Study on space charge effect in an electrostatic ion analyzer applied to measure laser produced ions Q. Y. Jin, H. Y. Zhao, S. Sha, J. J. Zhang, Zh. M. Li, W. Liu, X. Zh. Zhang, L. T. Sun, and H. W. Zhao Citation: Review of Scientific Instruments 85, 033307 (2014); doi: 10.1063/1.4869015 View online: http://dx.doi.org/10.1063/1.4869015 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Space-charge compensation measurements in electron cyclotron resonance ion source low energy beam transport lines with a retarding field analyzera) Rev. Sci. Instrum. 85, 02A739 (2014); 10.1063/1.4854315 Shot-to-shot reproducibility in the emission of fast highly charged metal ions from a laser ion sourcea) Rev. Sci. Instrum. 83, 02B302 (2012); 10.1063/1.3655528 The DCU laser ion source Rev. Sci. Instrum. 81, 043305 (2010); 10.1063/1.3374123 Magnetic field influence on laser-produced ion stream Rev. Sci. Instrum. 75, 1353 (2004); 10.1063/1.1711188 Generation of intense streams of metallic ions with a charge state up to 10+ in a laser ion source Rev. Sci. Instrum. 75, 1575 (2004); 10.1063/1.1691520
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 033307 (2014)
Study on space charge effect in an electrostatic ion analyzer applied to measure laser produced ions Q. Y. Jin,1,2 H. Y. Zhao,1,a) S. Sha,1 J. J. Zhang,1 Zh. M. Li,1,2 W. Liu,1,2 X. Zh. Zhang,1 L. T. Sun,1 and H. W. Zhao1 1 2
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
(Received 26 December 2013; accepted 8 March 2014; published online 25 March 2014) The abundance of different ions produced by laser ion sources is usually analyzed by an electrostatic ion analyzer (EIA). Ion current intensities in the range of several mA/cm2 at the position of the EIA have been achieved from the laser ion source developed by the Institute of Modern Physics; this indicates that a noticeable influence of space charge effect during the ion transmission will occur. Hence, while the parameters of the EIA or the beams are changed, such as ion species, current intensity, the ions’ transmission efficiency through the EIA is different, which will result in an uncertainty in the estimation of the ions’ yields. Special attention is focused on this issue in this paper. Ion’s transmissions through the EIA under different circumstances are studied with simulations and experiments, the results of which are consistent with each other. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4869015] I. INTRODUCTION
II. PRINCIPLE OF EIA
Laser Produced Plasma (LPP) has been intensively studied during the last decades due to its potential applications in various fields, one among which is providing pulsed high intensity and high charge state ion beams from solids for heavy ion accelerators.1, 2 For this application, the ion current intensity, abundance of ions with different charge state, and ion energy are very important parameters. Generally, the abundance and energy of different charge state ions produced by a laser ion source are analyzed by an electrostatic ion analyzer (EIA),3 which has an advantage in the diagnostics of LPP within low energy range (tens of eV to tens of keV). The structure of an EIA is very simple, and the cost is lower compared with other diagnostic instruments. The main drawback of EIA is the requirement of a large number of laser shots to obtain the results, which needs a good repeatability of laser system. In laser ion source experiments, a cylindrical parallel EIA combined with the time-of-flight method, is usually used as an energy analyzer with a deflection angle of 90◦ , which means that the measurement of ion velocity consists in measuring the time of flight of the ions passing the distance from the target to the detector. For diagnostics of laser produced ions using an EIA, the principle of operation and detailed understanding of the optical properties are required. The ability for an EIA to distinguish different ion species depends on its parameters, such as radius of deflection, slit width of entrance and exit, the width between two cylindrical plates. Therefore, appropriate parameter setting will give a high accuracy to estimate the abundance of different ion species produced by laser ion source. The simulation and experimental results about ion transmission during the EIA under different beam and EIA parameters will be discussed in this paper.
