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Energy distribution of ions extracted from A 5 GHz ECR-ion source
186
Nuclear
Instruments
ENERGY DISTRIBUTION
OF IONS EXTRACTED
H. KGHLER,
B.A. HUBER
Imtrtut fir
M. FRANK,
Expenme,ltul-PhY.~lk
AG II.
and
Ruhr-Unrccwitiit
and Methods
m Physu
Research 823 (1987) 186-189 North-Holland. Amsterdam
FROM A 5 GHz ECR-ION SOURCE
K. WIESEMANN
Bochum,
FRG
The energy distributions of ions in different charge states extracted from an ECR ion source were analvsed using a double hemispherical energy analyser. The effect of the finite ion temperature can be separated from other mechanisms broadening the energy distribution. It is found that the ion temperature has only a small influence on the energetic width of the extracted ion beams. Additional broadening is enhanced by large extraction gaps and a large fieldstrength in the extraction gap. Pressure inside the source and applied microwave power turn out to be of negligible influence.
1. Introduction In order to perform translational energy spectroscopic studies of electron capture reactions by highly ionized projectiles we have replaced a simple electron beam ion source [l] by an ECR ion source [2.3], which delivers beams of highly charged ions at rather low beam energies (extraction voltages < 500 eV). For this kind of studies large ion currents are not required. however, the width of the energy distribution of the extracted beam is a very important entity. So far only one measurement does exist [4], yielding values between 6 and 40 eV per charge. depending on the ion source conditions. In the following we will discuss some more detailed results on the energy distribution of extracted ions and its dependence on different plasma- and beam parameters.
2. Experiment The discharge is created by a 5 GHz-klystron (microwave power: 25OG300 W,,,) within the field of a hexapole (2X cm in length) superimposed to the main mirror field. In order to allow beam transportation at low kinetic energies the extraction system is housed in a shielding soft iron cylinder, which also serves to concentrate the magnetic mirror field. The mirror field. which by a proper shaping of the soft iron pieces is made slightly asymmetric in order to support the extraction, drops down very steeply near the extraction region. where it reaches very small values. If not otherwise specified, measurements were performed using for extraction an aperture 2 mm in diameter. The orifice diameter in the acceleration electrode, which is 4 mm apart, amounts to 3 mm. The ion beam formed by the aid of an einzel lens and zoom lens system - is focussed onto the entrance slit of a mass 0168-583X/X7/$03.50 Cc Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
spectrometer and may be adjusted by a steering plate unit. After passing a magnetic sector field the ion beam is shot into a translational energy spectrometer [5].
3. Results and discussion The ion source was operated at gas pressures between 10 ’ and 10 ’ Pa depending on the desired charge state and the source gas used. A typical mass/charge spectrum for ions extracted from an Ar discharge is displayed in fig. 1, showing charge states up to q = 13. The intensity of different peaks can be optimized by adjusting the voltages in the lens and deflection system indicating differences in the emittance or regions of extraction for different charge states. Whereas the main spectrum in fig. 1 was optimized for q = 8. the amplified part of the spectrum was measured with scttings optimized for the charge state y = 11. The energy distribution of the extracted ions was measured by the aid of two hemispherical condenser plates. By scanning the acceleration voltage in front of the analyser the energy spectra were measured at a kinetic energy of 5004 (eV). The counting rate as function of the acceleration voltage is shown in fig. 2a for doubly charged nitrogen ions. In fig. 2b the same curve is shown on a logarithmic scale. The energy distribution is rather asymmetric, showing the shape of an accelerated Maxwellian. Thus from the logarithmic slope of the high energy tail the ion temperature inside the plasma can be evaluated. A deviation of the halfwidth from that of an accelerated Maxwellian with this temperature indicates further processes increasing the energy width of the beam. In the following we, therefore, characterize the measured energy distributions by their energetic halfwidth (after correction for the finite resolution of the analyser) SE (fwhm) and the ion temperature kT, as well.
187
Fig. 1. Charge
atate spectrum
of Ar”
’ ions extracted
Fig. 3 shows the results for SE as function of the charge state 4, measured with ion source gases Ne. Ar and N?. We find the results in the atomic gases Ne and Ar to be quite similar, whereas the molecular case yields much narrower energy distributions. The error bars describe those changes which result from different voltage settings of the ion optic and ion beam guiding system as well as from the general reproduceability of the discharge conditions. For Ne and Ar we find an average increase of SE with the charge state 4, ranging from - 12 eV for
from the ECR-ion
source
singly charged ions to - 24 eV for Ar”+. However for low charge states, especially for q = 2, the values of the measured halfwidth seem to be strongly enhanced. In the case of nitrogen the SE values vary from - 3 to 6 eV if q is changed from 1 to 4, which is much smaller than in the rare gas case. However, this effect may partly be due to a larger extraction gap and hence weaker extraction field used for the nitrogen measurements. In order to distinguish between different mechanisms, which might influence the energetic halfwidth of
2eV
505.29
Fig. 2. Energy distribution
v
-
c!;
196.L8V
of doubly charged nitrogen ions ( p = 2 X 10 4 Pa: .Y = 230 W). (a) linear plot of the counting the deceleration voltage: (b) logarithmic plot of the energy distribution.
