A pulser R&D for the HEPS booster bumper magnet

The High Energy Photon Source (HEPS) is a fourth generation photon source, including a storage ring, a booster ring and a Linac. Due to the small dynamic aperture of the storage ring, a novel on-axis swap-out injection scheme was chosen. Here, the 6GeV booster acts as an accumulating ring during that injection process. To extract 6 GeV beam from the booster before injection into the storage ring, four slow bumper magnets are applied to assist the extraction kicker to accomplish. The bumper pulse magnetic field waveform is a half-sine wave with 1ms pulse bottom width. Depending on the simulation and test, a classic LC resonance circuit topology with IGBT switching in series with fast recovery diodes is adopted. In addition, an energy recycle circuit and capacitor charging circuit are designed, to decrease power loss and reduce the influence on the output pulse current waveform during the capacitor re-charge process. A pulsed power supply prototype has been completed, and the testing results show that the bumper pulser can fully meet the all requirements of HEPS booster high energy extraction system.


INTRODUCTION
The HEPS is a new generation synchrotron radiation light source based on typical low emittance storage ring.As shown in Fig1, the accelerator complex is including a c=1.4kmE=6GeV storage ring, a c=454m E=0.5~6GeV booster ring and a L=49m, E=0.5GeV Linac.A novel on-axis swap-out injection scheme is the baseline option for HEPS storage ring, limited by its small dynamic aperture.3][4][5] At most 10 bunches are injected in the booster ring in one second energy ramping cycle period from 500 MeV to 6 GeV.The beam is injected into and extracted from the booster bunch by bunch with 50Hz repetition rate.
In order to reduce the extraction kick strength, 4 sets of slow bumper magnets are applied to create a local bump to ease extraction.The Layout is showed in Fig 2 .Combining with the accelerator physical design and bumper magnet design， the parameters of the pulsed power sup-ply of bumper have been confirmed, which are showed in Table 1.
Fig. 1 The layout of HEPS accelerator complex Fig. 2 The Layout of the booster extraction

DESIGN SCHEME AND SIMULATION ANALYSIS
Compared with other fast pulse power supplies of injection and extraction system， the impulse speed of bumper pulser is relatively slow.So the most mature technology-LC resonance circuit， which is shown in Fig. 3, be-came the preferred scheme.
Fig. 3 RLC resonance circuit The parameters satisfy the second-order dynamic equation [6] .By deriving, the parameter formula of LC resonance circuit (undamped state) can be obtained, including the characteristic impedance z, the peak current i, and the half-sine wave bottom width τ. (Cvalue of resonant capacitance; L-value of circuit inductance; R-value of circuit resistance) [7] .
The actual circuit will have impedance, in order to get a half-sine waveform, so the circuit need to operate at underdamping state.
For the reliability, the solid-state switch is chosen.Because of its internal parasitic or integrated reverse diode, the reverse circuit cannot be restrained when the switch was closed.Therefore, the external reverse diode is necessary.The simulation is shown in Fig. 4.

Fig.4 Fundamental circuit simulation
Fig. 5 Waveform of fundamental circuit Although the inverse blocking diode is used， the inverse current will still appear because of charging branch, which is shown in Fig. 5.And the value of resistance is smaller, the inverse current will be bigger.But be limited by the time constant of circuit， it is unable to fulfil charging in 20ms.And then the actual peak current will be lower than the theoretical value, when the circuit operate steadily.
In addition, there will be 50% power wasted on the charging resistor.
For the 1ms bottom width half-sine wave pulse, the average power is not low.The wasted power will increase the output capacity of the charging power supply, and the charging resistor will heat up badly.
To solve these problems, an energy recycle branch and a charging switch are added in the circuit, which is shown in Fig. 6.

