A LARGE BORE QUADRUPOLE MAGNET FOR CALIBRATION OF ROTATING COILS (original) (raw)

A LARGE BORE QUADRUPOLE MAGNET FOR CALIBRATION OF ROTATING COILS

Kailash Ruwali #{ }^{\#}, S. William Amalraj, K. Sreeramulu, Ashok Kumar, Kushraj Singh, P. K. Kulshreshtha, R. S. Shinde, Accelerator Magnet Technology Division, Raja Ramanna Centre for Advanced Technology, Indore, India

Abstract

A large bore quadrupole magnet is designed, fabricated and magnetically characterized for calibration of rotating coils. The magnetic design was carried out using 2D POISSON code and OPERA 3D code. Two different excitation coils were designed having same number of turns but different conductor sizes to utilize the available space in the magnet geometry for obtaining maximum field gradient. The magnet design produces a field gradient of 3.2 T/m3.2 \mathrm{~T} / \mathrm{m} in the magnet aperture diameter of 310 mm . The systematic higher order multipoles which are obtained using 3D calculations are in concurrent with the measured data using Harmonic Bench Model 692. The details of magnetic design, magnet development and magnetic measurement results are discussed in this paper. Calibration of rotating coils was carried out using this magnet and the coil parameters obtained after calibration are presented.

INTRODUCTION

A quadrupole magnet having aperture diameter 310 mm and core length of 310 mm was designed, developed and magnetically characterized. This quadrupole magnet was required for the calibration of rotating coils having major coil radius of 138 mm . These rotating coils were designed and developed by M/s Danfysik A/S, Denmark and are integral part of Harmonic Bench Model 692 [1]. This bench will be used to characterize multipole magnets at RRCAT. In this paper, the design, fabrication, magnetic measurement results of the quadrupole magnet and rotating coil parameters obtained after calibration are discussed.

MAGNET DESIGN

The quadrupole magnet consists of four poles in its magnetic circuit. Each pole is associated with an excitation coil(s) to generate a magnetic field with a constant gradient in both horizontal plane and vertical plane. The magnitude of the magnetic field being zero at the centre of the magnet aperture and increases linearly with increasing distance from its centre. The strength is specified in terms of the gradient (Tesla/meter) of the magnetic flux density between the poles. The coil windings on the adjacent poles are connected electrically in series (provided the current direction is in opposite sign, with the adjacent pole windings) within the quadrupole magnet. The 1/81 / 8 cross-section of the quadrupole magnet is shown in Fig. 1. The pole tip profile is hyperbolic curve for obtaining good quadrupole

Figure 1: 1/8 Cross-section of magnet.
magnet field distribution. The shape of the pole tip profile does not exactly correspond to a true hyperbola and the poles have been truncated laterally to provide space for coil(s). In order to compensate for the finite pole width, shims are provided at the ends of the pole tip profile. The idea of using a short flat at the end of the pole profile (as shown in Fig. 1) is to improve the physical measurement of the pole spacing. The magnet pole ends are chamfered [2] for defining the magnetic length more precisely, preventing saturation and controlling transverse field distribution. The pole end chamfer of the quadrupole magnet is shown in Fig. 2. The cross-section of the quadrupole magnet was determined by computing the gradient distribution with the two-dimensional magneto static program POISSON [3]. The pole profile is a hyperbolic curve with a tangent to produce 3.2 T/m3.2 \mathrm{~T} / \mathrm{m} quadrupole field gradient with the higher order multipole field errors in the range of 5E-04. The poles were tapered to an angle of 37∘37^{\circ} and return back legs were optimized to get field values below 1.7 T , in order to reduce the

Figure2: Pole end shape.

Table 1: Major Parameters of the Quadrupole Magnet

hysteresis effect [4]. All 3-dimensional calculations were performed using OPERA-3D [5]. Table 1 gives the major parameters of the quadrupole magnet.

MAGNET DEVELOPMENT

The quadrupole magnet is required to be fabricated with stringent tolerances as this magnet will be used for the calibration of rotating coils. Pre-machined low carbon steel plates (carbon content ∼0.08%\sim 0.08 \% ) of ∼35 mm\sim 35 \mathrm{~mm} thick were used as the magnet core. In order to achieve the required core length of 310 mm,9310 \mathrm{~mm}, 9 machined plates (both sides) were stacked with an allowance of 4 mm in matting area and pole profile. These plates were then stacked, compressed using hydraulic press and then joined by welding to get each core quadrant as shown in Fig. 3. A total of 4 such quadrants were fabricated. The welded quadrants were then machined at matting surfaces and pole profile using 5 axes CNC machine. Table 2 summarizes the mechanical inspection report of the assembled quadrupole magnet. The difference between maximum and minimum in bore diameter and inter pole distance are 0.15 mm and 0.3 mm which are on higher side then the desired values of 0.1 mm and 0.2 mm respectively. This variation is attributed to the technical complexity involved in fabricating the large size,

Table 2: Mechanical Inspection Report

Figure 3: Welding, clamping bolts and L-clamps positions for the assembly.
heavyweight magnet. Also, this was the first time that such a large and complex magnet was fabricated in India.

