Measurement of the ripple of magnet power supply and its effect to the beam energy

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Radiat Detect Technol Methods (2017) 1:7 DOI 10.

1 Radiat Detect Technol Methods (2017) 1:7 DOI /s ORIGINAL PAPER Measurement of the ripple of magnet power supply and its effect to the beam energy Jian-Yong Zhang 1 Xiao Cai 1 Xiao-Hu Mo 1 Guang-Yi Tang 1 Shu-Jun Wei 1 Feng-Li Long 1 Bin Chen 1 M. N. Achasov 2,3 N. Yu. Muchnoi 2,3 Received: 20 January 2017 / Revised: 4 May 2017 / Accepted: 9 May 2017 / Published online: 1 June 2017 Institute of High Energy Physics, Chinese Academy of Sciences; China Nuclear Electronics and Nuclear Detection Society and Springer Science+Business Media Singapore 2017 Abstract Objective: This study is aimed at measuring the ripple of magnetic power supply of BEPCII and checking its effect to beam energy. Materials and methods: A sensor made of printed circuit board coils is designed and manufactured. The sensor was inserted into a good area region of the magnetic field with the surface perpendicular to the field force lines. The change of the magnetic field would be detected according to Faraday s law. Results: The experiment result indicates that the timedependent ripple of the magnetic field is in the magnitude of ppm. Conclusion: Such a small effect of the time-dependent ripplecan be negligibleto beam energy. Keywords Laser HPGe detector Beam energy measurement PACS Kv Fy This work is supported in part by National Natural Science Foundation of China (NSFC) under Contracts Nos.: , , , and B Jian-Yong Zhang jyzhang@mail.ihep.ac.cn Xiao-Hu Mo moxh@mail.ihep.ac.cn 1 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing , China 2 Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences, 11 Lavrentyev, Novosibirsk, Russia Novosibirsk State University, Novosibirsk, Russia Introduction The upgraded Beijing Electron Positron Collider (BEPCII) isa τ-charm factory with a center-mass energy ranging from 2.0 to 4.6 GeV and a design peak luminosity of cm 2 s 1 [1,2] at the center of mass energy of GeV. The design luminosity was achieved on the evening of April 5, The upgraded Beijing spectrometer detector (BESIII), with high efficiency and good resolution for measuring both charged and neutral particles, started data taking in 2008 [3,4]. The largest charm and charmonium data samples in the world were collected, and more than a hundred papers have been published these years. After large amounts of data are acquired and analyzed, the statistical uncertainties in physics analysis become smaller and smaller, while the systematic uncertainties play more and more prominent roles [5 7]; especially, the uncertainty due to the measurement of beam energy is a big concern. Starting from the year 2007, a high accuracy beam energy measurement system (BEMS) located at the north crossing point (NCP) of BEPCII was designed, constructed and put into the commissioning at the end of 2010 [8 11]. The layout schematic of the system is shown in Fig. 1. Two days were used to perform the ψ scan. The mass difference between the PDG (2010) value and the measured result by BEMS is 1 ± 36 kev, the deviation of which indicates that the relative accuracy of BEMS is at the level of [10]. However, during the J/ψ scan in 2013, the offline data analysis discloses an energy deviation of 0.6 MeV for the measured J/ψ mass by virtue of BEMS from that given by PDG (2012). The deviation is too large to be explained by the uncertainty of both BEMS measurement and PDG value. We have checked many possible reasons which could affect BEMS detection carefully, but we did not find such a fac-

