Status of the PLS-II Magnet Design and Fabrication

Transcription

1 Journal of the Korean Physical Society, Vol. 56, No. 6, June 2010, pp Status of the PLS-II Magnet Design and Fabrication D. E. Kim, K. H. Park, H. G. Lee, H. S. Han, Y. G. Jung, H. S. Suh, Y. D. Joo and K. R. Kim Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang (Received 24 November 2009, in final form 12 January 2010) The only 3rd generation synchrotron light source in Korea, which is operated by the Pohang Accelerator Laboratory (PAL), is planning a major upgrade of the PLS to meet the more demanding requirements of the synchrotron light users. The main features of the major upgrade are (1) increasing the electron beam energy from 2.5 GeV to 3.0 GeV for higher energy X-ray photons, (2) decreasing the electron beam emittance from 18.9 nm to 5.8 nm to increase the photon brilliance, and (3) increasing the number of straight sections to increase the number of insertion devices from 10 to 20 to meet the demand for insertion devices. In the upgraded PLS (PLS-II), there will be 24 combined function dipole magnets, 96 quadrupole magnets, and 96 sextupole magnets with some auxiliary magnets for electron beam injection. In this report, the physical design features, and the mechanical aspects of the magnet design are described. PACS numbers: Lc Keywords: Magnet, PLS, PLS-II, Injection, Dipole, Quadrupole, Sextupole DOI: /jkps I. INTRODUCTION The magnet system of PLS-II consists of 24 gradient magnets, 96 quadrupoles, and 96 sextupole magnets which have additional windings for horizontal correctors, vertical correctors, and skew quadrupole excitations [1]. For the injection system, 4 pulsed kicker magnets and a septum magnet are needed [2]. The major changes from the PLS design are as follows: The combined function gradient magnet is adopted to save lattice space for an additional short straight section. Also, the dipole field is pushed to the limit to make it shorter with the smallest possible 34 mm vertical gap. The number of dipole magnets is reduced from 36 to 24, and the effective length is increased from 1100 mm to 1800 mm. Q1, Q2, and Q3 series were wired in pair across the straight sections, and Q4, Q5, and Q6 were connected in series for the whole 24 magnets for each family. In PLS-II, we have 4 families of quadrupoles, which will be powered in series with independent auxiliary windings for operational flexibility. Therefore, all quadrupole coils will be redesigned for optimum current and cooling. For PLS-II, the maximum required field gradient reaches 24.5 T/m, compared to the design limit of 18 T/m. A preliminary 2D magnetic analysis showed that the existing quadrupoles could be operated at 24.5 T/m with small changes in the higher order harmonic contents. However, the adequacy of the existing quadrupole will be determined after detailed rotating coil measurements and beam dynamics simulations. All 70 combined function horizontal/vertical correctors are removed, and the corrector functions are combined with the sextupole magnets. The number of sextupole magnets is changed from 48 magnets with 2 families to 96 magnets with 4 families. Among the 96 magnets, 48 sextupole magnets are recycled from the PLS, and 48 new combined sextupole magnets will be needed. The kicker magnets for the injection will be mostly recycled. The straight section for the injection system will be readjusted to accommodate the increased septum length, and the increased kicker separation. The septum magnet will be redesigned with a smaller gap and a smaller leakage field, which are essential for top-up operation. The existing PLS quadrupoles will be mostly recycled. In the PLS, we had 6 families of quadrupoles The schematic layout of the PLS-II magnet system is with 24 magnets per each family (Q1 to Q6). shown in Fig. 1 where half of the superperiod is shown. The details of the magnet system for the PLS-II will be dekim@postech.ac.kr; Fax: detailed in the following section