Fig.6 Energy recovery simulation
The energy recycle circuit is in parallel with the resonant capacitor, and a set of diodes, which can limit the current only flow to the resonant capacitor, are in series with the energy storage inductor.The time constant of the energy recycle circuit needs to 10 times to the time constant of discharging circuit, so the inductance of energy storage inductor needs 100 times to the inductance of bumper magnet [7] , and the inductor must be unsaturated.But due to this inductor needs to be put into a 3U case (height 13.34cm), the inductance is finally deter-mined to be 30mH.The added charging switch is closed, when the circuit operates in pulse generation state.The driving timing is shown in Fig. 7.It is distinctly shown from the simulation result (Fig. 8), the output pulse has been greatly improved.

CIRCUIT DESIGN AND TEST
Depending on the calculate and simulation, the type and quantity of power devices are determined, including one IGBT (type FZ600R12KS4) acts as the main dis-charging switch, the other IGBT(type IXYX100N120B3) acts as the charging switch, and totally 12 pieces fast recovery diodes(type IXYS-DSEI30-12A; 6 pieces for each group) .The testing procedure is shown in Fig. 9.
Fig. 9 Test circuit Fig. 10 Test waveform The main output pulse is relatively slow, for this bandwidth, a 300A DCCT can satisfy test requirements.But for the current of the energy recycle circuit, all of our Pearson sensors have saturated, so the test waveform is distortion.In the Fig. 10, we can distinctly see, the high voltage terminal voltage of resonant capacitor was charged in time by the energy storage inductor of the energy recycle circuit.Not only the output pulse is significantly improved, also the output power of charging power supply has been decreased and the temperature of the charging resistance has been effectively reduced.

DESIGN OF THE FORMAL CIRCUIT AND PROTOTYPE CONSTRUCTION
On the basis of experimental circuit, a clock control unit to realize the precise control of the two driving signals is added in the final circuit.And it is also important to isolate the earth potential of different devices effectively.Which is shown in Fig. 11.Besides, utilizing the internal bus capacitor assembly method of magnet power supply to place all the diodes to the PCB's double layers, it became convenient to connect with the heat sink.Finally, the 3D model and formal prototype can be seen in Fig. 12~13.

PERFORMANCE TEST
After all the preliminary work, the prototype is used to complete the magnet field tests at the real loadthe Bumper Magnet.And during the testing procedure, the acceptance of the prototype performance index has been accomplished.The peak current can reach 212A easily, the output pulse bottom width can be controlled within 1ms, the stability of peak current is less than ±0.165%.All the results can fully meet the requirements, which are shown in Fig. 14.

CABLE SELECTION
For conveniently maintaining, all the pulsers are put outside the tunnel with at least 40m cables.But to the LC resonance design, the parasitic inductance in the loop directly affects the output performance, especially the inductance of cables.How to select cables is particularly important.For RF coaxial cables, it can accurately calculate the parasitic inductance (250nH/m), and has no special requirements for laying.But the shielding layer needs to be used as a cable, and the shielding layer's resistance is a little bigger.It does not only lead the out-put waveform distortion, but also affect the peak current even leading to insufficient power of the charging power supply.Which is shown in Fig. 15.For obtaining 212A peak current, the charging power supply has put out 670V, approximately reaches its rated output.Two RF coaxial cables work in parallel can solve the problem, but it is hard to connect with the bumper magnet.Fig. 15 Test with RF coaxial cable Above all, it is need to customize a kind of special twisted-pair shielded cable, which is shown in Fig. 16.It is shown in Fig. 17, with 550V charging voltage does the pulser for the bumper magnet can obtain the 212A peak current.And depending on the bottom width (960ns), the parasitic inductance of the new twisted-pair shielded cable is 7uH/50m.Besides, no matter how to laying the cable, the parasitic inductance is almost unchanged.

CONCLUSION
Based on LC resonance design scheme, the pulser for the HEPS booster bumper magnet has been researched and developed.The energy recycle circuit has been utilized successfully, and this scheme can also be expanded to similar situations.With multiple structure modifications, the prototype has been completed.Besides, the development of cables helps the pulser can be put outside the tunnel.It can greatly reduce maintenance time.Finally, all performance index can fully meet the requirements of physical design. .1088/1742-6596/2687/2/022025

Table . 1
Design parameters of the pulsed power supply