Two different coil configurations (coil type A and coil type B) have been adopted to utilize the available space and to obtain maximum field gradient using a power supply of rating 1000 A and 28 V . There are 4 coils of each coil type and are connected electrically in series using copper bus bar. The coil type A and coil type B are connected electrically in parallel. Both the coil types have same number of turns ( 48 turns per pole) but different conductor size. The conductor sizes are 15×15×ϕ10 mm15 \times 15 \times \phi 10 \mathrm{~mm} and 7×7×ϕ5 mm7 \times 7 \times \phi 5 \mathrm{~mm}. The coils were wound using oxygen free high conductivity hollow copper conductor. The grade of copper used is C 10200 (OF grade) as per ASTM B-170 having purity 99.99%99.99 \%. The conductor was cleaned thoroughly to eliminate dust, rust and oil etc. before wrapping of the fibre tape with 50%50 \% overlap for inter-turn insulation, then wound the required turns using a coil winding former on a machine for maintaining the coil geometry. The wound coils were epoxy resin impregnated/casted under vacuum using moulds. The finished coils were inspected for the geometrical dimensions, electrical parameters (resistance, inductance, inter-turn insulation tests etc.). Two common hydraulic cooling headers were made and laid around the coils in order to connect them with parallel connections.

MEASUREMENT RESULTS AND COIL CALIBRATION

Before carrying out the magnetic measurements, magnet is demagnetized completely by energizing the magnet in forward-reverse direction. After this, magnet is cycled from 0 to peak current 5 times and then measurements were carried out. All the measurements were taken in forward direction of hysteresis curve.

Magnetic field pattern was measured using 3 axes hall probe manipulator. Hall mapping of the magnet was carried out using LPT-141-2S Hall probe from Danfysik in magnet mid plane ( 140 mm either side of magnet centre) till the magnetic field goes to about 0.5%0.5 \% along the magnet length on both sides of the magnet.

Figure 4: Measured magnetic field pattern along axial direction at various radial distances.

Figure 5: Photo of magnet during magnetic measurement using Harmonic Bench Model 692.

The measured vertical field pattern along axial direction (along the length of the magnet) at various radial distances (horizontal plane) is shown in Fig. 4. From measured data, we found that the magnetic length is 0.451 m and integrated quadrupole strength is 0.89292 T at 600 A.

The higher order harmonics present in the magnet aperture was measured using Harmonic Bench Model 692. Figure 5 shows the photo of quadrupole magnet during magnetic measurement using Harmonic Bench Model 692. The rotating coil method is a standard and widely accepted method of measuring accelerator magnets [6]. A long coil is placed along the central axis of a magnet, and rotated about that axis. A voltage is induced in the coil equal to the rate of change of the normal component of flux integrated over the coil’s surface. Decomposing the measured voltage by Fourier analysis breaks it down into components. Subsequent analysis of a complete rotating coil measurement leads to the extraction of magnet parameters including integrated field gradient, harmonic contents, magnet hysteresis effects and field repeatability [7]. The rotating coil which is meant for the magnetic measurement of the quadrupole magnets are configured to measure the higher harmonics with the dipole and quadrupole signals bucked out [8]. Figure 6 shows the coil cross section of a tangential coil. The coil consists of a main coil (Coil 1) and bucking coils

Figure 6: Cross-section of a tangential coil.

Figure 7: Comparison of theoretical and measured systematic higher order multipoles.
(Coil 2 and Coil 3). Main coil has 40 turns and bucking coils (Coil 2 and Coil 3) have 20 and 10 turns respectively. Bucking coil is used to cancel out the main field component and allow higher harmonics to be measured more accurately. The integrated strength measured using hall probe was used to deduce the rotating coil parameters and are mentioned in Table 3. The measured systematic higher order multipoles are in agreement with 3D calculations and are shown in Fig. 7.

Table 3: Design and calibrated parameters of rotating coil

ACKNOWLEDGEMENT

We acknowledge the meticulous attention to the details paid and hard work done by Shri. Bhim Singh and Shri. Navin Awale during fabrication and testing.

REFERENCES

[1] Harmonic Bench Model 692, M/s Danfysik A/S, Denmark.
[2] Masayuki Kumada et al., “Optimization on the end shaping of a quadrupole magnet”, Nuclear Instruments and Methods 211 (1983) 283-286.
[3] POISSION Group of Codes, LANL, New Mexico.
[4] Kailash Ruwali et al., “Quadrupole magnets for Indus2”, IEEE Transactions on Applied super conductivity, volume 14, No.2, June 2004, 406-408.
[5] Vector Fields Limited, 24 Bankside Oxford, England.
[6] R. P. Walker, “Magnetic Measurements”, in H. Winick, “Synchrotron Radiation Sources: A Primer”, World Scientific, 1995, 181-188.
[7] K. N. Henrichsen, “Magnetic Field Measurements in Beam Guiding Magnets”, Technical Note LHC-98008, CERN, Geneva Switzerland, 1998.
[8] Animesh K. Jain, “Harmonic Coils”, Proc. CERN Accelerator School on Measurement and Alignment of Accelerator and Detector Magnets, CERN-98-05.