Studies on low energy beam transport for high intensity high charged ions at IMPa) Y. Yang, L. T. Sun, Q. Hu, Y. Cao, W. Lu, Y. C. Feng, X. Fang, X. Z. Zhang, H. W. Zhao, and D. Z. Xie Citation: Review of Scientific Instruments 85, 02A719 (2014); doi: 10.1063/1.4832935 View online: http://dx.doi.org/10.1063/1.4832935 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The direct injection of intense ion beams from a high field electron cyclotron resonance ion source into a radio frequency quadrupolea) Rev. Sci. Instrum. 85, 02A740 (2014); 10.1063/1.4861405 Transport and emittance study for 18 GHz superconducting-ECR ion source at RCNPa) Rev. Sci. Instrum. 83, 02A335 (2012); 10.1063/1.3671748 Design of a compact, permanent magnet electron cyclotron resonance ion source for proton and H 2 + beam productiona) Rev. Sci. Instrum. 81, 02A321 (2010); 10.1063/1.3267838 Intense beam production of highly charged heavy ions by the superconducting electron cyclotron resonance ion source SECRAL (invited)a) Rev. Sci. Instrum. 79, 02A315 (2008); 10.1063/1.2804900 Intense heavy ion beam production from IMP LECR3 and construction progress of a superconducting ECR ion source SECRAL Rev. Sci. Instrum. 75, 1410 (2004); 10.1063/1.1690473
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 02A719 (2014)
Studies on low energy beam transport for high intensity high charged ions at IMPa) Y. Yang,1,2,b) L. T. Sun,1 Q. Hu,1 Y. Cao,1 W. Lu,1,2 Y. C. Feng,1 X. Fang,1,2 X. Z. Zhang,1 H. W. Zhao,1 and D. Z. Xie1 1 2
Institute of Modern Physics, CAS, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100039, China
(Presented 12 September 2013; received 6 September 2013; accepted 16 October 2013; published online 27 November 2013) Superconducting Electron Cyclotron Resonance ion source with Advanced design in Lanzhou (SECRAL) is an advanced fully superconducting ECR ion source at IMP designed to be operational at the microwave frequency of 18–24 GHz. The existing SECRAL beam transmission line is composed of a solenoid lens and a 110◦ analyzing magnet. Simulations of particle tracking with 3D space charge effect and realistic 3D magnetic fields through the line were performed using particle-in-cell code. The results of the beam dynamics show that such a low energy beam is very sensitive to the space charge effect and significantly suffers from the second-order aberration of the analyzing magnet resulting in large emittance. However, the second-order aberration could be reduced by adding compensating sextupole components in the beam line. On this basis, a new 110◦ analyzing magnet with relatively larger acceptance and smaller aberration is designed and will be used in the design of low energy beam transport line for a new superconducting ECR ion source SECRAL-II. The features of the analyzer and the corresponding beam trajectory calculation will be detailed and discussed in this paper. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4832935] I. INTRODUCTION
SECRAL1 (Superconducting Electron Cyclotron Resonance ion source with Advanced design in Lanzhou) was built at IMP (Institute of Modern Physics) in 2002 and has since been installed and commissioned at HIRFL (Heavy Ion Research Facility in Lanzhou). Until now, SECRAL has produced several highly charged ion beams with record high intensity, such as 455 eμA of 129 Xe27+ and 395 eμA of 209 Bi31+ by 24 GHz frequency heating. However, experiments have shown that the beam quality of high-current ion beams extracted from the ECR and transported through the low energy beam line is rather poor,2 with low transmission through the line. For low energy beam with high intensity, nonlinear effects, such as space charge effect and the aberration of the magnetic elements, are important reasons of beam quality degradation. Unfortunately, the existing low energy beam line was initially designed with Trace-3D program without taking any nonlinear effects into account. In order to investigate the possible causes of the bad transmission of SECRAL line, resimulations were carried out using Particle-In-Cell code and possible improvement was proposed. On this basis, a new low energy beam transport line, including a new 110◦ analyzing magnet, is designed and will be used for a new superconducting ECR ion source SECRAL-II.