Fig. 1 is a cross-section drawing of the EIA. R1, R2, R, and R are the inner, outer, mean radius of the deflection plates, and the gap width between the two cylindrical plates, respectively; |V1| = V2 = U/2 is the voltage applied on the plates; s1 and s2 are the width of the entrance and exit slits, respectively. When the EIA is given a voltage U, only ions with certain energy to charge ratio can get through it. The relationship4 is given by
a) Electronic mail: [email protected].
0034-6748/2014/85(3)/033307/4/$30.00
E/Z = keU,
(1)
where E is ion’s energy, Z is the charge state, k = R/2R, e is elementary charge. For the ions transmitted through EIA, the horizontal trajectories can be written in a matrix form5 ⎤ ⎡ ⎡ ⎤ x0 x ⎢ ⎢ ⎥ ⎥ (2) ⎣ x ⎦ = M ⎣ x0 ⎦ , E E0 where x0 , x0 , and E0 are the ions’ initial coordinate to central orbit, incident angle, and energy spread before entering plates, respectively; x, x , and E are the ions’ coordinate position, angle, and energy spread after getting out the cylindrical plates, respectively; M is the transmission matrix of EIA. Because the change of energy spread is negligible, M can be expressed as5 √ √ ⎤ √ ⎡ R sin π2 2 R2 (1 − cos π2 2) cos π2 3 2 √ √ √ ⎥ ⎢ √ M = ⎣ − 2 sin( π 2) √1 sin π 2 ⎦. cos π 2 2
R
0
2
0
2
2
1
(3) In the theoretical calculation, ions’ incident angle is smaller by two orders compared to other parameters, so ions’ trajectories are mainly determined by x0 and E0 . The real EIA parameters, R1, R2, R, and R used in the simulation and experiments are 90 mm, 110 mm 100 mm, and 20 mm,
85, 033307-1
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033307-2
Jin et al.
Rev. Sci. Instrum. 85, 033307 (2014) TABLE I. Parameters used for EIA simulation. Current density (μA/mm2 )
Energy to charge state ratio (eV)
Ta1+ Ta5+
1.625 8.124
1000 2500
Al1+ Al11+
8.124 32.497
500 1800
Ion
FIG. 1. Cross-section view of an EIA with a deflection angle 90◦ .
respectively. While s1 = 1 mm, s2 = 4 mm, assuming they are very close to the plates, the maximum energy spread of ions getting through the plates is 3.06% theoretically. In fact, the distance between s1 (and s2) and plates, and also the space charge (SC) effect should be taken into consideration, hence the real energy spread should be lower than 3.06%. Between s1 (and s2) and plates is a slit with a width 20 mm, which is used to shield fringe field. The potential distribution is also displayed in Fig. 1.
get through EIA except Ta5+ . As to Ta1+ ions with energy 2.5 keV, it is the same situation as above. For each element, two E/Z values are chosen to compare the transmission efficiency at different particle energy. Then the transmission efficiency under different parameters is simulated. The software CST particle studio is used to simulate the effect of different beam parameters on particle transmission. In the simulations, the effect of electrons is not taken into consideration since plasma will separate into ion and electron components immediately on entering the electrode gap. The simulation results of Ta1+ , Ta5+ ions are displayed in Figure 2. Figure 2(a) shows the results of E/Z = 2500 eV. Ions getting through EIA with a current density of 1.625 μA/mm2 will increase as slit1 increases, while ions with a current density 8.124 μA/mm2 will decrease due to a stronger space
III. SIMULATION RESULTS
At the location of EIA, ion current intensities in the range of several mA/cm2 have been achieved from our laser ion source6, 7 with a Nd:YAG laser, whose energy shot on target is less than 2 J, FWHM 8-10 ns, and wavelength 1064 nm. This indicates that space charge effect on the ions’ transmission during the EIA cannot be neglected.4 Actually, in the experiments only the s1 and s2 can be changed, and by narrowing the width of slits and hence limiting the current intensity getting through the EIA, the influence of space charge effect can be decreased. In the simulation, transmission efficiency of heavy element Ta and light element Al ions with different charge states, different current intensities, and different energy to charge ratios are compared. The detailed parameters used in the simulation are given in Table I. Here, the current density is used as a correction for space charge effect, but not the real current density of any charge state, and it is set to be close to the total current of all charge states arriving entrance slit s1 during experiments. For example, when EIA is given a certain voltage in the experiments, only ions with the same E/Z = 2.5 keV can get through EIA, such as Ta5+ with an energy of 5 × 2.5 keV, Ta1+ with an energy of 2.5 keV, but when Ta5+ ions with energy 5 × 2.5 keV get into EIA, the ions (all of the other charge states) with the energy 5 × 2.5 keV will also get into EIA at the same time. So the space charge effect will come from total current of all charge states, even if the other charge state ions with the energy of 5 × 2.5 keV cannot
FIG. 2. Simulation results of Ta ions under different EIA and beam parameters of (a) E/Z = 2500 eV and (b) E/Z = 1000 eV.
