Magnetic Field Design for a 2.45-GHz ECR Ion Source with Permanent Magnets

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1 Journal of the Korean Physical Society, Vol. 55, No. 2, August 2009, pp Magnetic Field Design for a 2.45-GHz ECR Ion Source with Permanent Magnets J. Y. Park Department of Physics, Pusan National University, Busan and Korea Basic Science Institute, Busan H. S. Lee, M. S. Won, B. S. Lee, J. P. Kim, J. H. Yoon, J. S. Bae and J. K. Bang Korea Basic Science Institute, Busan J. K. Ahn Department of Physics, Pusan National University, Busan (Received 26 December 2008) We optimize the magnetic field configuration for a 2.45 GHz electron cyclotron resonance (ECR) ion source with permanent magnets, which is designed to construct a compact heavy-ion accelerator for intensive studies on nucleosynthesis. The magnetic system consists of two radially magnetized solenoids, a hexapole magnet, and a central ring magnet. We report magnetic field measurements of the permanent-magnet ECR ion source and compare with the results with those obtained using an Opera3D calculation. PACS numbers: n, N, Di Keywords: ECR, Ion Source, Permanent magnet, Plasma I. INTRODUCTION The need for high currents of multiply charged ions led to the development of an electron cyclotron resonance ion source (ECRIS) [1]. The main features of the ECRIS source can be summed up by the following points: highly charged ions produced in continuous and pulsed modes, feasibility to accelerate elements up to uranium, easy adjustment of the electron temperature and density, robustness, and long-life with no wearing parts like filaments. Due to the unique capability of producing highly charged ion beams, the ECRIS has been widely utilized in a variety of research areas ranging from atomic and nuclear physics to material sciences. In an ECR source, highly charged ions are produced stepwise by electron impact ionization. During the bombardment of electrons, the ions or atoms are ionized, leading to charges up to the maximum charge with all electrons stripped off. A highly charged ion generates bremsstrahlung X- ray photons when it is decelerated or stops in materials. The maximum X-ray photon energy can readily reach several hundreds of MeV for highly charged ions. Extreme ultra-violet (EUV) light can also be emitted in atomic transitions of highly charged Xe ions. ahnjk@pusan.ac.kr; Fax: There has been a rapid improvement in the performance, such as achievable charge states and beam intensity, of ECR ion sources. Empirical scaling laws show that the maximum achievable charge state and intensity grow with increasing microwave frequency. At the same time, the magnetic fields should be increased to fulfill the resonance conditions and to have good confinement. That requires superconducting magnets with very high frequencies, for example, 18 GHz, 28 GHz, or even higher. On the other hand, ECR ion sources can be miniaturized with permanent magnets only. This has the advantages of low power consumption and low operating cost [2 7]. In the paper, we report optimization of the magnetic field configuration for a 2.45-GHz ECRIS with permanent magnets. II. DESIGN OF THE ECR MAGNETIC FIELD CONFIGURATION Our 2.45-GHz ECRIS consists of two radially magnetized solenoids, a hexapole magnet, and a central ring magnet. The two solenoid magnets make a mirror field to confine the ECR plasma axially while the hexapole magnet confines it radially. For a solenoid field, the radial magnetization is known to be stronger than the axial one. We have searched for an optimum configuration of -409-

2 -410- Journal of the Korean Physical Society, Vol. 55, No. 2, August 2009 Fig. 1. Calculated magnetic field strengths from Eq. (1). Top-right plot (a) shows the axial magnetic field strengths according to the inner radius (R 1) of the solenoid magnet with a fixed thickness of 3 cm. Bottom-left plot (b) indicates the axial magnetic field strengths according to the thickness of the solenoid magnet. Bottom-right figure (c) represents axial magnetic field strengths in terms of the distance between two solenoid magnets. Fig. 2. Profiles of the multicusp field in terms of the numbers of cusp. the solenoid geometry by varying the thickness, the inner and outer radii, and the distance between the two solenoid magnets. The axial magnetic field strength at a given point z along the symmetric axis is given by B z (z) = B r 2 ( 1 a 1 1 a 2 1 b b 2 ) + ln (1 + b 1)(1 + a 2 ) (1 + b 2 )(1 + a 1 ), (1) where B r represents a residual magnetic induction, a 1 = 1 + (z + l)/r 2, a 2 = 1 + (z l)/r 2, b 1 = 1 + (z + l)/r1, and b 2 = 1 + (z l)/r 1. l denotes the half thickness of the solenoid [8]. Calculated magnetic field strengths are shown in Fig. 1. The maximum strength of the axial magnetic field varies with the radial thickness, the axial thickness, and the distance between the two solenoid magnets. The minimum field strength is not very sensitive to the radial thickness of the solenoid, while its depends heavily on the axial thickness and the distance between the two solenoid magnets. The optimized configuration of the

