THE LINAC RF-SYSTEM FOR CHINA SPALLATION NEUTRON SOURCE PROJECT

Transcription

1 CANS-XV 18 th Meeting of the nternational Collaboration on Advanced Neutron Sources April 25-29, 2007 Dongguan, Guangdong, P R China THE LNAC RF-SYSTEM FOR CHNA SPALLATON NEUTRON SOURCE PROJECT Linac RF Group, CSNS Project nstitute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, Beijing, , China Abstract China Spallation Neutron Source (CSNS) project will start in China. The construction goal of the CSNS first phase (CSNS ) is determined to the neutron power at 120kW. So the designed output proton energy of LNAC is 81 MeV. Five sets of klystron power sources are used to power the RF and the four DTL tanks. At 324MHz RF frequency and 2.5 MW peak power, the Toshiba E3740A klystron is one of the candidates. Now we have already made progress with some key technologies for Linac RF system. The digital feedback system of low level RF control was successfully applied during the beam commissioning of the ADS 3.5 MeV RF accelerator at a high beam duty 7.15%. The system also provided satisfactory control on the cooling water temperature for the cavity resonance of the RF accelerator. The proposed AC series resonance high-voltage power supply for the klystron is also presented. 1. NTRODUCTON The CSNS (China Spallation Neutron Source) accelerator is the first large-scale, high-power proton accelerator project to be constructed in China. The CSNS is designed to accelerate proton beam pulses to 1.6 GeV kinetic energy at 25 Hz repetition rate, striking a solid metal target to produce spallation neutrons [1]. At phase of the project, the accelerator complex is designed to deliver a beam power of 120 kw, the DTL output beam energy to 81 MeV. Five sets of klystron power sources are used to power the RF and four DTL tanks. For future phase of the upgrade project, another three klystron power sources and three DTL tanks will be needed to accelerate the beam to 132 MeV, and the CSNS beam power will be raised to 240 kw. The layout of CSNS linac RF system is shown in Fig 1. Additionally, three solid state RF amplifiers are used to drive two MEBT bunchers and a LRBT debuncher. One-RF-unit-per-cavity independent RF control design is adopted. H S LEBT 3 MeV MEBT RF Buncher1 Buncher2 CSNS DTL 81 MeV CSNS DTL 132 MeV LRBT Debunche RNG Solid State Solid State Amplifier Amplifier Solid State Amplifier < 1 MW klystron 2.5 MW klystron ea. Spare Klystron Figure 1: Layout of CSNS linac RF system 2. KLYSTRONS 256

2 Originally, the RF frequency of the CSNS linac was proposed to be MHz following ADS (Accelerator Driven Sub-critical) program. The ADS research program was launched in 1999 under the support of the National 973 Plan, the Ministry of Science and Technology, China. Under this program, a four-vane type RF accelerator was built. ts RF power source was a MHz, cw 1 MW klystron and waveguide equipment assisted by CERN. However, the pulsed feature of the CSNS beam demands a linac RF source of higher peak power for efficiency. The RF frequency for the linac is thus changed to 324 MHz, the same as that of J-PARC, so that the same klystron can be used for the RF and DTL as a high power pulsed RF source. The available klystron like J-PARC type is made by TOSHBA company, type No. E3740A (Fig. 2, a). ts average output power is 93 kw. At 2.5 MW output power and 25Hz repetition rate, the pulse width can be up to 1.5 ms (Fig. 2, b). Basic requirements for CSNS klystron RF power source is given in Table 1. Furthermore, another probable candidate is a 324MHz klystron from CP company (3MW pulse, duty factor up to 6%). t is under development. 2.0 Pulse Width tp [ms] pps 65 pps 50 pps (a) 0.2 Solid line & Point: Measurement value Dotted line : Theoretical value Peak Output Power po [Mw] (b) Figure 2: TOSHBA E3740A klystron Table 1: Basic requirements for CSNS klystron RF power source Frequency Klystron peak output power at saturation Repetition rate RF pulse width Efficiency Gain 324 MHz 2.5 MW ( < 1 MW for RF) 25 Hz 1 ms (Max.) 55% (Min.) 50 db (Min.) 3. AC SERES RESONANCE HGH VOLTAGE POWER SUPPLY FOR KLYSTRON The proposed 120 kv /50A high voltage power supply for the klystron is shown in Fig. 3. We will try a new type of power supply, which, so far, no one has ever applied as klystron power supply. Actually, the principle of the circuitry is very traditional -- L C Alternating Current (AC) series resonance. t consists of five basic units: GBT frequency converter, exciting transformer, L C AC series resonance loop circuit, high voltage diodes, energy storage capacitor bank. 257