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Rev. Sci. Instrum. 85, 033307 (2014)
FIG. 5. EMT signal amplitudes for Ta ions with different charge state at different EIA slit width.
The simulation results of Al1+ , Al11+ ions are displayed in Figure 3, which are much different from the results of Ta. Figure 3(a) shows the result of E/Z = 1800 eV. For Al1+ ions, no matter what the current density it is, the transmission efficiency will decrease as slit increases. Al11+ ions with current density 8.124 μA/mm2 will increase as slit increases, while the current density is 32.497 μA/mm2 , it will increase first, and then decrease because of higher energy. The same situation appears in Figure 3(b) on Al11+ ion with current density 8.124 μA/mm2 , E/Z = 500 eV. As to other parameters simulated in Figure 3(b), it shows a decrease in tendency with the increase of slit because of lower particle energy. IV. EXPERIMENTAL RESULTS FIG. 3. Simulation results of Al ions under different EIA and beam parameters of (a) E/Z = 1800 eV and (b) E/Z = 500 eV.
charge effect. As to the ions of different charge state (Ta1+ , Ta5+ ) with the same current intensity and E/Z, the particle transmission efficiency of Ta5+ is much larger than Ta1+ . Because Ta5+ ions have higher energy, the effect of space charge is weaker. Figure 2(b) shows the result of E/Z = 1000 eV. The particle transmission efficiency through EIA is much smaller than E/Z = 2500 eV and it also shows a tendency of decrease as a whole.
FIG. 4. Time of flight spectrum of Ta ions through EIA.
To verify the simulation results above, experiments under the similar conditions were carried out. In the experiments, a faraday cup (FC) with an aperture of 28 mm, 80 cm before slit s1, was used to measure the total ion current intensity produced by the laser ion source, and an electron multiplier tube (EMT) with an input aperture of 20 mm was put 20 cm downstream of slit s2 to measure the time of flight spectrum of different charge state ions through EIA. The peak current of Ta, Al measured by FC are 2.08 mA (3.38 μA/mm2 ), and 14.35 mA (23.317 μA/mm2 ), respectively. So it is appropriate to assume a current density as listed in Table I as a correction for space charge effect in the simulation above, and time
FIG. 6. EMT signal amplitudes for Al ions with different charge state at different EIA slit width.
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033307-4
Jin et al.
Rev. Sci. Instrum. 85, 033307 (2014)
cally for different ion species, ion energy, and EIA parameters with simulations and experiments. It is shown that the transmission efficiency varies a lot as the above-mentioned parameters are changed due to the very strong space charge effect. From the simulations and experiments, the following conclusions about the influence of SC on ion transmission in the EIA could be drawn:
FIG. 7. EMT signal amplitudes for Ag ions with different charge state at different EIA slit width.