3 Magnetic Field Design for a 2.45-GHz ECR Ion Source J. Y. Park et al Fig. 3. Schematic view of the permanent magnets. mirror magnets is then found to be such that R 1 = 4 cm, 2l = 3 cm, and d = 20 cm. The radial field distribution is proportional to B = B 0 r N/2 1, where N is the number of cusps and r is the radial distance from the center of the device to the tip of the magnet. A high-order multicusp field for confining the plasma in the radial direction can increase the resonant volume in the radial direction. Hexapole-type multicusp magnets (N = 6) are most commonly used for radial confinement in conventional minimum-b ECR ion sources [8 10]. A stick-shaped permant magnet is generally used for the hexapole magnet. However, we have chosen a cylindrical shape due to the space limitation on our design of the whole system. The multicusp magnetic fields obtained by using the OPERA-3D simulator are plotted in Fig. 2 for three different N-pole magnets. A large number of cusps is known to create a more extended center-field region near r = 0. A schematic view of the magnetic structures is displayed in Fig. 3. Two solenoid magnets with an inner radius of 40 mm, an outer radius of 100 mm, and a thicknesses of 30 mm are placed at both ends, are at each end. A hexapole magnet consists of 6 pieces of alternating magnetized cusps. It has an inner radius of 30 mm, an outer radius of 50 mm, and a thicknesses of 150 mm. A movable ring magnet is placed to surround the hexapole magnet in the central region. The ring magnet has an Fig. 4. Calculated axial magnetic field with (solid line) and without (dashed line) a central ring magnet. The horizontal line represents the magnetic field for ECR condition. inner radius of 80 mm, an outer radius of 100 mm, and a thickness of 20 mm, which produces a flat axial magnetic field over the ECR zone for a volume-type ECRIS. Figure 4 illustrates the mirror magnetic field (B z ) strengths with and without a central ring magnet. The

4 -412- Journal of the Korean Physical Society, Vol. 55, No. 2, August 2009 Fig. 5. Magnetic field measurement system consisting of a Gauss meter and a movement system to measure the axial and the radial magnetic fields. Table 1. Magnetic design parameters of the permanentmagnet ECR ion source. Binj,ext Bmin Lmirror LECR Plasma chamber ID 0.24 T T 200 mm 50 mm 40 mm Fig. 6. The measured field distribution (triangles) is compared with the OPERA-3D simulation result (solid line). Closed circles indicate the magnetic field distribution measured with a shorter distance between solenoid mirror magnets, which give a better fit to the simulation result. solid line represents the mirror field of a volume-type ECRIS with a central ring magnet while the dashed line represents the surface mirror field without a central ring magnet. The horizontal line shows an ECR magnetic field of T at 2.45 GHz. The features of the magnetic design are given in Table 1. III. MAGNETIC FIELD MEASUREMENT We measured the magnetic field strength (Bz ) every 5 mm along the axial direction, as shown in Fig. 5. A gauss meter (F. W. BELL, Model 6010) was moved along the axial and the radial direction, by using a stepping-motor controller. The measured magnetic field distributions (symbols) are overlaid with the OPERA-3D simulation result (solid line) in Fig. 6. The measured field distribution with the simulation geometry (triangles) is found to be slightly off the simulation result, which could be due to non-uniform magnetization of the permanent magnets. Reducing the distance between the two solenoid magnets makes the field distribution fit the design better, as displayed by the circles. One should note that the ECR zone of Bmin = T is achieved over a length of 50 mm, which is the expected value. The maximum field (Bmax ) was measured to be 0.22 T, with a mirror ratio (Bmax /Bmin ) of 2.5. We also measured the uniformity of the axial magnetic field along the radial direction. Figure 7 shows the axial field distributions measured at r = 0 mm (closed circles), Fig. 7. Axial magnetic field distributions measured along the off-axis direction for small angles and at different azimuthal angles. r = 5 mm (open circles), and r = 10 mm (crosses). The axial field is found to be uniform over the radial volume to a few percent. To understand the systematic uncertainties in measuring the magnetic field strengths, we also calculated the axial magnetic field along the off-axis direction for small angles. The calculation started from the center of one end and moved towards positions 0 mm, 5 mm, 15 mm, and 20 mm away from the center of the other end, which corresponded to polar angle of 0, 1.9, 3.8, and 7.6