rate versus
V. PRODUCTION/APPLICATIONS
the extracted ion energy distributions, wc dctcrmined the ion temperature as function of the charge state from the exponential decay of the distribution functions. The result is shown in fig. 4. The measured ion temperatures vary between about 1 and 3 cV and show a alight increase for more highly charged ions. This latter increase, however. is considered to be due to the increasing influence of noise on the spectra of the higher charge states, i.e. as an artcfact. The ion temperaturc being essentially the same for all charge states. Whereas the results in molecular nitrogen and argon are rather similar. the ion temperatures in neon discharges seem to be larger by 1 to 2 eV. Assuming an ideal plasma boundary and a Maxwellian velocity distribution of the ions inside the plasma layer, the energetic halfwidth of an accelerated half Maxwellian distribution should be given by 2.5 kT,, if the acceleration voltage is iarge compared to kT, [6]. We have corrected the measured 6E values with regard to the ion temperature cont~bution in order to separate the influence of the ion extraction itself. If variations of the space potential in the extraction region are consid-
ered as possible broadening mechanism. the corrected halfwidth d should increase linearly with y. Therefore fig. 5 shows the ratios (d/q) versus 4. In the case of N, we find the predicted behaviour: (A/q) is nearly constant with a value of - 1.5 eV per charge. However, for both rare gases (3/q) shows a definite increase towards lower charge states. Whereas for 4 larger than 6 the values scatter around 2 eV per charge. (3/y) is enhanced by a factor of - 6 for q = 1.2. At present this finding is not very clear; as a possible reason nonlinear energy transfer from high to low charge states occurring in the extraction region may be considered, as in this region ions with different charge states have different velocity and stream through each other. The results described so far have been otained for the following parameter settings: gas pressure in the ion source: 10-s to 10m4 Pa for low charge states: 10~4-10-’ Pa for higher charge states: microwave power: 200 W; extraction voltage: 400-600 V; diameter of the extraction hole: 2 mm, ion energy : (500-600) y eV. In the following we wifl discuss, how the energetic halfwidth of the measured energy distribution is influenced by these parameters. In the case of NC and N’+ ions no significant pressure dependence over the range considered was observed. Likewise a change in the applied microwave power did not affect the energy distribution, These results seem plausible, as the ion temperature contributes to the total energetic width by a small amount only and the mean free path is always longer than the dimension of the discharge. Concerning the influence of the extraction voltage and geometry we find with constant geometry an almost linear increase of the halfwidth with the extraction voltage. Increasing the aperture diameter from 2 to 4 mm and the gap from 4 to 8 mm results in a slight increase of the measured halfwidths when equal voltages are
L,
0 Fig. 4. Ion temperature
XT, as function of y. 0: Ar: 0 NC; 0
NL.
:L ---7
.
6
_~(__,___ a lo
Fig. 5. Reduced energy width per charge (J/y) Ar; 0 Ne: 0 NJ.
1
9
wrws
y, 0
as a further
hint to the occurrence of nonlinear effects in the extraction region. The larger values measured by Meyer [4] may be partly due to the larger diameter of the used extraction aperture, which amounts to 8 mm.
This work has been supported Deutsche Forsehungsgemeinschaft, acknowledged.
by a grant for the which is gratefully
References
considered (see fig. 6). However, the decisive quality is the electric field, which at the same voltage is half of that in the small gap, when using the larger gap. Thus we see, that at the same fieldstrength the larger aperture results in more than twice the halfwidth as compared to that of the small aperture. This can not simply be explained by the increase of the drainage region, from where the ions stem. We, therefore. consider this finding
H.J. Kablert, K. Wiesemann and B.A. Huber, Ann. Phys. (Leipzig) 42 (1985) 133. See e.g. Proc. Int. ion Eng. Congress ISfAT 83 &i 1PplT 83 KYOTC) (1983) Voi. 1 (Institute El. Eng.. Tokyo. Japan, 19X3). Contributed papers of the 7. Workshop on ECR Ion Sources. Jfilich 1986, ed.. H. Beuscher. Jill-Cnnf-57 (July. 1986) and references therein. F.W. Meyer: Nucl. Instr. and Methods B9 (1985) 532. H.J. Kahlert. B.A. Huber and K. Wiesemann, J. Phvs. Bl6 (1983) 449. K. Wiesrmann, 1. Phys. 14 (1981) 1404.