2 7 Page 2 of 6 J.-Y. Zhang et al. Fig. 1 Simplified schematic of beam energy measurement system. The positron and electron beams are indicated. R1IAMB and R2IAMB are accelerator magnets, and the HPGe detector is represented by the dot at the center.the shielding wall of the beam tunnel is shown cross-hatched, and the laser is located out-side the tunnel R2IAMB Laser positrons 6.0m Lenses electrons HPGe 1.5m 1.8m 0.4m R1IAMB 3.75m tor which could cause such obvious deviation. The relation between the uncertainty of Compton backscattering process and the status of accelerator was studied quantitatively [14], which indicates that stability of accelerator plays an important role in energy determination. It is well known that high luminosity and high stability of BEPCII are a basis for good quality data taking for BESIII. The key factor for the BEPCII stability is the stability of magnet, which in turn involving the stability of power supply of magnet. The construction stability of the power supply is claimed at the level of In 2015, the power supply group of accelerator center measured the magnet current ripple using TeK 3014 C oscilloscope and TeK spectrum analyzer RSA 3303B; however, the measurement results was not consistent. Since the stability of magnet is a concern for the BESIII data taking and the energy deviation problem of BEMS, a careful measurement should be performed to clear up the issue. In this paper, the measurement of the ripple of the power supply of BEPCII magnet will be described in detail. The paper is organized as follows: The magnet lattice of BEPCII and corresponding magnet power supply are introduced in Magnet lattice of BEPCII and corresponding magnet power supply section. The methodology of the measurement is presented in Methodology section, including a general ideal, the design of coil, and the manufacture of the sensor. The measurement status is introduced in Measurement section. A short discussion is presented in the last section. Magnet lattice of BEPCII and corresponding magnet power supply BEPCII is the upgrade of BEPC, with two rings in the existing tunnel serving high energy physics and synchrotron radiation (SR) research [15]. Serving as a collider, BEPCII operates in the beam energy region from 1.0 to 2.3 GeV, and aim for high luminosity. The schematic storage rings of BEPCII are shown in Fig. 2a, and a example of magnet is shown in Fig. 2b. The inner ring and the outer ring cross at the north interaction point and the south interaction point, and Fig. 2 a Schematic diagram of the storage ring of BEPCII. b Magnet in the storage ring of BEPCII the positron beam and the electron beam run in a half of inner ring and a half of outer ring, and then collide at the south interaction point with the angle of 11 mrad. For the dedicated SR operation, electron beams circulate in the outer ring with a pair of horizontal bending coils in the super conducting

3 Measurement of the ripple of magnet power supply and its effect to the beam energy Page 3 of 6 7 Table 1 Detailed information of magnet and corresponding power supply of BEPCII in storage ring Magnet type Connection type P.S. type Number of P.S. Stability (8 h) Bending magnet Half ring series connection Silicon controlled Quadrupole magnet Separated power supply Switch mode Sextupole magnet Two magnets series connection Switch mode Correction magnet Separated power supply Switch mode Special magnet Separated power supply Switch mode magnets. In the northern crossing point, a bypass is designed to connect two halves of the outer ring. BEPCII requires the injector in two aspects [16]. One is the full energy of e and e + beams injected into the storage rings; the other one is with e + injection rate 50 ma/min. To realize the full energy top-off injection up to 1.89 GeV, the klystrons were replaced with new MW ones and the modulators upgraded with new pulse transformer oil tank assembly, pulse forming network, thyratrons, charging choke and DC power supplies. BEPCII reuses 44 BEPC bend magnets and 28 quads. A total of 267 new magnets, including 48 bends, 89 quads, 72 sextupoles, 4 skew quads and 54 dipole correctors, were produced. The magnets were measured with both rotating coils and straight wires. The results are in agreement with each other within There are 1 electric and 3 permanent wigglers in the storage rings serving as SR wavelength shifters. To provide required flexibility for BEPCII operation with various modes, each arc quadruple is excited with an independent power supply. There are approximately 400 magnet power supplies in the storage rings and transport lines. The detailed information of magnet and corresponding power supply are listed in Table 1. According to precision controlling and stability requirements to the magnet power supply of BEPCII, a long-time investigation on the front-end hardware was performed. A BNL-designed PSC/PSI (Power Supply Controller/Interface) was adopted [17] to control power supplies with high precision. It not only has a good performance, but also its commercial products are available. The PSC/PSI is a very simple integrated system that provides all functions (current setting, value reading, control and report) with a single board. The PSI was installed in the power supply. The PSC may reside in the VME-64x crate or in a chassis. The PSI and the PSC are connected with a pair of fiber optic cables that provide electrical isolation. One PSC can control up to six PSIs. The PSI has one 16-bit analog output, four 16-bit analog inputs, fifteen digital commands and sixteen digital read back. The PSI and power supply are connected with two cables: one for the analog signals and the other for the digital signals. A PSC has a RS-232 interface and a VME interface. It can be accessed and controlled by VME bus and a serial port. Methodology General idea Assume one has spatially uniform magnetic field which experiences harmonic oscillation in time according to B = B 0 cos(ωt), (1) where ω = 2πν, and ν is the integer multiple of power frequency. A flat current loop with surface S placed into such a field perpendicular to the field force lines, the magnetic flux can be given by: φ = B S, (2) an induction voltage will be produced according to Faraday s law, and the voltage on the loop end can be written as: U = dφ = ωsb 0 sin(ωt) = U 0 sin(ωt), (3) dt where U 0 = ωsb 0. Using the multi-layer coils, as long as the area is large enough, the change of the magnetic field will be detected, even using a oscilloscope. The drift of the magnetic field caused by other reason such as temperature cannot be detected using such method. Concept design of coil A monitor made of printed circuit board (PCB) was designed as an square coil probe to measure the stability of the magnet field in the storage ring of BEPCII. The probe should be inserted into the good area region of the magnetic field 1 during experiment. The dimensions of the space between the magnet and vacuum chamber are 120, 3.3, and 150 mm respectively. For convenience, a PCB with 100 mm 100 mm, a line width of 4 mils 2 and a gap width 1 The good area region of magnetic field means the magnetic force lines go through the vacuum box uniformly and vertically. 2 Mils is a unit of length, 1 mils means one thousand of inch, namely mm.