of flight spectrum is also measured around peak current intensity. The reason to measure the spectrum around the peak current is that only within this area the phenomena in the simulation can be observed obviously. Figure 4 is an EMT output signal of Ta ions through the EIA. Peak amplitudes of the different charge states in the spectrum are measured to compare the influence under different beam and EIA parameters. Figure 5 shows the results of Ta experiments under the conditions of E/Z = 2682.5 eV. Amplitudes of Ta2+ -Ta5+ will decrease as slit increases because of lower energy and higher current, and it is the same as the simulation result of Figure 2(a). The current intensity of Ta6+ is lower, so it shows an increasing tendency. Though current intensity of Ta7+ looks high enough, it also has a high energy, so the fluctuation is not that large. From Figure 7, we can also see that no matter which one of the slits is changed; it will affect the ions’ transmission efficiency. More detailed results will be presented in the experiments of Al and Ag. Figure 6 shows the results of Al experiments with E/Z = 1810 eV. It presents the similar trend with the simulation results shown in Figure 3(a). For lower charge states, such as Al7+ , Al8+ , Al9+ , the amplitudes decrease as slit increases. As to higher charge state ions, with the increasing of slit width, the amplitudes will increase first, and then decrease. Figure 7 is the results of Ag by changing the width of slit s2. It shows the same tendency just like changing slit s1. The peak amplitudes of lower charge states, such as Ag4+ -Ag10+ , will reduce with the increase of slit2 width. While for higher charge states, like Ag11+ -Ag14+ , even with larger current intensities than lower charge states, only small fluctuations are detected by the EMT because of their higher energy. These results indicate that even for the very limited current that are transmitted by the EIA, the space charge effect still plays an important role, which causes the divergence of the ion beam in the path between the slit s2 and the EMT. V. DISCUSSION AND CONCLUSIONS
Transmission efficiency of laser produced ion beams during an electrostatic ion analyzer has been studied systemati-
(1) At a certain EIA voltage, the influence of SC on lower charge states is more evident than on higher charge states. (2) The influence of SC is more apparent at lower EIA voltage compared with that at higher voltage. (3) Due to the above terms, the transmitted current by the EIA could decrease with the increasing slit widths especially in the case of lowly charge ions or low EIA voltages. These phenomena we observed are consistent with the formula in Ref. 2 about the space-charge limited ion current in the ribbon-shaped beam model8 2Ze 1/2 h(yf − yi ) 3/2 I = 4ε0 E . (4) m l2 Here, yi h is the initial cross-section of the beam and yf h the expanded cross-section after the interaction length l; ε0 is the permittivity of vacuum. Therefore, for laser produced plasma measurement one need to use an EIA with caution even it is for analyzing the relative charge state distribution. To limit the total current entering the EIA and hence the space charge effect at the beginning stage, the width of the input slit should be set as thin as possible. In our case, this value was set as 0.2 mm. Narrowing the output slit and shortening the distance between the EIA and downstream detector (like an EMT) would help to minimize the influence of space charge out of the analyzer. ACKNOWLEDGMENTS
This work is supported by China Natural Science Foundation (NSF) (Grant Nos. 11275239 and 11221064) and 100 Talents Program of the Chinese Academy of Sciences (CAS) (Grant No. Y214160BR0). 1 A.
