Optimization stability and performance of the TPS storage ring dipole magnet power supply

This article focuses on the current status of the dipole magnet power supply (DMPS) and improvement in the Taiwan Photon Source (TPS) storage ring. The DMPS provides a stable and precise magnetic field for the storage ring operation. Appropriate wiring methods are employed to minimize magnetic field interference across the entire TPS area to meet the demands of high-energy output and overcome the challenges of operating at high currents. The target energy is set at 850 V and 750 A, utilizing a single-pole switching voltage regulator as a high-precision constant current source output structure. The system incorporates a closed control loop that uses the Direct Current Current Transducer (DCCT) to provide current signal feedback to the system. FPGA (Field-Programmable Gate Array) calculates PID (Proportional-Integral-Derivative) compensation values, generating a 2.1 kHz pulse width modulation (PWM) signal to regulate the output current. At the same time, insulated gate bipolar transistor (IGBT) modules are switching components. However, even after several years of practical operation, the stability and performance of the DMPS in the storage ring still require improvements. To enhance long-term output current stability and address peripheral issues, the current TPS utilizes the Beam Orbit Feedback (FOFB) system to suppress and fine-tune the magnetic field and compensate for the impact of temperature drift on the DMPS's output current. This improvement ensures a more stable circulation of the photon beam within the storage ring. By optimizing the temperature control circuit of the main control card, the long-term output current stability has been successfully enhanced to within ± 10 ppm. Simultaneously, the FOFB system reduces uncertainties in adjusting the X-axis beam position, improving beam stability and quality. Furthermore, relevant protective measures have been implemented to ensure robust system operation. Ultimately, these improvement measures have successfully met TPS's stringent requirements for the DMPS, enabling the synchrotron accelerator light source to operate at higher performance levels and fostering advanced scientific research. The results of these upgrades underscore the success of the power supply enhancements, making significant contributions to the overall improvement of the Taiwan Photon Source facility.


Introduction
The article focuses on the status and upgrades of the DMPS in the storage ring of TPS.TPS is a third-generation 3 GeV synchrotron light source in Taiwan, successfully commissioned on December 16, 2014.The electron beam is efficiently accelerated from 150 MeV to 3 GeV with a repetition frequency of 3 Hz.By the end of 2015, the TPS storage ring achieved a stable beam current of 500 mA and has been operating successfully for nearly a decade.TPS has received widespread recognition for its world-class beam quality and performance standards.Continuous improvements and system updates are crucial for maintaining and enhancing research facilities [1,2].In the TPS storage ring, the dipole magnets are periodically distributed around the ring with a circumference of 518 meters, divided into 1/24 segments, each consisting of 2 dipole magnet units.The entire storage ring consists of 48 dipole magnet units, which play the role of deflecting electrons.A single DMPS unit in series provides the energy supply for these magnets [3][4][5].The overall TPS control system requires the stability of the beam position to be less than 1/10 of the beam size.The FOFB system has been successfully implemented to achieve sub-micron orbit stability.FOFB effectively improves the orbit stability, with suppression bandwidths of horizontal and vertical planes reaching 300 Hz [6][7][8].However, concerning the relationship between the actual bend magnet current and the tuning of the beam position in the X direction, the FOFB system is used to compensate for the instability generated by the dipole magnet current, effectively stabilizing the situation of the beam particles.Moreover, the dipole magnet current's frequency spectrum can interfere with the FOFB -1 -system's tune-X behavior.Therefore, this paper primarily investigates the issue of the instability of the output current from the DMPS.To understand the impact of changes in the dipole magnet current on the position of the beam particles, experiments were conducted under top-up mode conditions by increasing the bend magnet current by 100 mA and closing the FOFB system to observe changes in the beam size and position.The experimental results, as shown in figure 2, indicate that with a 100 mA increase in the bend magnet current, the  position of the beam particles moved from −56.5 μm to −57.5 μm, resulting in a one μm variation.The instability of the DMPS output directly affects the beam particles' position.To improve the stability of the output current from the DMPS, we conducted a detailed analysis of the circuit structure and control loop of DMPS to identify areas for improvement.Ultimately, by adjusting the compensation temperature of the main control card, we successfully ensured a more stable circulation of the photon beam in the storage ring and achieved a long-term output current stability within ±10 ppm during the week [9][10][11][12].
Additionally, the article describes the experiences with DMPS operation over the years, including installing power cabling, generating noise from low-frequency power switch switching, and implementing leak and fire detection systems.These measures aim to ensure a safer and more stable operation of DMPS in the TPS storage ring, which is crucial for particle accelerators [13].