2 Status of the PLS-II Magnet Design and Fabrication D. E. Kim et al Table 1. Parameters of the combined function dipole magnet. Parameters Value Unit Number of Magnets 24 EA Bend Angle 15 degree Central Field Tesla Nominal Gradient T/m Gap at the Center 34 m Effective Magnetic Length (along the arc) 1.8 m Magnetic Efficiency 0.95 Ampere Turns ka Number of Turns 48 Nominal Current A Current Density 4.57 A/mm 2 Voltage Drop/Magnet 38.1 Volt Conductor Size/ Cooling Hole Diameter 11.8/10.3/6.0 mm Conductor Area mm 2 Number of Pancakes/Pole 4 Resistance 21.5 mohm Power per Magnet 15.3 kw Inductance/Magnet 29 mh Number of Cooling Channels 4 Coolant Flow Rate 10.9 Liters/min Coolant Temperature Rise 20.2 K Coolant Pressure Drop 700 kpa Table 2. Requirements for the integrated multipole components for the combined function dipole magnet. Multipole component Systematic error Random errors Sextupole 0.54 T/m 2 ±0.11 T/m 2 Octupole 141 T/m 3 ±71 T/m 3 Decapole T/m 4 ± T/m 4 tion shielding wall, the circumference of the ring is nearly identical to that of the PLS. To squeeze the lattice space for the additional straight section, the space between the magnets is minimized, and the use of a combined function dipole is essential in saving lattice space. The combined function dipole magnet (also called the gradient magnet) has a dipole field combined with a focusing quadrupole field. With the built-in quadrupole field, the number of quadrupole magnets can be reduced. In PLS-II, the gap at the center of the magnet is 34 mm, which is very small compared to the previous gap of 58 mm. The central field is increased significantly to T, compared to the previous T, at 2.5 GeV. This dipole field is superposed with the focusing field gradient of T/m. Also, for the PLS-II DBA lattice, there are only 24 bending magnets, and each bending magnet should bend 15 degrees compared to the previous 10- degree bending for TBA. The effective magnetic length of the gradient magnet is m. The major parameters of the gradient magnet are summarized in Table 1. Also, the allowed integrated multipole errors based on the beam dynamics simulation are listed in Table 2. The sextupole, octupole, and decapole components are defined using the following convention. The vertical magnetic field B y at midplane with nominal excitation is expressed as a function of the transverse coordinate as follows: B y (x, s) = B 0 (s) + B 1 (s)x + 1 2! B 2(s)x ! B 3(s)x ! B 4(s)x 4 + Fig. 1. Schematic layout of the PLS-II magnet system. Half of the cell is shown. Blue magnets are the combined function dipole magnets, red magnets are quadrupoles, and yellow ones are sextupole magnets. II. COMBINED FUNCTION DIPOLE MAGNET Many small rings are adopting combined function dipole magnets to reduce the lattice space [3 6]. Since PLS-II should be accommodated in the existing radia- Here, s is a coordinate along the orbit and x is the transverse coordinate perpendicular to s and y. B 2 (s), B 3 (s), and B 4 (s) represent the sextupole, octupole, and decapole components, respectively. The values listed in the Table 2 are the integrated value of B 2 (s), B 3 (s), and B 4 (s) along the electron trajectory. The systematic error represents the allowed average value and the random part represents the magnet-to-magnet fluctuation. There were a few limitations related to the design of the gradient magnet: Since the effective magnetic length is long and the bending radius is small, the sagitta of the orbit is very large. The half sagitta reaches 28.7 mm. If we make the core of the magnet straight along the

3 Journal of the Korean Physical Society, Vol. 56, No. 6, June 2010 Fig. 2. Three dimensional drawing of the combined function dipole magnet. Two bus bars for parallel connections are shown. orbit, we need to increase the pole width to include the sagitta, which results in a bulky, and heavy magnet. To avoid this problem, the pole should be curved following the trajectory. Manufacturingwise, this approach might be challenging, but the magnet will be more compact and cost effective. Fig. 3. Typical flux shape of the combined function dipole magnet at nominal excitation. Using one lamination for the core is more cost effective and accurate, but the thickness of the coil pancake is limited to the minimum opening in the gap region. In our case, the center gap is 34 mm, and the minimum opening is 28.8 mm. Therefore, the thickness of the pancake should be less than 28.8 mm for the coil assembly. Another restriction is the very small longitudinal