Balabaev, V. Balanusta, S. Kondrashev, K. Konukov, A. Lozhkin et al., Rev. Sci. Instrum. 73, 773 (2002). 2 M. Okamura, T. Katayama, R. A. Jameson, T. Takeuchi, T. Hattori, and N. Hayashizaki, Nucl. Instrum. Methods Phys. Res. B 188, 216 (2002). 3 S. Shan, Q. Y. Jin, Z. M. Li, X. H. Guo, L. C. Zhou et al., Chin. Phys. C 37, 117006 (2013). 4 E. Woryna, P. Parys, J. Wolowski, and W. Mroz, Laser Part. Beams 14, 293 (1996). 5 H. Q. Xia and Ch. L. Liu, in Theory of Beam Transport, edited by X. M. Cao (Xi’an Jiaotong University Press, Xi’an, 1991), p. 124. 6 H. Y. Zhao, Q. Y. Jin, S. Sha, J. J. Zhang, Z. M. Li et al., Rev. Sci. Instrum. 85, 02B910 (2014). 7 S. Sha, H. W. Zhao, X. H. Guo, Z. L. Zhang, X. Fang et al., Rev. Sci. Instrum. 83, 02B303 (2012). 8 H. Bader, K. Rohr, and H. Weber, J. Phys. D: Appl. Phys. 13, L149–L151 (1980).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 134.129.164.186 On: Sat, 20 Dec 2014 08:05:58
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 134.129.164.186 On: Sat, 20 Dec 2014 08:05:58
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 033307 (2014)
Study on space charge effect in an electrostatic ion analyzer applied to measure laser produced ions Q. Y. Jin,1,2 H. Y. Zhao,1,a) S. Sha,1 J. J. Zhang,1 Zh. M. Li,1,2 W. Liu,1,2 X. Zh. Zhang,1 L. T. Sun,1 and H. W. Zhao1 1 2
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
(Received 26 December 2013; accepted 8 March 2014; published online 25 March 2014) The abundance of different ions produced by laser ion sources is usually analyzed by an electrostatic ion analyzer (EIA). Ion current intensities in the range of several mA/cm2 at the position of the EIA have been achieved from the laser ion source developed by the Institute of Modern Physics; this indicates that a noticeable influence of space charge effect during the ion transmission will occur. Hence, while the parameters of the EIA or the beams are changed, such as ion species, current intensity, the ions’ transmission efficiency through the EIA is different, which will result in an uncertainty in the estimation of the ions’ yields. Special attention is focused on this issue in this paper. Ion’s transmissions through the EIA under different circumstances are studied with simulations and experiments, the results of which are consistent with each other. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4869015] I. INTRODUCTION
II. PRINCIPLE OF EIA
Laser Produced Plasma (LPP) has been intensively studied during the last decades due to its potential applications in various fields, one among which is providing pulsed high intensity and high charge state ion beams from solids for heavy ion accelerators.1, 2 For this application, the ion current intensity, abundance of ions with different charge state, and ion energy are very important parameters. Generally, the abundance and energy of different charge state ions produced by a laser ion source are analyzed by an electrostatic ion analyzer (EIA),3 which has an advantage in the diagnostics of LPP within low energy range (tens of eV to tens of keV). The structure of an EIA is very simple, and the cost is lower compared with other diagnostic instruments. The main drawback of EIA is the requirement of a large number of laser shots to obtain the results, which needs a good repeatability of laser system. In laser ion source experiments, a cylindrical parallel EIA combined with the time-of-flight method, is usually used as an energy analyzer with a deflection angle of 90◦ , which means that the measurement of ion velocity consists in measuring the time of flight of the ions passing the distance from the target to the detector. For diagnostics of laser produced ions using an EIA, the principle of operation and detailed understanding of the optical properties are required. The ability for an EIA to distinguish different ion species depends on its parameters, such as radius of deflection, slit width of entrance and exit, the width between two cylindrical plates. Therefore, appropriate parameter setting will give a high accuracy to estimate the abundance of different ion species produced by laser ion source. The simulation and experimental results about ion transmission during the EIA under different beam and EIA parameters will be discussed in this paper.
Fig. 1 is a cross-section drawing of the EIA. R1, R2, R, and R are the inner, outer, mean radius of the deflection plates, and the gap width between the two cylindrical plates, respectively; |V1| = V2 = U/2 is the voltage applied on the plates; s1 and s2 are the width of the entrance and exit slits, respectively. When the EIA is given a voltage U, only ions with certain energy to charge ratio can get through it. The relationship4 is given by
a) Electronic mail: [email protected].