Storage ring dipole magnet power supply
The NSRRC power team installed the DMPS in the TPS storage ring purchased from Eaton Yale Power.Before testing the TPS DMPS performance, we ensured that all power requirements were correctly set, including cooling water, controller settings, replacement controls, wiring, and other engineering tasks, to meet TPS specifications (except for noise).To enhance its performance, we measured all current -2 - output performances to ensure they met TPS requirements, including sound noise.We performed the installation step by step.We built a dedicated DMPS soundproof room, effectively resolving the noise issue by reducing low-frequency switch noise from 97 dB to 54 dB, equivalent to background environmental noise [14].When TPS successfully operated at 3 GeV, the storage ring current was 400 mA.The TPS DMPS engineering installation was proven successful.However, the load consists of 48 dipole magnets and cables.We used three parallel lines (each with a cross-sectional area of 325 mm 2 ) to connect all the dipole magnets.The TPS storage ring has a circumference of 518 m, and the return cable length is approximately 600 m, requiring three power lines.The current density of the cable at the maximum output of DMPS is 0.718 amperes per square millimeter.However, the DMPS output cable installed in TPS can withstand a current density of 4 amperes per square millimeter, meeting the electrical requirements and safety specifications of DMPS.We connected the return cable in series with all the dipole magnets.Installing the return cable is crucial in the TPS storage ring dipole system.The cable connections in the TPS storage ring are shown in figure 3, and installing the return cable effectively reduces parasitic inductance, achieving the expected function [15,16].The unipolar power supply is specifically designed for the TPS storage ring dipole magnets.The converter can provide 750 amperes of current and up to 850 volts of voltage.Figure 4 shows the technical circuit diagram of the DMPS, which can be divided into sections such as input electromagnetic interference (EMI) filter and Soft start circuit, AC-DC and DC-DC converter, output filtering, and control cabinet.High-precision DCCT is used as the feedback current signal to the controller, and FPGA is used to implement PI compensator calculations, generating PWM modulation signals to drive intelligent SkiiP IGBT modules, achieving an output current of 750 amperes with long-term stability within ±10 ppm.

INPUT EMI filter, soft start, and rectifier
The system block diagram of the storage ring DMPS is shown in figure 5.It can be observed that the DMPS power supply is connected to a 380 V, 60 Hz three-phase input power source through the EMI filter terminal.The output of the EMI filter is connected to the circuit breaker (CB)1, while the primary input isolation is provided by contactors CR1 and CR2.CR1 is the main contactor, while CR2 is the charging contactor.CR2 contactor is in series with resistors (R1-R3) to form a soft-start circuit, reducing the inrush current of the transformer (TR)1.Isolation transformer TR1 provides line voltage to meet the required load voltage and is appropriately sized to meet the required output voltage and current.The transformer configuration consists of a primary connection in a delta connection and a secondary connection in a delta-star connection.This 12-pulse connection eliminates the fifth and seventh harmonics in the input line, resulting in the total harmonic distortion (THD) of the input current being less than 10 %.The voltage THD depends on the system's short-circuit ratio (for a fast short-circuit ratio of 10, the voltage THD is approximately 5 %).The control power is derived from the 380 V line-to-line power supply provided through the control transformer TR2.

AC/DC rectifier and DC/DC converter
The Rectifier section of theDMPS consists of two sets of three-phase diode bridge rectifiers (BRA and BRB) configuration, forming a twelve-pulse diode rectifier.The twelve-pulse diode BRA and BRB are connected to each secondary of TR1 through fuses (FS1 to FS6), as shown in the first half of figure 6, to convert three-phase AC input into DC.The two outputs of the rectifier are connected in series to provide the required DC voltage.The output of the diode rectifier is followed by the filter CF1, which mainly reduces the diode rectifier's output ripple voltage and current.These capacitors are part of the IGBT module.Additionally, the output of the DC rectifier introduces the IGBT module.The IGBT module comprises a DC capacitor filter (CF1), intelligent SkiiP IGBT modules, and water-cooled heat sinks.The SkiiP IGBT module has current monitoring capabilities and provides active current sharing and current protection between modules.PWM technology controls the output current.The current monitoring feature of the switching modules provides an IGBT frequency of 2.1 kHz.