a) Contributed paper, published as part of the Proceedings of the 15th
International Conference on Ion Sources, Chiba, Japan, September 2013. b) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. 0034-6748/2014/85(2)/02A719/3/$30.00
II. THE LOW ENERGY BEAM TRANSPORT LINE OF SECRAL
Figure 1 shows the layout of the low energy beam line of SECRAL. It mainly consists of a solenoid lens, a unclamped double-focusing 110◦ analyzing magnet, two slits mounted on both the horizontal and the vertical directions and beam diagnosis apparatuses, such as moveable Faraday cup and emittance measuring device. The solenoid lens is directly attached on the extraction flange of the source body to control the size of the beam going into the analyzing magnet. The analyzing magnet has a geometrical acceptance of 200 mm in the horizontal and 120 mm in the vertical direction, the bending radius is of 600 mm and edge angle is of 39.15◦ . III. SIMULATED AND EXPERIMENTAL RESULTS
To perform the beam transport simulation, 209 Bi31+ was chosen as the design particle. Figure 2 shows the measured bismuth beam charge state distribution obtained with SECRAL for an ECR platform voltage of 23 kV at the location of the detector chamber (shown in Fig. 1). Allison scanner is used for the 2D emittance measurement. About 395 eμA 209 Bi31+ beam is obtained while the total current extracted from the ion source is up to 10 emA. Simulation was carried out for multi-species beam containing ions of six charge states, 209 Bi32+ , 209 Bi31+ , 209 Bi30+ , 209 Bi29+ , 16 O2+ , 16 3+ O , which have relatively high currents and dedicate significant space charge effect. The space charge compensation is considered with a factor of 75%. In the simulation, we assumed all ion species have the same initial conditions with a emittance of 100 π mm mrad and Twiss parameters α = −0.5, β = 173 mm/mrad.3 These particles are initially distributed
85, 02A719-1
© 2013 AIP Publishing LLC
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02A719-2
Yang et al.
Rev. Sci. Instrum. 85, 02A719 (2014)
FIG. 1. Layout of the multi-component ion beam transport line.
in a 4D water-bag transverse hyperspace with uniform momentum spread of 0.2%. The realistic 3D fields of the solenoid and the analyzing magnet were calculated with TOSCA and imported in the simulation. Figures 3(a) and 3(b) show the calculated and experimental emittance patterns at the location of the detector chamber, respectively. Nonlinear forces degrade the beam quality, with obvious beam phase space distortion and emittance blowup. Values of measured emittance of the beam in horizontal and vertical are of about 400 π mm mrad and 470 π mm mrad, respectively. Simulated values are very corresponding with experimental results, with about 400 π mm mrad in horizontal and 450 π mm mrad in vertical. Effect of space charge has been studied and discussed in previous works,2 indicated that such a low energy beam is very sensitive to it. However, there has not been an effective method to increase the space charge compensation for high charged ion beams. Another significant factor, causing beam emittance blowup, is the spherical aberration of the magnets. Magnetic field distribution in an ideal model of dipole magnet can be written as Bx (x, y, z) = Bz (x, y, z) = 0,
(1)
By (x, y, z) = By .
(2)
In the second-order Taylor’s expansion, the above distribution can be described as follows:
FIG. 3. (a) Calculated beam phase space pattern of 209 Bi31+ in the simulation. (b) Measured beam phase space.