Nuclear
Instruments
ENERGY DISTRIBUTION
OF IONS EXTRACTED
H. KGHLER,
B.A. HUBER
Imtrtut fir
M. FRANK,
Expenme,ltul-PhY.~lk
AG II.
and
Ruhr-Unrccwitiit
and Methods
m Physu
Research 823 (1987) 186-189 North-Holland. Amsterdam
FROM A 5 GHz ECR-ION SOURCE
K. WIESEMANN
Bochum,
FRG
The energy distributions of ions in different charge states extracted from an ECR ion source were analvsed using a double hemispherical energy analyser. The effect of the finite ion temperature can be separated from other mechanisms broadening the energy distribution. It is found that the ion temperature has only a small influence on the energetic width of the extracted ion beams. Additional broadening is enhanced by large extraction gaps and a large fieldstrength in the extraction gap. Pressure inside the source and applied microwave power turn out to be of negligible influence.
1. Introduction In order to perform translational energy spectroscopic studies of electron capture reactions by highly ionized projectiles we have replaced a simple electron beam ion source [l] by an ECR ion source [2.3], which delivers beams of highly charged ions at rather low beam energies (extraction voltages < 500 eV). For this kind of studies large ion currents are not required. however, the width of the energy distribution of the extracted beam is a very important entity. So far only one measurement does exist [4], yielding values between 6 and 40 eV per charge. depending on the ion source conditions. In the following we will discuss some more detailed results on the energy distribution of extracted ions and its dependence on different plasma- and beam parameters.
2. Experiment The discharge is created by a 5 GHz-klystron (microwave power: 25OG300 W,,,) within the field of a hexapole (2X cm in length) superimposed to the main mirror field. In order to allow beam transportation at low kinetic energies the extraction system is housed in a shielding soft iron cylinder, which also serves to concentrate the magnetic mirror field. The mirror field. which by a proper shaping of the soft iron pieces is made slightly asymmetric in order to support the extraction, drops down very steeply near the extraction region. where it reaches very small values. If not otherwise specified, measurements were performed using for extraction an aperture 2 mm in diameter. The orifice diameter in the acceleration electrode, which is 4 mm apart, amounts to 3 mm. The ion beam formed by the aid of an einzel lens and zoom lens system - is focussed onto the entrance slit of a mass 0168-583X/X7/$03.50 Cc Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
spectrometer and may be adjusted by a steering plate unit. After passing a magnetic sector field the ion beam is shot into a translational energy spectrometer [5].
3. Results and discussion The ion source was operated at gas pressures between 10 ’ and 10 ’ Pa depending on the desired charge state and the source gas used. A typical mass/charge spectrum for ions extracted from an Ar discharge is displayed in fig. 1, showing charge states up to q = 13. The intensity of different peaks can be optimized by adjusting the voltages in the lens and deflection system indicating differences in the emittance or regions of extraction for different charge states. Whereas the main spectrum in fig. 1 was optimized for q = 8. the amplified part of the spectrum was measured with scttings optimized for the charge state y = 11. The energy distribution of the extracted ions was measured by the aid of two hemispherical condenser plates. By scanning the acceleration voltage in front of the analyser the energy spectra were measured at a kinetic energy of 5004 (eV). The counting rate as function of the acceleration voltage is shown in fig. 2a for doubly charged nitrogen ions. In fig. 2b the same curve is shown on a logarithmic scale. The energy distribution is rather asymmetric, showing the shape of an accelerated Maxwellian. Thus from the logarithmic slope of the high energy tail the ion temperature inside the plasma can be evaluated. A deviation of the halfwidth from that of an accelerated Maxwellian with this temperature indicates further processes increasing the energy width of the beam. In the following we, therefore, characterize the measured energy distributions by their energetic halfwidth (after correction for the finite resolution of the analyser) SE (fwhm) and the ion temperature kT, as well.
187
Fig. 1. Charge
atate spectrum
of Ar”
’ ions extracted
Fig. 3 shows the results for SE as function of the charge state 4, measured with ion source gases Ne. Ar and N?. We find the results in the atomic gases Ne and Ar to be quite similar, whereas the molecular case yields much narrower energy distributions. The error bars describe those changes which result from different voltage settings of the ion optic and ion beam guiding system as well as from the general reproduceability of the discharge conditions. For Ne and Ar we find an average increase of SE with the charge state 4, ranging from - 12 eV for
from the ECR-ion
source
singly charged ions to - 24 eV for Ar”+. However for low charge states, especially for q = 2, the values of the measured halfwidth seem to be strongly enhanced. In the case of nitrogen the SE values vary from - 3 to 6 eV if q is changed from 1 to 4, which is much smaller than in the rare gas case. However, this effect may partly be due to a larger extraction gap and hence weaker extraction field used for the nitrogen measurements. In order to distinguish between different mechanisms, which might influence the energetic halfwidth of
2eV
505.29
Fig. 2. Energy distribution
v
-
c!;
196.L8V
of doubly charged nitrogen ions ( p = 2 X 10 4 Pa: .Y = 230 W). (a) linear plot of the counting the deceleration voltage: (b) logarithmic plot of the energy distribution.