Output filter and current feedback detection components
The output stage of the DMPS begins with an LC filter (LO1, CO1), as shown in the second half of figure 6.This filter provides the output voltage that meets the required specifications for ripple voltage.After this filter, a damping network (CO3, RO4) is used to reduce transient overvoltage.The Detector Isolation Board (VTB3) is used to sense the unfiltered voltage, which serves as a feedback signal for the internal voltage loop.The Detector Isolation Board (VTB) is used to sense the output voltage and provides an isolated signal to the control board for local and remote monitoring and protection.Resistors RG1 and VTB5 provide ground fault protection signals.Two precision ultra-stable DCCTs give current output signals to the control board.One of these signals is the current feedback (IM) for the high-gain current regulator, while the user uses the second DCCT signal.

Control rack
The Control Rack is the control core for the storage ring's DMPS, encompassing various functionalities such as communication, current control modulation loops, essential local control, electrical protection circuits, and digital signal protection.It consists of six functional control boards, which are as follows: (

Optimize the temperature controller and enhance interlock protection
This section focuses on enhancing the output current stability of the DMPS.It involves optimizing the components of the current control loop on the COMMC control card and temperature control.
Additional sensing components will also be installed to strengthen DMPS protection against water leakage and fire hazards, thereby improving the overall performance and operational safety of the DMPS.

Optimize the temperature control loop of the COMMC
Study the reasons for the output current instability of the DMPS.The circuit diagram in figure 7 shows that the high-precision current feedback loop component uses a DCCT in the current output mode to avoid interference in the feedback current signal during transmission.The feedback current signal is sent to the COMMC, enclosed in a temperature-controlled copper box, and converted into a voltage compensator circuit.This implementation utilizes temperature control to maintain high precision and low-temperature coefficient resistance in low-temperature environments, reducing variations in the feedback current caused by temperature changes and ensuring a more stable performance of the DMPS output current.-7 -    Additionally, we directly installed the high-precision, low-temperature coefficient resistors on the surface of the temperature-controlled copper box to improve heat dissipation performance.This forces the resistors to be at the temperature set point, reducing the time required for the thermal equilibrium of the DMPS output current.

Enhance external interlock protection
Most electrical and safety interlock protection settings were already considered during the initial design of the DMPS.However, after the experimental installation at the TPS facility, we have added the following external interlock protections based on the current situation: (1) TPS tunnel door open interlock: in compliance with the radiation safety requirements, whenever the tunnel door control is opened, the DMPS must immediately shut down to ensure the safety of maintenance personnel.
(2) Dipole magnet water flow and temperature protection: the dipole magnets utilize water-cooling with ionized water as the coolant for heat dissipation.Each magnet is equipped with temperature and water flow sensors.In case of abnormal temperature or water flow in any attraction without apparent reasons, the DMPS operation must be stopped.An internal inspection of the dipole magnets in the TPS storage ring is required to ensure the facility's safety.
(3) DMPS water leakage detection: since the DMPS adopts a water-cooling method for temperature reduction, numerous cooling water pipes are distributed throughout the system.To prevent damage to the power supply caused by water pipe or joint aging and leakage during prolonged operation, water leakage detectors are installed within the DMPS.
-9 -(4) Arc and smoke detectors: the primary purpose is to prevent the high-voltage capacitors inside the DMPS from experiencing excessive resistance due to frequent charge and discharge, resulting in overheating and self-ignition of the capacitors.In such cases, smoke may also be generated.
The DMPS operation must be immediately halted to minimize damage.
These additional external interlock protections enhance the safety and performance of the DMPS, safeguarding both the equipment and personnel during its operation.
4 Output current ripple, long-term stability, and cycling magnetic field operation of the DMPS Following the adjustment of the DMPS temperature regulation settings, several experiments needed to be conducted for validation by utilizing the weekly TPS shutdown and maintenance period.These experiments encompassed the following aspects: (1) Output Current Ripple Measurement involves observing oscillatory behavior in the DMPS output current.The goal was to assess whether any oscillations were present in the output current.
(2) Stability Measurement: the stability of the DMPS output current over an extended duration was measured using a DCCT and a high-precision ammeter.This aimed to evaluate the effectiveness of the improvements made.
(3) Cycling Mode Test: a test was conducted to ensure that the modification in the temperature regulation settings would not adversely affect the Cycling control mode.
(4) Top-up Mode Performance Observation: TPS operation in the 400 mA top-up mode was observed, focusing on the relationships between beam particle position, bending magnet current, and Tune-X.The primary objective was to determine whether the DMPS output current could achieve excellent stability and consequently reduce the output power of Tune-X from the FOFB system.
By conducting these experiments, we aimed to validate the impact of adjusting the DMPS temperature regulation settings during the TPS weekly shutdown and maintenance interval.