y xy + 2ξ 2 , Bx (x, y, z) = By −n ρ ρ (x 2 − y 2 ) x n y2 +ξ By (x, y, z) = By 1 − n + , ρ 2 ρ2 ρ2 Bz (x, y, z) = 0,
(3)
(4) (5)
where ρ is the bending radius of the beam central trajectory and By is the vertical component of magnetic field along the central trajectory, ρ ∂By , (6) n=− By ∂x x=0,y=0 2 2 ρ ∂ By . (7) ξ= 2!By ∂x 2 x=0,y=0 Here, n and ξ present the quadrupole and sextupole component strength in dipole, respectively. Particle motion in the magnetic field can be described by equations including second order terms: x + (1 − n) =
1 x ρ2
1 ρ 1 + (2n − 1 − ξ ) 3 x 2 + ρ ρ ρ
1 xx ρ
1 ρ x ρ2 ρ 3 3 1 1 1 −n +2 ξ y2 + 0.5 ρ ρ ρ 1 1 2 1 ρ 2 + yy − 0.5 y − , ρ ρ ρ ρ + 0.5x 2 + (2 − n)
FIG. 2. Bismuth beam charge state distribution obtained with SECRAL.
(8)
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02A719-3
Yang et al.
Rev. Sci. Instrum. 85, 02A719 (2014)
FIG. 6. Calculated phase space pattern of 209 Bi31+ for SECRAL-II LEBT. FIG. 4. Magnetic field map of the compensating sextupole.
2 3 1 1 1 y +n y = 2(ξ − n) xy + xy ρ ρ ρ 2 1 1 ρ . (9) − x y +n y ρ ρ ρ According to the above equations, high-order factors of the magnetic field of the dipole will lead to nonlinear effect on the particle with aberration. IV. SEXTUPOLE CORRECTION FOR SECRAL LINE
We have analyzed the magnetic field distribution of the analyzing magnet in SECRAL line, it is shown that there is a large skew sextupole component on the edge of the analyzing magnet, which will cause a large distort in both horizontal and vertical phase space distributions. To compensate the influence of the skew sextupole component, a normal sextupole was added just downstream the analyzing magnet in the simulation. Figure 4 shows the map of the magnetic field of the normal sextupole. The calculated phase space distributions with sextupole correction are shown in Fig. 5. With less serious aberration in phase space, the emittance grows less fast, with values of the emittance of about 250 π mm mrad and 230 π mm mrad in horizontal and vertical planes, respectively. V. A NEW LEBT FOR SECRAL-II
Based on the analysis of the SECRAL low energy beam transport (LEBT), another LEBT is designed for a new super-
conducting ECR ion source SECRAL-II. The newly designed LEBT has a similar structure with the existing one (Fig. 1), except for the analyzing magnet which is replaced by a new one. The new analyzing magnet has a relatively large geometrical acceptance of 350 mm in the horizontal direction and 220 mm in the vertical direction, and the bending radius is of 600 mm. The beam optics, according to the simulation, requires the analyzing magnet has an entrance edge angle of 44◦ and exit edge angle of 36.5◦ . Beam transport simulation for the new LEBT of SECRAL-II is carried out with the design particle 209 Bi31+ . Figure 6 illustrates the simulation results. With much larger geometrical acceptance, effect of the spherical aberration of the analyzing magnet is less serious, resulting in a smaller factor in emittance growth. The calculated emittance is of about 160 π mm mrad in horizontal and 130 π mm mrad in vertical directions, respectively. VI. SUMMARY AND OUTLOOK
Simulations of bismuth beam passing through the low energy beam line of SECRAL have been performed. The simulations predict a large aberration of analyzing magnet, especially existing in second-order aberration, which will lead to beam phase space distortion and emittance growth. A suitable sextupole could compensate the aberration to some degree and reduce the emittance growth. Future work should include the experiment to verify the prediction. A new analyzing magnet for SECRAL-II LEBT is designed and the corresponding beam simulation is performed. The analyzer has a relatively large geometric acceptance through which the beam has a slight aberration and small emittance growth. ACKNOWLEDGMENTS
This work is supported by the 100 Talents Program of the CAS (No. Y214160BR0) and NSF (Contract No. 11221064). 1 H.
FIG. 5. Simulated results with sextupole correction.