0034-6748/2014/85(3)/033307/4/$30.00
E/Z = keU,
(1)
where E is ion’s energy, Z is the charge state, k = R/2R, e is elementary charge. For the ions transmitted through EIA, the horizontal trajectories can be written in a matrix form5 ⎤ ⎡ ⎡ ⎤ x0 x ⎢ ⎢ ⎥ ⎥ (2) ⎣ x ⎦ = M ⎣ x0 ⎦ , E E0 where x0 , x0 , and E0 are the ions’ initial coordinate to central orbit, incident angle, and energy spread before entering plates, respectively; x, x , and E are the ions’ coordinate position, angle, and energy spread after getting out the cylindrical plates, respectively; M is the transmission matrix of EIA. Because the change of energy spread is negligible, M can be expressed as5 √ √ ⎤ √ ⎡ R sin π2 2 R2 (1 − cos π2 2) cos π2 3 2 √ √ √ ⎥ ⎢ √ M = ⎣ − 2 sin( π 2) √1 sin π 2 ⎦. cos π 2 2
R
0
2
0
2
2
1
(3) In the theoretical calculation, ions’ incident angle is smaller by two orders compared to other parameters, so ions’ trajectories are mainly determined by x0 and E0 . The real EIA parameters, R1, R2, R, and R used in the simulation and experiments are 90 mm, 110 mm 100 mm, and 20 mm,
85, 033307-1
© 2014 AIP Publishing LLC
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033307-2
Jin et al.
Rev. Sci. Instrum. 85, 033307 (2014) TABLE I. Parameters used for EIA simulation. Current density (μA/mm2 )
Energy to charge state ratio (eV)
Ta1+ Ta5+
1.625 8.124
1000 2500
Al1+ Al11+
8.124 32.497
500 1800
Ion
FIG. 1. Cross-section view of an EIA with a deflection angle 90◦ .
respectively. While s1 = 1 mm, s2 = 4 mm, assuming they are very close to the plates, the maximum energy spread of ions getting through the plates is 3.06% theoretically. In fact, the distance between s1 (and s2) and plates, and also the space charge (SC) effect should be taken into consideration, hence the real energy spread should be lower than 3.06%. Between s1 (and s2) and plates is a slit with a width 20 mm, which is used to shield fringe field. The potential distribution is also displayed in Fig. 1.
get through EIA except Ta5+ . As to Ta1+ ions with energy 2.5 keV, it is the same situation as above. For each element, two E/Z values are chosen to compare the transmission efficiency at different particle energy. Then the transmission efficiency under different parameters is simulated. The software CST particle studio is used to simulate the effect of different beam parameters on particle transmission. In the simulations, the effect of electrons is not taken into consideration since plasma will separate into ion and electron components immediately on entering the electrode gap. The simulation results of Ta1+ , Ta5+ ions are displayed in Figure 2. Figure 2(a) shows the results of E/Z = 2500 eV. Ions getting through EIA with a current density of 1.625 μA/mm2 will increase as slit1 increases, while ions with a current density 8.124 μA/mm2 will decrease due to a stronger space
III. SIMULATION RESULTS
At the location of EIA, ion current intensities in the range of several mA/cm2 have been achieved from our laser ion source6, 7 with a Nd:YAG laser, whose energy shot on target is less than 2 J, FWHM 8-10 ns, and wavelength 1064 nm. This indicates that space charge effect on the ions’ transmission during the EIA cannot be neglected.4 Actually, in the experiments only the s1 and s2 can be changed, and by narrowing the width of slits and hence limiting the current intensity getting through the EIA, the influence of space charge effect can be decreased. In the simulation, transmission efficiency of heavy element Ta and light element Al ions with different charge states, different current intensities, and different energy to charge ratios are compared. The detailed parameters used in the simulation are given in Table I. Here, the current density is used as a correction for space charge effect, but not the real current density of any charge state, and it is set to be close to the total current of all charge states arriving entrance slit s1 during experiments. For example, when EIA is given a certain voltage in the experiments, only ions with the same E/Z = 2.5 keV can get through EIA, such as Ta5+ with an energy of 5 × 2.5 keV, Ta1+ with an energy of 2.5 keV, but when Ta5+ ions with energy 5 × 2.5 keV get into EIA, the ions (all of the other charge states) with the energy 5 × 2.5 keV will also get into EIA at the same time. So the space charge effect will come from total current of all charge states, even if the other charge state ions with the energy of 5 × 2.5 keV cannot
FIG. 2. Simulation results of Ta ions under different EIA and beam parameters of (a) E/Z = 2500 eV and (b) E/Z = 1000 eV.