Output Current Ripple
The harmonic components of the DMPS output current were analyzed using a Dynamic Signal Analyzer (DSA) with a high-resolution 1600-line scanning capability (HP 35670 A), covering a frequency range from 0 to 6.4 kHz.The harmonic content of the output current in the proposed DMPS prototype is depicted in figure 11.At 2.1 kHz, the DMPS exhibits the maximum current ripple, with a harmonic component of 17.2 mA attributed to the IGBT module modulation frequency.At 4.2 kHz, the DMPS also demonstrates a harmonic part of 9.8 mA.No oscillations were observed at other frequency points, resulting in a total harmonic distortion (THD) of the output current maintained within 30 mA peak-to-peak, indicating a lower ripple performance.

Long-term stability
Testing the stability of the DMPS output current is a crucial indicator.The Ultrastab Saturn current sensor and an eight 1/2-digit high-resolution digital voltmeter were employed to sample data every 10 seconds under a latency (612.735A) output current condition.As shown in figure 12, the output current stability measurement of the DMPS was conducted over 12 hours, during which the variation range of the current ripple remained within ten mA.The stability of the output current was maintained within ±10 ppm, indicating that the DMPS possesses excellent output current stability capability.

Cycling magnetic field operation
After each shutdown maintenance of DMPS, a cycling magnetic field operation procedure is carried out when re-injecting the electron beam.This procedure primarily involves a ramp-up rate of 50 amperes per second, causing the DMPS output current to vary from 0 to 700 amperes, continuously repeating this process five times.This process eliminates residual magnetism on the dipole magnets, as depicted in figure 13.A manual test of the cycling mode's DMPS current waveform is conducted once, taking 25 seconds for the entire cycling mode.Only after confirming proper functionality can the conditions and state be set for injection in the TPS storage ring.

Top-up Mode Performance Observation
Finally, the relationship among beam particle position, bending magnet current, and Tune-X during a week of TPS operation in the 400 mA top-up mode is observed, as shown in figure 14.The output current of the bending magnet power supply becomes more stable, significantly reducing the tuning correction variation of Tune-X.The difference in DMPS output current performance between the default and newly adjusted settings of the temperature control box is illustrated in figure 15.Within a week, the drift in the output current of the bending magnet power supply improves from 50 mA to 10 mA.This effectively reduces the disturbance of DMPS output current on the Beam current, thus substantially enhancing the stability of the Beam current.-12 -

Conclusions
This study focused on the optimization stability and performance of the DMPS in the TPS facility.Various measures were implemented to achieve this goal.Optimization of DMPS Control Core: the Control Rack, the DMPS control core, was optimized to cover communication, current control modulation circuits, essential local control, electrical protection circuits, and digital signal protection.Incorporating six functional control boards, including the CSSB, CARG, CAFB, CDFB, COMMC, and the SkiiP Interface Board, enhanced control and safety.
(1) Enhanced External Interlock Protections: additional external interlock protections were introduced to improve safety further.These included the TPS tunnel door open interlock to immediately shut down the DMPS upon opening of the tunnel door, dipole magnet water flow and temperature protection for safeguarding against anomalies, DMPS leakage detection to prevent damage caused by water pipe or joint failures, and arc and smoke detectors to stop the DMPS operation in case of capacitor overheating promptly.
(2) Temperature Control Adjustment: the temperature control box setting was fine-tuned during TPS weekly shutdown maintenance time, significantly improving the output current stability of the bending magnet power supply.The output current drift was reduced from 50 to 10 mA over a week, ensuring more consistent and reliable performance.
(3) Low Output Current Ripple: the analysis of the DMPS output current using a high-resolution DSA revealed low harmonic components within a frequency range of 0 to 6.4 kHz.The total harmonic distortion remained below 30 mA peak-to-peak, indicating excellent low-ripple performance.
(4) Long-term Stability: stability is a critical indicator of DMPS performance.DCCT and a highresolution DVM monitored the output current stability at 612.735A output current conditions -13 -every 10 seconds.The results demonstrated that the output current stability remained within ten mA throughout the one-week measurement period and within ±10 ppm, confirming outstanding output current performance.
(5) Cycling Magnetic Field Operation: after each shutdown maintenance, the DMPS underwent a cycling magnetic field operation procedure involving repetitive cycles of 0-700 A to 700-0 A at a ramp rate of 50 A/s five times.This process effectively eliminated residual magnetism on the dipole magnets, ensuring consistent conditions and states during electron beam injection in the TPS storage ring.
In conclusion, the comprehensive enhancements and optimizations have significantly improved the stability, safety, and performance of the DMPS, contributing to the overall efficiency and reliability of the TPS facility.