W. Zhao, L. T. Sun, X. Z. Zhang, Z. M. Zhang, X. H. Guo, W. He, P. Yuan, M. T. Song, J. Y. Li, Y. C. Feng et al., Rev. Sci. Instrum. 77, 03A333 (2006). 2 Y. Cao, W. Lu, W. H. Zhang, S. Sha, Y. Yang, B. H. Ma, H. Wang, Y. H. Zhu, J. W. Guo, X. Fang et al., Rev. Sci. Instrum. 83, 02B726 (2012). 3 L. T. Sun, D. Leitner, G. Machicoane, E. Pozdeyev, V. Smirnov, S. B. Vorozhtsov, D. Winklehner, and Q. Zhao, Rev. Sci. Instrum. 83, 02B705 (2012).
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: 130.88.90.110 On: Sun, 21 Dec 2014 05:39:01
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: 130.88.90.110 On: Sun, 21 Dec 2014 05:39:01
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 02A719 (2014)
Studies on low energy beam transport for high intensity high charged ions at IMPa) Y. Yang,1,2,b) L. T. Sun,1 Q. Hu,1 Y. Cao,1 W. Lu,1,2 Y. C. Feng,1 X. Fang,1,2 X. Z. Zhang,1 H. W. Zhao,1 and D. Z. Xie1 1 2
Institute of Modern Physics, CAS, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100039, China
(Presented 12 September 2013; received 6 September 2013; accepted 16 October 2013; published online 27 November 2013) Superconducting Electron Cyclotron Resonance ion source with Advanced design in Lanzhou (SECRAL) is an advanced fully superconducting ECR ion source at IMP designed to be operational at the microwave frequency of 18–24 GHz. The existing SECRAL beam transmission line is composed of a solenoid lens and a 110◦ analyzing magnet. Simulations of particle tracking with 3D space charge effect and realistic 3D magnetic fields through the line were performed using particle-in-cell code. The results of the beam dynamics show that such a low energy beam is very sensitive to the space charge effect and significantly suffers from the second-order aberration of the analyzing magnet resulting in large emittance. However, the second-order aberration could be reduced by adding compensating sextupole components in the beam line. On this basis, a new 110◦ analyzing magnet with relatively larger acceptance and smaller aberration is designed and will be used in the design of low energy beam transport line for a new superconducting ECR ion source SECRAL-II. The features of the analyzer and the corresponding beam trajectory calculation will be detailed and discussed in this paper. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4832935] I. INTRODUCTION
SECRAL1 (Superconducting Electron Cyclotron Resonance ion source with Advanced design in Lanzhou) was built at IMP (Institute of Modern Physics) in 2002 and has since been installed and commissioned at HIRFL (Heavy Ion Research Facility in Lanzhou). Until now, SECRAL has produced several highly charged ion beams with record high intensity, such as 455 eμA of 129 Xe27+ and 395 eμA of 209 Bi31+ by 24 GHz frequency heating. However, experiments have shown that the beam quality of high-current ion beams extracted from the ECR and transported through the low energy beam line is rather poor,2 with low transmission through the line. For low energy beam with high intensity, nonlinear effects, such as space charge effect and the aberration of the magnetic elements, are important reasons of beam quality degradation. Unfortunately, the existing low energy beam line was initially designed with Trace-3D program without taking any nonlinear effects into account. In order to investigate the possible causes of the bad transmission of SECRAL line, resimulations were carried out using Particle-In-Cell code and possible improvement was proposed. On this basis, a new low energy beam transport line, including a new 110◦ analyzing magnet, is designed and will be used for a new superconducting ECR ion source SECRAL-II.