4 7 Page 4 of 6 J.-Y. Zhang et al. But from practical point of view, the inner rings make the small contribution for the total area and increase the manufacture cost, so it is reasonable to begin from a certain ring, say from kth ring. Therefore, the area of one layer becomes A d = Δ 2 (S m S k ). (7) Manufacture of circuit Fig. 3 The PCB sensor. a Design of the coil; b, c are the connection of multiple layers with plan view and side view separately. d Photograph of PCB sensor of 4 mils chosen as the manufacture parameter. About 200 rings is expected for one layer, as shown in Fig. 3a. Multiple layers scheme is effective to extend the rings and increase the effective area. Consequently, 10 layers are cascaded. The connection method from plan view and side view is shown in Fig. 3b, c separately. The area for one layer can be evaluated as follows. The innermost area of layer is denoted as (2Δ) 2, which is equal to the square of 2(a + b) (that is Δ = a + b). Here a and b indicate, respectively, the line width and the gap between two adjacent lines. For the next area, it is equal to (4Δ) 2, and similarly, the area encircled by the jth ring will be (2 jδ) 2, refer to Fig. 3c for schematic structure of coil. The sum of all the area gives the total area of one layer, viz. A = (2Δ) 2 + (4Δ) 2 + +(2 jδ) 2 +. (4) Considering the detection efficiency and manufacture ability, the design of coil probe has been optimized. Too narrow of a width and a gap width will cause the circuit break from corrosion. According to the engineer s suggestion, the suitable dimension of line width and gap width is 5 and 6 mils separately. The distance of line to the center of PCB gets closer, the effective area of the coil around smaller. To satisfy the high detection efficiency requirement, a square with the side length of l = 26.5mm is left blank at the center of the PCB. Taking into account a solder joints between layers, input hole, output hole should be left in the square, only 125 rings were manufactured for one layer. By virtue of about dimensions, a = 5 and b = 6 mils. The k is estimated as l/(2δ) 47, while m = k = 172. With these numbers, the area of one layer can be figured out in the light of Eq. (7) tobe0.523m 2. The thickness of every layer is about 0.15 mm. Multiple layers are welded, which will help to improve the detection efficiency. Therefore, 16-layer coil probe is manufactured. The final PCB sensor is shown in Fig. 3d. The total effective area is about 8.37 m 2 theoretically. Using a standard magnet, the effective area of the sensor is calibrated by comparing the measured induction voltage and the equivalent area is determined as 8.64 m 2. Measurement The sensor has compact structure and is sensitive to the instantaneous change of the magnetic field. The contribution of ripples with different frequency can be stripped from the measurement result using Fourier transform. As BEMS is installed at the north crossing point of the storage ring of BEPCII, the ripple of the magnet field nearby If the total number of rings is m, the area is A m = Δ 2 S m, (5) where S m = (2m) 2 = 2 m(m + 1)(2m + 1). (6) 3 Fig. 4 The setup of the experiment