3 Figure 3: Proposed AC series resonance high voltage power supply for klystron By an GBT frequency converter, the 50 Hz, three-phase 380 V power from the mains is converted to 100 Hz, single-phase power. The exciting transformer step up the 100 Hz low voltage input power to about 2.5 kv. The inherent resonance frequency of L C AC series resonance loop circuit is just 100 Hz. So, AC voltage of 120 kv peak on the resonance capacitor C can be obtained. When the AC high voltage wave becomes negative, after high voltage diodes, the energy storage capacitor bank C Z can be charged. Fig. 4 shows a simulation result. n this case, the klystron cathode voltage is -106 kv, and the discharging pulse is at 1 ms width, 25 Hz repeated rate. That means, every four times the energy storage capacitor bank is charged, once discharging pulse occurs. They must be exactly synchronized. Figure 4: Simulation waveform for the discharging pulse at 1 ms width, 25 Hz repeated rate. The green is AC high voltage wave on the resonance capacitor C. The blue is the klystron cathode Voltage. The red is the klystron beam current wave. This scheme avoids step-up high voltage transformers and multiphase high voltage rectifiers. ts circuitry structure simplicity leads to maintenance convenience, lower trip rate. Meanwhile, this scheme can be easily upgraded to 50 Hz repeated rate discharging operation for CSNS phase in the future [2]. t also has good voltage droop compensation. The modulator provides a trigger pulse to the klystron modulation anode. The high voltage DC pulse discharging flattop is optimized by properly selecting the modulator switching time so that klystron cathode voltage droop is compensated by the rising sinusoidal waveform of 100 Hz AC negative half wave. By many simulation analyses, Optimized parameters of resonance loop are as following: L = 1.6 H, C = uf, non-loaded quality factor of L C AC series resonance loop circuit 0 >= 250 required (in this case, efficiency η = 87%, power factor cosθ= 0.99). 258

4 A scaling circuit (Fig. 5) was set up to demonstrate the principle of the CSNS type linac HV power supply. n this L C AC series resonance power supply test, we achieved 14 kv voltage and 14 A pulse current. A satisfactory flattop was obtained by optimizing the pulse trigger timing (Fig. 6). Now, the prototype fabrication is underway by Wuhan FANKE Transformer Manufacture Co., Ltd. Frequency Converter nductor Capacitor bank Figure 5: A scaling demonstration apparatus test of the CSNS type linac HV power supply. Triggering a little bit before the vertex (the best compensation) Figure 6: Voltage droop compensation 4. DGTAL LOW LEVEL RF CONTROL n the first half year of 2006, the ADS program RF accelerator was under beam commissioning. We just take advantage of this opportunity to carry out R&D of a digital low level RF () prototype for CSNS. So, its frequency is MHz for the ADS program. 4.1 Hardware and Algorithms A block diagram of ADS RF system is shown in Fig. 7. An accelerating electric field stability of -1% in amplitude and -1 degree in phase is required for the RF system. This field control, which is implemented by a combination of feedback (FB) and feed-forward (FF) algorithms, is the most important key function in the system. The two 14bit-ADCs (AD6645) and two 14bit-DACs (AD9764) are installed on FPGA board. The digital FB and FF is carried out through one chip of FPGA (STRATX series by ALTERA Co.). The DSP (C6000 series by T Co.) is in charge of communications and translating values between / and Amplitude/Phase. We adopt AD8345 as modulator. The algorithms in FPGA can be seen in Fig. 8. The RF pulse width is 1.4 ms. The set tables (FB and FF) are given every 1.36 μs during the 1.4 ms RF pulse. 259