a) Contributed paper, published as part of the Proceedings of the 15th
International Conference on Ion Sources, Chiba, Japan, September 2013. b) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. 0034-6748/2014/85(2)/02A719/3/$30.00
II. THE LOW ENERGY BEAM TRANSPORT LINE OF SECRAL
Figure 1 shows the layout of the low energy beam line of SECRAL. It mainly consists of a solenoid lens, a unclamped double-focusing 110◦ analyzing magnet, two slits mounted on both the horizontal and the vertical directions and beam diagnosis apparatuses, such as moveable Faraday cup and emittance measuring device. The solenoid lens is directly attached on the extraction flange of the source body to control the size of the beam going into the analyzing magnet. The analyzing magnet has a geometrical acceptance of 200 mm in the horizontal and 120 mm in the vertical direction, the bending radius is of 600 mm and edge angle is of 39.15◦ . III. SIMULATED AND EXPERIMENTAL RESULTS
To perform the beam transport simulation, 209 Bi31+ was chosen as the design particle. Figure 2 shows the measured bismuth beam charge state distribution obtained with SECRAL for an ECR platform voltage of 23 kV at the location of the detector chamber (shown in Fig. 1). Allison scanner is used for the 2D emittance measurement. About 395 eμA 209 Bi31+ beam is obtained while the total current extracted from the ion source is up to 10 emA. Simulation was carried out for multi-species beam containing ions of six charge states, 209 Bi32+ , 209 Bi31+ , 209 Bi30+ , 209 Bi29+ , 16 O2+ , 16 3+ O , which have relatively high currents and dedicate significant space charge effect. The space charge compensation is considered with a factor of 75%. In the simulation, we assumed all ion species have the same initial conditions with a emittance of 100 π mm mrad and Twiss parameters α = −0.5, β = 173 mm/mrad.3 These particles are initially distributed
85, 02A719-1
© 2013 AIP Publishing LLC
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02A719-2
Yang et al.
Rev. Sci. Instrum. 85, 02A719 (2014)
FIG. 1. Layout of the multi-component ion beam transport line.
in a 4D water-bag transverse hyperspace with uniform momentum spread of 0.2%. The realistic 3D fields of the solenoid and the analyzing magnet were calculated with TOSCA and imported in the simulation. Figures 3(a) and 3(b) show the calculated and experimental emittance patterns at the location of the detector chamber, respectively. Nonlinear forces degrade the beam quality, with obvious beam phase space distortion and emittance blowup. Values of measured emittance of the beam in horizontal and vertical are of about 400 π mm mrad and 470 π mm mrad, respectively. Simulated values are very corresponding with experimental results, with about 400 π mm mrad in horizontal and 450 π mm mrad in vertical. Effect of space charge has been studied and discussed in previous works,2 indicated that such a low energy beam is very sensitive to it. However, there has not been an effective method to increase the space charge compensation for high charged ion beams. Another significant factor, causing beam emittance blowup, is the spherical aberration of the magnets. Magnetic field distribution in an ideal model of dipole magnet can be written as Bx (x, y, z) = Bz (x, y, z) = 0,
(1)
By (x, y, z) = By .
(2)
In the second-order Taylor’s expansion, the above distribution can be described as follows:
FIG. 3. (a) Calculated beam phase space pattern of 209 Bi31+ in the simulation. (b) Measured beam phase space.
y xy + 2ξ 2 , Bx (x, y, z) = By −n ρ ρ (x 2 − y 2 ) x n y2 +ξ By (x, y, z) = By 1 − n + , ρ 2 ρ2 ρ2 Bz (x, y, z) = 0,
(3)
(4) (5)
where ρ is the bending radius of the beam central trajectory and By is the vertical component of magnetic field along the central trajectory, ρ ∂By , (6) n=− By ∂x x=0,y=0 2 2 ρ ∂ By . (7) ξ= 2!By ∂x 2 x=0,y=0 Here, n and ξ present the quadrupole and sextupole component strength in dipole, respectively. Particle motion in the magnetic field can be described by equations including second order terms: x + (1 − n) =
1 x ρ2
1 ρ 1 + (2n − 1 − ξ ) 3 x 2 + ρ ρ ρ
1 xx ρ
1 ρ x ρ2 ρ 3 3 1 1 1 −n +2 ξ y2 + 0.5 ρ ρ ρ 1 1 2 1 ρ 2 + yy − 0.5 y − , ρ ρ ρ ρ + 0.5x 2 + (2 − n)
FIG. 2. Bismuth beam charge state distribution obtained with SECRAL.