clearance between the gradient magnet and the sextupole magnet, which is located next to it. The longitudinal clearance between the iron yoke is small, and the coils from both magnets should not physically interfere with each other during installation. Also, adequate space for the installation should be secured. This severely restricts the coil width and results in a high-aspect-ratio coil that is tall and upright, minimizing the longitudinal extent of the coils. The 3D drawings of the magnet are shown in Fig. 2. The final coil consists of 4 layers of identical pancakes. The thickness of a pancake is mm, which is barely enough for assembly. The coil extends only mm from the end of the core. A simple 2D FEM(Finite Element Method) analysis are carried out using a pole contour which is conformal mapped from the SPEAR3 pole profile [4]. The conformal-mapped shims are adjusted to result in an optimum flux distribution in the good field region. The flux shape is shown in Fig. 3. The multipole contents are well within the physical requirements for this ideal geometry. However, there is a possibility of end saturation which may result in a worse higher harmonic content and higher saturation. Also, the manufacturing tolerance will also Fig. 4. Field uniformity of the gradient magnet for various excitation currents. contribute to the multipole content. The uniformity of the gradient magnet is defined using the following formula: B By (x) (B0 G0 x) =. B B 0 G0 x Here, B0 is the magnetic field at the magnetic center, and G0 is reference field gradient at the center of the magnet. The field uniformity is optimized at the nominal excitation, and the profile is different for slightly different excitation (I/In = 1.00, 1.016, 1.044, 1.106), as shown in Fig. 4. This implies that the field uniformity profile depends on the saturation characteristics of the pole material; therefore, the final field profile will be very sensitive to the material properties because the magnet is operated in the well-saturated region. The calculated 2-dimensional magnetic efficiency for the dipole component at nominal excitation is about 97.2% while that of the quadrupole component is 97.5%. The magnetic efficiency will change depending on the

4 Status of the PLS-II Magnet Design and Fabrication D. E. Kim et al Fig. 5. Field distribution of the gradient magnet at the midplane, as calculated by usingopera-3d. exact material used and the final yoke design. To assess the 3-dimensional effects, preliminary 3D OPERA calculations were carried out. Using the symmetry of the problem, the upper half of the magnet was modeled with a 3-mm mesh size near the air gap. The iron yoke followed the curved electron beam curvature. No efforts to optimize the end contribution like chamfering have been tried yet. The contour plot at the bottom clearly shows the curved, gradient structure of the field distribution. This model will be elaborated to estimate the required end chamfering after the prototype measurements. The results of the 3D calculation are shown in Fig. 5. Fig. 6. Typical end view of the quadrupoles. tape, with half overlapping. Fiberglass is chosen for the ground wrapping because it becomes transparent after epoxy impregnation, thereby allowing for the detection and the filling of trapped air bubbles that could contribute to electrical shorts. After the coils have been wound and tested, the coils are epoxy encapsulated in a vacuum-tight mold. Finally, the epoxy is cured in an electric oven. The manufacturing procedure for the other magnets are practically the same. III. MANUFACTURING OF THE MAGNETS The core is a laminated structure using 1.0-mm ultralow-carbon steel, which is custom-made by POSCO, or S1010 equivalent. The thickness variation across the lamination is ±3 µm, and the carbon content is less than 50 ppm. The hardness of the lamination is 55 ± 3 on the Rockwell Bscale. The magnetic induction at 100 Oe is 1.9 Tesla, and the coercive force is 0.8 Oe after magnetic excitation up to 100 Oe. Punching is performed with an 800-ton mechanical press at 20 C. The burr height is less than 12 µm, and the standard deviation of the pole profile from the ideal contour along the edge of the pole tip is 25 µm. The coils for the