5 Fig.7 Block diagram of ADS RF system Set_point_table_ Cavity_read_table_ FR - Kp P 开环 / 闭环 FF_table_ FFbeam_table_ 射频开关 0 饱和限制 Cavity F a1 b1 A/D c1 d1 Sample-clk FR - K Kp P FF_table_ a2 b2 c2 d2 射频开关 0 饱和限制 Cavity_read_table_ K FFbeam_table_ 开环 / 闭环 Set_point_table_ Forward F A/D a1 b1 c1 d1 FR FR Forward_read_table_ Forward_read_table_ Sample-clk Reflected F a1 b1 A/D c1 d1 Sample-clk FR FR Reflected_read_table_ Reflected_read_table_ Fig. 8 Algorithms in FPGA Besides the above fundamental functions for the system, the other two are the cavity resonance control and fast RF high power protection. The cavity mainly suffers from temperature fluctuation that leads to cavity distributed inductance and capacitance change, so, the resonance frequency will drift away from MHz-- detuning. n order to keep it in a desired resonance frequency range, especially for beam duration, cavity resonance control plays an important role. Cavity input phase and cavity field phase are measured by FPGA board, then, phase difference between them is calculated by DSP. The phase drift corresponds with frequency drift. When the phase drift exceeds the preset threshold due to cavity detuning, DSP will control the tuner or other tuning mechanism through PLC until the detuning phase goes back to set-phase. The original design version of the cavity resonance control is moveable slug tuner control. But in order to ensure a good flatness of cavity longitudinal field, a cooling water tuning system is substituted for the former (Fig. 9). At present, the resonance frequency is controlled by stabilizing outlet temperature of cooling water of RF cavity, i.e. direct temperature closed-loop control. Now its temperature control resolution is Further mproved design is under preparation. 260

6 Fig. 9 Cooling water tuning system for the cavity resonance control n order to protect the high-power components such as klystron, circulator and RF window in case of RF arc and over RF reflection, the RF drive and beam shall be inhibited within 1μs of the detection and shall remain inhibited for the remainder of the pulse. They shall be re-enabled for the following pulse. But if detections take place three times in a second, The RF drive and beam shall be permanently blocked. 4.2 Performance Prior to beam commissioning, the comparison test between with and without FB control were carried out. As shown in the left two waveforms of Fig. 10, in the case of no FB control, due to klystron beam voltage sag, the amplitude and phase decrease respectively by 6.4% and 19 degrees in the 1.4 ms pulse. The right two waveforms of Fig. 10 show the amplitude and phase with FB control. The variations of the both components at flattop were very small. The corresponding errors of the amplitude and phase were ±0.4% and ±0.5, respectively. No FB Control With FB Control 6.4% Amplitude Droop Setpoint Amplitude Flatness ±0.4% 19 Degrees Phase Droop Setpoint Phase Flatness ±0.5 Klystron beam voltage sag leads to amplitude and phase droop. Fig. 10 Performance of feedback control with no beam During beam commissioning, the same comparison test between with and without FB control were also carried out. The measurement waveforms of no FB control are shown in Fig.11. n Fig. 11 (a), the upper one is cavity field amplitude of RF, and the lower one is a reflected RF power from RF. t is obvious that the beam loading is very heavy. The two waveforms in Fig. 11 (b) are detected from two beam transformers. The upper one is input beam, and the lower one is output beam. t is of bad quality and stability. Transmission efficiency is not so high. After FB control gets into the act, the cavity field amplitude becomes flat (Fig. 12, a). t leads to good beam quality and stability (Fig. 12, b). n this case, the RF input and output beam are 48.6 ma and 44.5 ma respectively. Transmission efficiency is 91.6%. 261

7 (a) (b) Figure 11: RF beam commission without FB control (a) (b) Figure 12: RF beam commission with FB control Next step, we will examine the function of FF plus FB control. t can improve the transient response of the front and trailing edge of the beam loading. Also, the stability of long time operation must be studied. ACKNOWLEDGEMENTS The authors would like to thank the members in the digital development group of nstitute No. 23 of China Aerospace Science and ndustry for the construction of the digital system. Thanks are also due to S. Michizono (KEK), Z. Fang (KEK), E. Chishiro(JAEA), T. Kobayashi(JAEA), S. Yamaguchi(KEK), S. Fukuda(KEK) for many useful discussions. REFERENCES [1] J. Wei et al, China Spallation Neutron Source Project: Design terations and R&D Status, J. Korean Phys. Soc. (2006) [2] J. Wei et al, CSNS Technical note (2007) 262