on focusing degree
α
(B = 5d,
V
a
=600V,
V
b
=1.8kV,
P
= 6
×
10
−4
Pa).
Fig. 1. Geometry parameters of extraction
electrode.
1–plasma; 2–emitting surface; 3–quartz pipe;
4–extraction electrode; 5–assembling angle
θ;
6–waist.
Firstly, effects of
D/d, B/d,
plate voltage
V
a
of
oscillator (oscillator power is determined by its plate
Fig. 3. Effect of
B/d
on focusing degree
α
(D = 1.8d,
b
= 0.9D,
V
a
=600V,
V
b
=1.8kV,
P
= 6
×
10
−4
Pa).
904
( HEP & NP )
30
forms emitting surface (Fig. 1). The extraction ef-
ficiency of ion source is related with the shape of
emitting surface. The curved electric field makes ions
assemble before they enter extraction electrode. As-
sembling angle is
θ.
The appearance of ion beam is
approximately like a taper. Because the entrance of
extraction electrode and space charges disperse ions,
ion beam has a waist in extraction electrode. The
distance between the waist and the upper end of ex-
Fig. 4. Effect of oscillator voltage
V
a
on focus-
ing degree
α
(b = 0.9D,
B
= 5d,
V
b
=1.8kV,
P
= 6
×
10
−4
Pa).
traction electrode is determined by the geometry pa-
rameters of extraction electrode. The best emitting
surface should make the waist be in the middle of ex-
traction electrode to make ion beam just shave the
edges of the entrance and the exit of extraction elec-
trode (Fig. 6(b)). In this case, those are evident that
the ions striking the extraction electrode are the least,
the focusing degree is the largest and the extraction
efficiency is the highest.
D/d
is the best in this case.
Adding
b
can reduce the probability that ions strike
the upper surface of extraction electrode so that the
focusing degree rises. But it simultaneously makes
the electric field between the emitting surface and
the extraction electrode decrease so that the extrac-
tion force of extraction electrode reduces. Therefore,
extraction beam weakens obviously. When the pa-
rameters except
B
don’t change, the distance between
the waist and the upper end of extraction electrode
doesn’t change. Reducing
B
is equal to only reducing
the distance between the waist and the lower end of
extraction electrode so that the probability that ions
strike the lower part of extraction electrode becomes
small. So
α
rises in line with
B/d
reducing. But when
B/d
decreases, the gas resistance becomes small and
the pressure in discharging bottle reduces so that ion
density decreases. This can make the curvature ra-
dius of emitting surface become small and ion beam
2
Fig. 5. Effect of extraction voltage
V
b
on fo-
cusing degree
α
(b = 0.9D,
B
= 5d,
V
a
=600V,
P
= 6
×
10
−4
Pa).
Secondly, the effects of the inner diameter of ex-
traction electrode on
α
and
I
M
are measured with
extraction electrodes of different inner diameters by
adjusting other parameters to the best values. Ta-
ble 1 shows that
α
doesn’t basically change with
d
and that
I
M
roughly increases in proportion to the
square of
d.
Table 1. Effects of inner diameter
d
of extrac-
tion electrode on extraction voltage
I
M
and fo-
cusing degree
α
(D = 1.8d,
b
= 0.9D,
B
= 5d,
V
a
=600V,
V
b
=1.8kV,
P
= 6
×
10
−4
Pa).
d/mm
1
I
M
/µA
α
55
1.47
1.5
132
1.62
229
1.54
over focus. So the tendency of
α
rising slows down
when
B/d
is smaller than some value.
In the case that the geometry proportions of ex-
traction electrode don’t change, the shape of emitting
surface and the relative position of waist in extraction
electrode don’t change too.
I
M
and
I
K
simultaneously
increase or decrease with
d
rising or reducing, so the
focusing degree doesn’t change with
d.
Adding
d
is
3
Analysis
In the action of electric potential, the edge of
the plasma in ion source presents concave shape and
9
RF
905
equal to adding the area which ion beam goes
through, so
I
M
increases in proportion to
d
square.
of ion. The formula shows that the total electric cur-
rent emitting from emitting surface is in proportion
to three-second square of extraction voltage. In low
extraction voltage, focusing degree and total beam
intensity increase with extraction voltage rising, so
extraction beam intensity increases evidently. In high
extraction voltage, focusing degree decreases with ex-
traction voltage rising though total beam intensity
still increases in proportion to three-second square
of extraction voltage, so the tendency of extraction
beam intensity increasing slows down. This has been
proved in the experiments. Thus, raising extraction
beam intensity by adding extraction voltage when the
tendency of extraction beam intensity increasing al-
ready slows down isn’t beneficial to ion source. That
can distinctly reduce the lifetime of extraction elec-
trode.
Fig. 6. Three focusing states of ion beam.
(a) weak focusing; (b) the best focusing;
(c) over focusing.
When ion beam is in the best focusing state,
adding of oscillator power or pressure can make the
ion density in discharging bottle rise, the curvature
radius of emitting surface large, waist move down
(Fig. 6(a)), ion beam weakly focus and ions striking
the upper part of extraction electrode increase. On
the contrary, reducing of oscillator power or pressure
can make ion beam over focus. Extraction voltage is
opposite to oscillator power. In the best state, contin-
uous adding of extraction voltage can make the cur-
vature radius of emitting surface small and ion beam
over focus. On the contrary, it can make ion beam
weakly focus. So there are the best oscillator power
and the best extraction voltage. Those can be seen
from experimental results that adding
D/d
is benefi-
cial to ion beam focusing from over focusing and that
reducing
D/d
is beneficial to ion beam focusing from
weakly focusing.
The total electric current emitting from emitting
surface can be gotten according to the emitting for-
mula of concentric spherical extraction electrode
2
I
b
=
9
2e
m
p
1
2
3
4
Conclusion
Geometry parameters of extraction electrode, ex-
traction voltage, oscillator power and pressure are the
factors affecting the extraction property of an ion
source. They influence one another. Change of ge-
ometry parameters influences not only the focusing
ability of extraction electrode but also the pressure
in discharging bottle and the distribution of extrac-
tion voltage. There is the best
D/d
on condition that
other parameters don’t change.
D/d
rising is ben-
eficial to ion beam focusing from over focusing. Fo-
cusing degree increases and extraction beam intensity
drops with
b/D
increasing. Focusing degree rises with
B/d
decreasing and the rising tendency slows down
when
B/d
is smaller than 4. The suitable ranges of
D/d, b/D
and
B/d
are respectively 1.6—2.1, 0.7—
1.1 and 4—7 when considering comprehensively the
focusing degree, the extraction beam intensity and
the pressure. Change of extraction voltage and os-
cillator power has opposite effect on the ion source
performance and they have respectively best opera-
tion values.
V
b
2
1
−
cosθ
.
(−β)
2
In the formula,
θ
is the assembling angle of emitting
surface (Fig. 1).
β
is the function of
r/r
0
.
r
0
is curva-
ture radius of emitting surface , and
r
is the distance
between the curvature center of emitting surface and
the upper end of extraction electrode.
m
p
is the mass
906
( HEP & NP )
30
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