gradient magnet are constructed of 4 flat pancakes on each pole. The coils are fabricated from a rectangular hollow conductor of 11.8 mm (width) 10.3 mm (height) with 6.0 mm diameter cooling channel. Each pancake is composed of 6-turns of 2-layers and is wound from the bottom layer to the top layer without joints. The bare conductor is insulated with Mylar tape, 0.08-mm thick and 20-mm wide, and Dacron tape, 0.13-mm thick and 20-mm wide. The insulated coils are wound using a one-axis winding machine. After completion of winding, the coils are removed from the winding form and ground wrapped with1 layer of mm-thick and 20-mm-wide fiberglass IV. QUADRUPOLE MAGNETS There are four different types of quadrupoles used in the PLS-II, and these are designated as Q1, Q2, Q3, and Q4. The major parameters of these magnets are shown in Table 3, and a typical schematic diagram of these magnets is shown in Fig. 6. The quadrupole magnets of the PLSI will be mostly recycled for PLS-II. However, there are a few required changes which are listed below; In the PLS storage ring, the Q1, Q2, and Q3 magnets were powered in pairs across the straight section by one power supply, and the Q4, Q5, and Q6 magnets were powered in series of 24 magnets to save power supply costs. Compromising between more flexibility for accelerator operation and the economy of serially-powered quadrupoles, the main coils of the PLS-II quadruples will be powered in series, with a small auxiliary independently powered coil for maximum operational flexibility. The maximum field gradient for PLS was 18 Tesla/m. For PLS-II, we need a maximum field gradient of 24.5 Tesla/m. To check the multipole contents at this level of excitation, we carried out a simple 2D FEM analysis. Neglecting the C-shape

5 Journal of the Korean Physical Society, Vol. 56, No. 6, June 2010 Table 3. Major parameters of the quadrupole magnet. Parameters Value Magnet ID Q1/Q4 Q2 Q3 Number of Magnets EA Magnetic Length m Max. Field Gradient Tesla/m Aperture Radius 36 mm Core Length mm Ampere Turns per Pole ka Magnetic Efficiency Current Density 16 Conductor Size/Cooling Hole Diameter 9.5/9.5/5.5 mm Conductor Area mm 2 Power/Magnet kw Nominal Current A Voltage Drop/Magnet Volt Resistance/Magnet mohm Inductance/Magnet mh Number of Cooling Channels 4 Coolant Flow Rate liters/min Coolant Velocity m/sec Coolant Temperature Rise K Coolant Pressure Drop 700 kpa Unit higher harmonic content. The existing quadrupoles will be measured with T/m excitation to check the field quality in the near future. Since we are planning to operate the existing quadrupole at higher excitation, the number of cooling circuits will be increased to limit the maximum temperature rise to less than 20 K. The longitudinal space between the magnets is very small. Therefore, clamping structures and other mechanical structures will be redesigned to fit in the given lattice space. A small auxiliary winding, which is about 6% of the main winding, will be added for operational flexibility, like ID matching and beam based alignment. Fig. 7. Two dimensional flux distribution in the quadrupole magnet at nominal excitation. structure, only one pole was simulated. The flux shape is shown in Fig. 7, and the 2-dimensional multipole contents were calculated. The multipole contents were well within the physical requirements. However, there is a possibility of end saturation, which may result in a worse V. SEXTUPOLE MAGNETS There were two different types of sextupole magnets in the PLS; each series was powered in series. In PLS- II, there are 4 types of sextupoles and 2 sextupoles from each type are used for each super-period, resulting in 96 sextupole magnets. The existing 48 PLS sextupole magnets will be reused without modifications. The other 48 sextupoles, with some spares, will be manufactured with