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033307-3
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Rev. Sci. Instrum. 85, 033307 (2014)
FIG. 5. EMT signal amplitudes for Ta ions with different charge state at different EIA slit width.
The simulation results of Al1+ , Al11+ ions are displayed in Figure 3, which are much different from the results of Ta. Figure 3(a) shows the result of E/Z = 1800 eV. For Al1+ ions, no matter what the current density it is, the transmission efficiency will decrease as slit increases. Al11+ ions with current density 8.124 μA/mm2 will increase as slit increases, while the current density is 32.497 μA/mm2 , it will increase first, and then decrease because of higher energy. The same situation appears in Figure 3(b) on Al11+ ion with current density 8.124 μA/mm2 , E/Z = 500 eV. As to other parameters simulated in Figure 3(b), it shows a decrease in tendency with the increase of slit because of lower particle energy. IV. EXPERIMENTAL RESULTS FIG. 3. Simulation results of Al ions under different EIA and beam parameters of (a) E/Z = 1800 eV and (b) E/Z = 500 eV.
charge effect. As to the ions of different charge state (Ta1+ , Ta5+ ) with the same current intensity and E/Z, the particle transmission efficiency of Ta5+ is much larger than Ta1+ . Because Ta5+ ions have higher energy, the effect of space charge is weaker. Figure 2(b) shows the result of E/Z = 1000 eV. The particle transmission efficiency through EIA is much smaller than E/Z = 2500 eV and it also shows a tendency of decrease as a whole.
FIG. 4. Time of flight spectrum of Ta ions through EIA.
To verify the simulation results above, experiments under the similar conditions were carried out. In the experiments, a faraday cup (FC) with an aperture of 28 mm, 80 cm before slit s1, was used to measure the total ion current intensity produced by the laser ion source, and an electron multiplier tube (EMT) with an input aperture of 20 mm was put 20 cm downstream of slit s2 to measure the time of flight spectrum of different charge state ions through EIA. The peak current of Ta, Al measured by FC are 2.08 mA (3.38 μA/mm2 ), and 14.35 mA (23.317 μA/mm2 ), respectively. So it is appropriate to assume a current density as listed in Table I as a correction for space charge effect in the simulation above, and time
FIG. 6. EMT signal amplitudes for Al ions with different charge state at different EIA slit width.
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033307-4
Jin et al.
Rev. Sci. Instrum. 85, 033307 (2014)
cally for different ion species, ion energy, and EIA parameters with simulations and experiments. It is shown that the transmission efficiency varies a lot as the above-mentioned parameters are changed due to the very strong space charge effect. From the simulations and experiments, the following conclusions about the influence of SC on ion transmission in the EIA could be drawn:
FIG. 7. EMT signal amplitudes for Ag ions with different charge state at different EIA slit width.