rate versus
V. PRODUCTION/APPLICATIONS
the extracted ion energy distributions, wc dctcrmined the ion temperature as function of the charge state from the exponential decay of the distribution functions. The result is shown in fig. 4. The measured ion temperatures vary between about 1 and 3 cV and show a alight increase for more highly charged ions. This latter increase, however. is considered to be due to the increasing influence of noise on the spectra of the higher charge states, i.e. as an artcfact. The ion temperaturc being essentially the same for all charge states. Whereas the results in molecular nitrogen and argon are rather similar. the ion temperatures in neon discharges seem to be larger by 1 to 2 eV. Assuming an ideal plasma boundary and a Maxwellian velocity distribution of the ions inside the plasma layer, the energetic halfwidth of an accelerated half Maxwellian distribution should be given by 2.5 kT,, if the acceleration voltage is iarge compared to kT, [6]. We have corrected the measured 6E values with regard to the ion temperature cont~bution in order to separate the influence of the ion extraction itself. If variations of the space potential in the extraction region are consid-
ered as possible broadening mechanism. the corrected halfwidth d should increase linearly with y. Therefore fig. 5 shows the ratios (d/q) versus 4. In the case of N, we find the predicted behaviour: (A/q) is nearly constant with a value of - 1.5 eV per charge. However, for both rare gases (3/q) shows a definite increase towards lower charge states. Whereas for 4 larger than 6 the values scatter around 2 eV per charge. (3/y) is enhanced by a factor of - 6 for q = 1.2. At present this finding is not very clear; as a possible reason nonlinear energy transfer from high to low charge states occurring in the extraction region may be considered, as in this region ions with different charge states have different velocity and stream through each other. The results described so far have been otained for the following parameter settings: gas pressure in the ion source: 10-s to 10m4 Pa for low charge states: 10~4-10-’ Pa for higher charge states: microwave power: 200 W; extraction voltage: 400-600 V; diameter of the extraction hole: 2 mm, ion energy : (500-600) y eV. In the following we wifl discuss, how the energetic halfwidth of the measured energy distribution is influenced by these parameters. In the case of NC and N’+ ions no significant pressure dependence over the range considered was observed. Likewise a change in the applied microwave power did not affect the energy distribution, These results seem plausible, as the ion temperature contributes to the total energetic width by a small amount only and the mean free path is always longer than the dimension of the discharge. Concerning the influence of the extraction voltage and geometry we find with constant geometry an almost linear increase of the halfwidth with the extraction voltage. Increasing the aperture diameter from 2 to 4 mm and the gap from 4 to 8 mm results in a slight increase of the measured halfwidths when equal voltages are
L,
0 Fig. 4. Ion temperature
XT, as function of y. 0: Ar: 0 NC; 0
NL.
:L ---7
.
6
_~(__,___ a lo
Fig. 5. Reduced energy width per charge (J/y) Ar; 0 Ne: 0 NJ.
1
9
wrws
y, 0
as a further
hint to the occurrence of nonlinear effects in the extraction region. The larger values measured by Meyer [4] may be partly due to the larger diameter of the used extraction aperture, which amounts to 8 mm.
This work has been supported Deutsche Forsehungsgemeinschaft, acknowledged.
by a grant for the which is gratefully
References
considered (see fig. 6). However, the decisive quality is the electric field, which at the same voltage is half of that in the small gap, when using the larger gap. Thus we see, that at the same fieldstrength the larger aperture results in more than twice the halfwidth as compared to that of the small aperture. This can not simply be explained by the increase of the drainage region, from where the ions stem. We, therefore. consider this finding
H.J. Kablert, K. Wiesemann and B.A. Huber, Ann. Phys. (Leipzig) 42 (1985) 133. See e.g. Proc. Int. ion Eng. Congress ISfAT 83 &i 1PplT 83 KYOTC) (1983) Voi. 1 (Institute El. Eng.. Tokyo. Japan, 19X3). Contributed papers of the 7. Workshop on ECR Ion Sources. Jfilich 1986, ed.. H. Beuscher. Jill-Cnnf-57 (July. 1986) and references therein. F.W. Meyer: Nucl. Instr. and Methods B9 (1985) 532. H.J. Kahlert. B.A. Huber and K. Wiesemann, J. Phvs. Bl6 (1983) 449. K. Wiesrmann, 1. Phys. 14 (1981) 1404.