(8)
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02A719-3
Yang et al.
Rev. Sci. Instrum. 85, 02A719 (2014)
FIG. 6. Calculated phase space pattern of 209 Bi31+ for SECRAL-II LEBT. FIG. 4. Magnetic field map of the compensating sextupole.
2 3 1 1 1 y +n y = 2(ξ − n) xy + xy ρ ρ ρ 2 1 1 ρ . (9) − x y +n y ρ ρ ρ According to the above equations, high-order factors of the magnetic field of the dipole will lead to nonlinear effect on the particle with aberration. IV. SEXTUPOLE CORRECTION FOR SECRAL LINE
We have analyzed the magnetic field distribution of the analyzing magnet in SECRAL line, it is shown that there is a large skew sextupole component on the edge of the analyzing magnet, which will cause a large distort in both horizontal and vertical phase space distributions. To compensate the influence of the skew sextupole component, a normal sextupole was added just downstream the analyzing magnet in the simulation. Figure 4 shows the map of the magnetic field of the normal sextupole. The calculated phase space distributions with sextupole correction are shown in Fig. 5. With less serious aberration in phase space, the emittance grows less fast, with values of the emittance of about 250 π mm mrad and 230 π mm mrad in horizontal and vertical planes, respectively. V. A NEW LEBT FOR SECRAL-II
Based on the analysis of the SECRAL low energy beam transport (LEBT), another LEBT is designed for a new super-
conducting ECR ion source SECRAL-II. The newly designed LEBT has a similar structure with the existing one (Fig. 1), except for the analyzing magnet which is replaced by a new one. The new analyzing magnet has a relatively large geometrical acceptance of 350 mm in the horizontal direction and 220 mm in the vertical direction, and the bending radius is of 600 mm. The beam optics, according to the simulation, requires the analyzing magnet has an entrance edge angle of 44◦ and exit edge angle of 36.5◦ . Beam transport simulation for the new LEBT of SECRAL-II is carried out with the design particle 209 Bi31+ . Figure 6 illustrates the simulation results. With much larger geometrical acceptance, effect of the spherical aberration of the analyzing magnet is less serious, resulting in a smaller factor in emittance growth. The calculated emittance is of about 160 π mm mrad in horizontal and 130 π mm mrad in vertical directions, respectively. VI. SUMMARY AND OUTLOOK
Simulations of bismuth beam passing through the low energy beam line of SECRAL have been performed. The simulations predict a large aberration of analyzing magnet, especially existing in second-order aberration, which will lead to beam phase space distortion and emittance growth. A suitable sextupole could compensate the aberration to some degree and reduce the emittance growth. Future work should include the experiment to verify the prediction. A new analyzing magnet for SECRAL-II LEBT is designed and the corresponding beam simulation is performed. The analyzer has a relatively large geometric acceptance through which the beam has a slight aberration and small emittance growth. ACKNOWLEDGMENTS
This work is supported by the 100 Talents Program of the CAS (No. Y214160BR0) and NSF (Contract No. 11221064). 1 H.
FIG. 5. Simulated results with sextupole correction.
W. Zhao, L. T. Sun, X. Z. Zhang, Z. M. Zhang, X. H. Guo, W. He, P. Yuan, M. T. Song, J. Y. Li, Y. C. Feng et al., Rev. Sci. Instrum. 77, 03A333 (2006). 2 Y. Cao, W. Lu, W. H. Zhang, S. Sha, Y. Yang, B. H. Ma, H. Wang, Y. H. Zhu, J. W. Guo, X. Fang et al., Rev. Sci. Instrum. 83, 02B726 (2012). 3 L. T. Sun, D. Leitner, G. Machicoane, E. Pozdeyev, V. Smirnov, S. B. Vorozhtsov, D. Winklehner, and Q. Zhao, Rev. Sci. Instrum. 83, 02B705 (2012).
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