5 Magnetic Field Design for a 2.45-GHz ECR Ion Source J. Y. Park et al Fig. 8. Off-axis magnetic field distributions for small angles (left) and the axial field distributions for different azimuthal angles (right). degrees, respectively. The distributions for the off-axis magnetic field with small angles are shown in the bottom left of Fig. 8. A possible shift on the order of 1 mm off the axis was shown to have little effect on the magnetic field distribution. We also studied any possible change in the magnetic field distribution for different azimuthal angles. As shown in the bottom right of Fig. 7, no azimuthal angle dependence was observed. Based on the optimized magnetic structure with all permanent magnets, we constructed and tested a compact ECRIS with a 2.45 GHz microwave generator, as shown in Fig. 9. The maximum 1 kw macrowave passes through the antenna to the plasma chamber. The characteristics of the entire ECRIS will be discussed elsewhere. The permanent-magnet ECRIS will be primarily used for high-energy X-ray generation and for high-intensity heavy-ion production. If this is to be achieved, heavyions from the ECR source should be accelerated up to several hundreds of kev. A single-end van de Graaff accelerator will accelerate the beam before it is replaced with a radio frequency quadrupole (RFQ) and a drift tube linac (DTL). Intense 12 C beam acceleration is of great interest in studying the 12 C(α, γ) 16 O reaction by using inverse kinematics. We plan to build a superconducting 28 GHz ECRIS that will deliver high-intensity beams of highly-charged heavy ions to the RFQ and DTL accelerator section. Accelerated 7 Li ions will interact with a hydrogen gas jet at an energy of a few MeV/u, Fig. 9. ECR ion source system with a microwave generator and a beam extraction line. which will then generate high-intensity fast neutrons at forward angles, as shown in Fig. 10 [11].

6 -414- Journal of the Korean Physical Society, Vol. 55, No. 2, August 2009 Fig. 10. Schematic view of the low-energy heavy ion accelerator with the permanent-magnet 2.45 GHz ECRIS and the superconducting 28 GHz ECRIS. IV. CONCLUSION We optimized the magnetic field configuration for a 2.45 GHz ECRIS with permanent magnets. The configuration was designed to construct a compact heavy-ion accelerator for intensive studies on nucleosynthesis reactions. The optimized design had a 5-cm-long ECR zone with two radially magnetized solenoids, a hexapole magnet, and a central ring magnet. The magnetic field strengths in the axial and the radial directions were measured and were shown to be in a good agreement with the results from OPERA-3D calculations. ACKNOWLEDGMENTS This work was supported by a Korea Research Foundation grant funded by Korean Government (MOEHRD), Basic Research Promotion Fund (KRF C00005), and the authors are grateful to the Korea Basic Science Institute (E28100) for financial support. REFERENCES [1] R. Geller, Electron Cyclotron Resonance Ion Sources and ECR Plasmas (IOP, Bristol, 1996). [2] G. D. Alton and D. N. Smithe, Rev. Sci. Instr (1994) [3] T. Nakagawa et al., Jpn. J. Appl. Phys. 35, L1124 (1996). [4] G. D. Alton, Nucl. Instr. Meth. A 382, 276 (1996). [5] G. D. Alton, F. W. Meyer, Y. Liu, J. R. Beene and D. Tucker, Rev. Sci. Instr. 69 (1998)729 [6] R. Trassl et al., Physica Scripta T 73, 380 (1997). [7] R. Trassl et al., Physica Scripta T 80, 504 (1999). [8] Q. L. Peny et al., J. Magnetism and Magnetic Materials, 268, 165 (2004). [9] Y. Liu and G. D. Alton et al., Rev. Sci. Instr. 69, 1311 (1998). [10] Y. K. Kwon and C. S. Lee, J. Korean Phys. Soc. 39, 604 (2001). [11] J. Y. Park and J. K. Ahn, Sae Mulli 54, 171 (2007).