5 Measurement of the ripple of magnet power supply and its effect to the beam energy Page 5 of 6 7 Fig. 5 The distribution of measurement results. a The dependence of induction voltage on measurement time. b The dependence of dipole field ripple on the frequency after Fourier transform should be concerned. The experiment was arranged on June 15, The measurement was taken by inserting the sensor to the gap between the vacuum chamber and the magnet yoke. Several measurements were carried out in magnet gaps near the crossing point, such as R1IMB01, R1IMB02, R1OMB02, R2OMB01 and R1IMB02. During the experiment, the beam energy is set at 2.09 GeV and the direct current field is measured using portable tesla meter. In the magnet field area, the value is about 0.78 T, while the background is about 1.3 Gauss. An oscilloscope is connected with the sensor to detect the voltage signal. The surface of the sensor is covered by aluminum foil to reduce noise. Even if the effective area of the sensor is large, the detected signal is small. Therefore, a differential preamplifier with gain of 100 is used to amplify the signals. The setup of the experiment can be found in Fig. 4. Labview software is used in the data acquisition. The sampling frequency is 250 KS/s, and 100 K points are included in one event. The typical distribution of the detection signal is shown in Fig. 5, in which (a) is the induction voltage detected by oscilloscope, (b) is the frequency distribution after Fourier transform. Discussion The dependence of the ripple of dipole field on the frequency is shown in Fig. 5b. It is clear that the dominant ripple is observed at the 50 Hz, and its amplitude is about 1 µv. The second maximum is shown at 100 Hz, whose amplitude is 1/10 of the maximum. The amplitudes of other peaks are much smaller. The time-dependant ripple of magnetic field is in the magnitude of µv, whose contribution to the fluctuation of magnetic field is about 1 ppm. Considering the other drift due to temperature, humidity and so on, the amplitude is no more than [18] as measured by power supply group. The large deviation of 0.3 MeV of BEMS cannot be caused by the ripple of power supply of magnet. There must be other reasons which cause such large deviation. References 1. Q. Qin, Z. Duan, N. Huang et al., Beam dynamic issues in the BEPCII luminosity commisioning, in: Proceedings of IPAC 10, Kyoto, Japan, p Preliminary Design Report of Accelerator BEPC, Second version, (in Chinese) 3. M. Ablikim et al. (BESIII Collaboration). Nucl. Instrum. Methods A 614, 345 (2010) 4. K.-T. Chao, Y.-F. Wang, Int. J. Mod. Phys. A (Suppl. Issue 1) 24, 1 (2009) 5. C.D. Fu, X.H. Mo, Chin. Phys. C 32, 776 (2008) 6. X.H. Mo, Nucl. Phys. B (Proc. Suppl.) 169, 132 (2007) 7. Y.K. Wang, X.H. Mo, C.Z. Yuan et al., Nucl. Instrum. Methods A 583, 479 (2007) 8. M.N. Achasov, V.E. Blinov, A.V. Bogomyagkova et al., Nucl. Phys. B (Proc. Suppl.) 189, (2009)

6 7 Page 6 of 6 J.-Y. Zhang et al. 9. X.H. Mo, E.V. Abakumova, M.N. Achasov et al., Chin. Phys. C 34, (2010) 10. E.V. Abakumova, M.N. Achasov, V.E. Blinov et al., Nucl. Instrum. Methods A 659, (2011) 11. J.Y. Zhang, E.V. Abakumovab, M.N. Achasov et al., Nucl. Phys. B (Proc. Suppl.) , (2012) 12. M. Ablikim et al. (BESIII Collaboration). Phys. Rev. D 90, (2014) 13. X.H. Mo, C.D. Fu, J.Y. Zhang et al., Chin. Phys. C 32, 995 (2008) 14. X.H. Mo, Chin. Phys. C. 38(10), (2014). arxiv: [hep-ph] 15. F.A. Harris, Int. J. Mod. Phys. A 24, 377 (2009) 16. C. Zhang, Chin. Phys. C 33(Suppl. II), (2009) 17. J. Liu, C. H. Wang, J. Zhao, A test system for BEPCII power supply control, in: Proceedings of PCaPAC2005, Hayama, Japan 18. Private communication

τ lepton mass measurement at BESIII

τ lepton mass measurement at BESIII J. Y. Zhang 1* on behalf of BESIII collaboration 1 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China * jyzhang@ihep.ac.cn November