of flight spectrum is also measured around peak current intensity. The reason to measure the spectrum around the peak current is that only within this area the phenomena in the simulation can be observed obviously. Figure 4 is an EMT output signal of Ta ions through the EIA. Peak amplitudes of the different charge states in the spectrum are measured to compare the influence under different beam and EIA parameters. Figure 5 shows the results of Ta experiments under the conditions of E/Z = 2682.5 eV. Amplitudes of Ta2+ -Ta5+ will decrease as slit increases because of lower energy and higher current, and it is the same as the simulation result of Figure 2(a). The current intensity of Ta6+ is lower, so it shows an increasing tendency. Though current intensity of Ta7+ looks high enough, it also has a high energy, so the fluctuation is not that large. From Figure 7, we can also see that no matter which one of the slits is changed; it will affect the ions’ transmission efficiency. More detailed results will be presented in the experiments of Al and Ag. Figure 6 shows the results of Al experiments with E/Z = 1810 eV. It presents the similar trend with the simulation results shown in Figure 3(a). For lower charge states, such as Al7+ , Al8+ , Al9+ , the amplitudes decrease as slit increases. As to higher charge state ions, with the increasing of slit width, the amplitudes will increase first, and then decrease. Figure 7 is the results of Ag by changing the width of slit s2. It shows the same tendency just like changing slit s1. The peak amplitudes of lower charge states, such as Ag4+ -Ag10+ , will reduce with the increase of slit2 width. While for higher charge states, like Ag11+ -Ag14+ , even with larger current intensities than lower charge states, only small fluctuations are detected by the EMT because of their higher energy. These results indicate that even for the very limited current that are transmitted by the EIA, the space charge effect still plays an important role, which causes the divergence of the ion beam in the path between the slit s2 and the EMT. V. DISCUSSION AND CONCLUSIONS
Transmission efficiency of laser produced ion beams during an electrostatic ion analyzer has been studied systemati-
(1) At a certain EIA voltage, the influence of SC on lower charge states is more evident than on higher charge states. (2) The influence of SC is more apparent at lower EIA voltage compared with that at higher voltage. (3) Due to the above terms, the transmitted current by the EIA could decrease with the increasing slit widths especially in the case of lowly charge ions or low EIA voltages. These phenomena we observed are consistent with the formula in Ref. 2 about the space-charge limited ion current in the ribbon-shaped beam model8 2Ze 1/2 h(yf − yi ) 3/2 I = 4ε0 E . (4) m l2 Here, yi h is the initial cross-section of the beam and yf h the expanded cross-section after the interaction length l; ε0 is the permittivity of vacuum. Therefore, for laser produced plasma measurement one need to use an EIA with caution even it is for analyzing the relative charge state distribution. To limit the total current entering the EIA and hence the space charge effect at the beginning stage, the width of the input slit should be set as thin as possible. In our case, this value was set as 0.2 mm. Narrowing the output slit and shortening the distance between the EIA and downstream detector (like an EMT) would help to minimize the influence of space charge out of the analyzer. ACKNOWLEDGMENTS
This work is supported by China Natural Science Foundation (NSF) (Grant Nos. 11275239 and 11221064) and 100 Talents Program of the Chinese Academy of Sciences (CAS) (Grant No. Y214160BR0). 1 A.
Balabaev, V. Balanusta, S. Kondrashev, K. Konukov, A. Lozhkin et al., Rev. Sci. Instrum. 73, 773 (2002). 2 M. Okamura, T. Katayama, R. A. Jameson, T. Takeuchi, T. Hattori, and N. Hayashizaki, Nucl. Instrum. Methods Phys. Res. B 188, 216 (2002). 3 S. Shan, Q. Y. Jin, Z. M. Li, X. H. Guo, L. C. Zhou et al., Chin. Phys. C 37, 117006 (2013). 4 E. Woryna, P. Parys, J. Wolowski, and W. Mroz, Laser Part. Beams 14, 293 (1996). 5 H. Q. Xia and Ch. L. Liu, in Theory of Beam Transport, edited by X. M. Cao (Xi’an Jiaotong University Press, Xi’an, 1991), p. 124. 6 H. Y. Zhao, Q. Y. Jin, S. Sha, J. J. Zhang, Z. M. Li et al., Rev. Sci. Instrum. 85, 02B910 (2014). 7 S. Sha, H. W. Zhao, X. H. Guo, Z. L. Zhang, X. Fang et al., Rev. Sci. Instrum. 83, 02B303 (2012). 8 H. Bader, K. Rohr, and H. Weber, J. Phys. D: Appl. Phys. 13, L149–L151 (1980).
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