6 Status of the PLS-II Magnet Design and Fabrication D. E. Kim et al Table 4. Major parameters of the sextupole magnet Parameters Value Unit Magnet ID S1 S2/S3/S4 Number of Magnets EA Magnetic Length m Max. Second Derivative 550 Tesla/m 2 Aperture Radius 39 mm Core Length mm Ampere Turns per Pole A Magnetic Efficiency Number of Turns 13 Current Density A/mm 2 Conductor Size/Cooling Hole Diameter 6.5/6.5/3.5 mm Conductor Area 32 mm 2 Power/Magnet kw Nominal Current A Voltage Drop/Magnet Volt Resistance/Magnet mohm Inductance/Magnet mh Number of Cooling Channels 3 Coolant Flow Rate liters/min Coolant Velocity m/sec Coolant Temperature Rise K Coolant Pressure Drop 700 kpa Maximum Skew Quak Grad T/m Maximu Horiz. Kick mrad Maximu Vert. Kick mrad the same core cross section as that of the existing sextupole magnet. The major parameters of these magnets are shown in Table 4. In addition to its primary function as a sextupole, this magnet should also operate as horizontal and vertical correctors and as a skew quadrupole magnet. The dedicated horizontal and vertical combined function correctors used in the PLS storage ring will be removed and the function will be incorporated in the combined function sextupole magnet to save lattice space. Still, there is an issue related to the strength of the correctors. The PLS design is based on a 2-mrad kick at a 2.0-GeV electron beam energy. This translates to a 1.33-mrad kick for 3.0- GeV electron beam for 200-mm long sextupole and mrad for 150-mm long sextupole magnets. The maximum 2nd derivative field of the PLS sextupole was 320 Tesla/m 2. For PLS-II, we need a maximum 2nd derivative field of 550 Tesla/m 2. To check the multipole contents at this higher level of excitation, we carried out a simple 2D FEM analysis. The existing sextupoles will be measured at an excitation level of 550 T/m 2 to check the field quality in near future. The temperature rise of the PLS-II sextupoles was estimated, and the temperature rise was be limited to 23.3 K in spite of the 550-T/m 2 excitation. The longitudinal space between the magnets is very small. Therefore, clamping structures and other mechanical structures will be redesigned to fit in the given lattice space. Due to the spacing problem, the inner coils will be used for the combined feature of the magnet, and the outer coil will be the main coil for sextupole excitations. A vertical steering field can be generated by applying a current I v at poles 1 and 6 and -I v at poles 3 and 4. Currents I h at poles 1 and 3, -I h at poles 4 and 6, 2 I h at pole 2, and -2 I h at pole 5 produce the horizontal steering field such that there will be no sextupole component in this excitation. The skew quadrupole component is created by using a current at the auxiliary coils of poles 2 and 5. VI. CONCLUSIONS In this report, the magnet system for PLS-II is described. The dipole magnets will be redesigned and replaced with a combined function gradient magnet to re-

7 Journal of the Korean Physical Society, Vol. 56, No. 6, June 2010 duce the longitudinal space. A magnetostatic analysis was carried out to meet the design requirements by minimizing the pole width and the weight of the magnet. Quadrupole magnets will be mostly recycled with modifications, adding 6% auxiliary windings for operational flexibility. Also, sextupole magnets will be recycled with 48 additional newly manufactured magnets with different lengths. The quadrupole will reach a 24.5-T/m field gradient, and the performance at this level will be measured by using a rotating coil system to verify that the quadrupoles are adequate for the PLS-II system. The sextupole will also operate at a 550 T/m 2 2nd derivative while it is designed for 320 T/m 2. The sextupoles will also be remeasured to check the validity for the PLS-II magnet system. ACKNOWLEDGMENTS REFERENCES [1] PLS-II Task force Team, Conceptual Design Report of PLS-II (Pohang Accelerator Laboratory, POSTECH, Pohang, 2009). [2] J. E. Milburn, S. H. Hong and S. S. Shin, Injection System for the PLS Storage Ring (Engineering note, Pohang Accelerator Laboratory, Pohang, 1994). [3] J. Tanabe, N. Andresen, R. Avery, R. Caylor, M. I. Green, E. Hoyer and K. Halbach, in Conf. Record of the 1989 IEEE Particle Accelerator Conference, Vol. 1, (1989), p [4] SPEAR3 Design Report (SSRL, SLAC, Stanford University, 2002). [5] L. Dallin, I. Blomqvist, D. Lowe and J. Swirksy, Gradinet Dipole Magnets for the Canadian Light Source (EPAC Paris, France, 2002) [6] M. Pont, E. Boter and M. Lopes, Magnets for the Storage Ring ALBA (EPAC2006, Edinburgh, Scotland, 2006). This work is supported by the Korean Ministry of Science and Technology and by POSCO.