Figure 1 .
Figure 1.The beam particle position, dipole magnet current, and Tune-X relationship without adjusting the temperature setting.

Figure 2 .
Figure 2. Influence of the DMPS output current variation on beam particle position.

Figure 3 .
Figure 3. Diagram of TPS storage ring dipole magnet cable connections.

Figure 5 .
Figure 5.The input circuit diagram of the storage ring DMPS.

Figure 6 .
Figure 6.System circuit architecture diagram of the TPS storage ring DMPS.

Figure 7 .
Figure 7. High-precision current feedback loop circuitry on the COMMC card.

Figure 8 .
Figure 8.The main control interface card is depicted, with the COMMC card and temperature control box on the left and the components of the temperature control box on the right side.

Figure 9 .
Figure 9. Measurement of the unadjusted state of the temperature control box is shown, with the temperature of the temperature-controlled copper box and heat sink on the left and the thermal imaging of the temperature control box measured on the right side.

Figure 8
Figure 8 depicts the composition of the COMMC card and the temperature control box, including high-precision low-temperature coefficient resistors, heat sinks, copper boxes, thermoelectric modules, and temperature sensors.The temperature control principle involves the installation of a high-precision temperature sensor on the surface of the copper box, converting the measurement signal into a temperature set value for closed-loop control.The heating module generates a compensation value to achieve the desired temperature.By default, the temperature-controlled copper box is set to 27 • C.However, as observed in the left side of figure 9, when the temperature set value is too low, the heat sink cannot operate properly, accumulating excessive heat.Due to thermal conduction characteristics, the temperature of the copper box rises from 27 • C to 41 • C, with a drift of approximately 3 • C within three hours.The temperature control box's instability affects the feedback resistor's accuracy and results in significant fluctuations in DMPS output current.The right side of figure 9 displays the thermal image of the temperature-controlled copper box under default settings, with the heat sink temperature reaching 75 • C. To address this issue, the temperature set value of the temperature-controlled copper box was optimized and adjusted to 38 • C, and a precision feedback resistor was affixed to the surface of the copper box to ensure enough temperature control for the feedback resistor, as shown in figure 10.The heat sink temperature decreased from 75 • C to 40 • C, and the temperature regulation of the

Figure 10 .
Figure 10.The temperature stability and thermal imaging after adjusting the temperature control box.

Figure 11 .
Figure 11.Output current spectrum analysis of the DMPS.

Figure 12 .
Figure 12.Output current stability of the DMPS during 12 Hours.

Figure 13 .
Figure 13.Cycling mode of the DMPS output current.

Figure 14 .
Figure 14.The beam particle position, DMPS output current, and Tune-X relationship with adjusting the temperature setting.

Figure 15 .
Figure 15.The relationship between temperature regulation box setting and DMPS output current.
1) Start Stop Control Board (CSSB) includes local/remote switching circuits, ON/OFF/Reset commands, and fault interlock signal integration.Digital Protection Board (CDFB): this board mainly provides protection for Magnetics (Transformer/inductors) over temperature, Cabinet doors Open, External Interlocks, Diode Fuses Blown, Capacitor fuse blown, Emergency Stop, Main water flow, and magnet Water Flow Low.It sends a single-fault interlock signal to the CSSB board.(5) Communication and Control Board (COMMC): the main functions of this board are to receive operational control commands from the local PC (via Ethernet), send interlock and status signals to the local PC (via Ethernet communication link), receive current set points from the local PC (via Ethernet communication link), and implement the current-loop and generate voltage-reference for the voltage-loop.(6) SkiiP Interface Board (SIB): this board provides the interface between control boards and the SkiiP Module, which includes IGBT gate signals, module current, and fault status.