WANG Chao-Peng(王超鹏),FANG Wen-Cheng(方文程),TONG De-Chun(童德春),GU Qiang(顾强),and ZHAO Zhen-Tang(赵振堂),

1Shanghai Institute of Applied Physics,Chinese Academy of Sciences,Shanghai 201800,China

2University of Chinese Academy of Sciences,Beijing 100039,China

3Department of Engineering Physics,Tsinghua University,Beijing 100084,China

Design and study of a C-band pulse compressor for the SXFEL linac

WANG Chao-Peng(王超鹏),1,2FANG Wen-Cheng(方文程),1TONG De-Chun(童德春),3GU Qiang(顾强),1and ZHAO Zhen-Tang(赵振堂)1,∗

1Shanghai Institute of Applied Physics,Chinese Academy of Sciences,Shanghai 201800,China

2University of Chinese Academy of Sciences,Beijing 100039,China

3Department of Engineering Physics,Tsinghua University,Beijing 100084,China

A C-band RF pulse compressor is in development at SINAP.It comprises of two resonant cavities,two mode convertors and a 3dB power divider.TE0.1.15mode is selected for obtaining higher quality factor Q0of the RF pulse compressor cavities,so that the power gain factor can be 3.2,which is supposed to multiply the RF power from 50MW to 160MW.In this paper,we report our work on C-band RF pulse compressor,namely the design simulation and cold test results.

C-band,Pulse compressor,High quality factor,High RF power

I.INTRODUCTION

A compact Soft X-ray Free Electron Laser test facility(SXFEL)is now underway at Shanghai Institute of Applied Physics(SINAP),Chinese Academy of Science.It will be neighboring the Shanghai Synchrotron Radiation Facility(SSRF),the f i rst third generation light source in China.To make SXFEL even more compact,the third section of linac shall be composed of four C-band microwave acceleration units,with a high acceleration gradient of 40MV/m[1].This shall be realized by using a C-band RF pulse compressor,a crucial component for the C-band high gradient acceleration system.It will be testedtogether with the C-band accelerating structure in the SXFEL facility.

RF pulse compression is used to multiply RF power for increasing acceleration gradient.At present,several types of RF pulse compressor have been utilized at different facilities all over the world.Since the successful efforts at Stanfors Linear Accelerator Center on the prototype of pulse compressor,the SLAC Energy Doubler(SLED)[2–4]on SLC,higher eff i ciency and f l at-top technology has been in rapid development for RF pulse compressors,such as SLED-II[5],small delay line RF pulse compressor[6,7](using coupled cavities),BPC[8],DLDS[9]and BOC[10].As a compact FEL facility,SXFEL uses high eff i ciency,high gradient C band acceleration unit.RF pulse compressor with f l at-top output pulse is the compromised proposal for both high eff i ciency and high accelerating gradient,which can be carried out by SLED-I controlled by AM-PM LLRF(low-level RF)modulation technology.

The proposed SLED-I comprises of two cavities,two mode convertors and a 3dB power divider.TE0.1.15mode of the circular cavity is selected for higher quality factor of 180000, and the power gain factor is targeted at 3.2.The 3dB power divider,optimized for higher power-transmitting,is effective to avoid breakdown problems.A four-ports mode convertoris used to connect the 3dB divider and resonant cavities,so that it can obtain more purity of TE01 mode than that of oneport or two-ports convertor,and so that only TE0.1.15mode exists for high quality factor.

In this paper,we present the design study of the C-band RF pulse compressor,with results of RF simulation and cold test of the resonant cavity.

II.CONFIGURATION AND PRINCIPLE OF RF PULSE COMPRESSOR

The C-band RF pulse compressor used for the C-band microwave acceleration unit of SXFEL is driven by a 50MW klystron,as shown in Fig.1.The RF pulses are compressed into shorter pulses.The RF power is thus increased by several timesanddividedintoequaltwopartsforthetwoaccelerating structures.

Fig.1.(Color online)The sketch of C-band microwave acceleration unit of SXFEL.

The C-band accelerating structure of the SXFEL test facility is required to operate at the gradient of 40MV/m.Cor-respondingly,the input power for each acceleration structure shouldbe70MW.Then,theklystronoutputpowerof50MW shall be enhanced to 160MW by the RF pulse compressor. Afterwaveguidedissipation,therewillbe75MWforeachacceleration structure.Considering the design and fabrication, SLED-I is one compromised option.As shown in Fig.2,it is composed of two identical high Q-value cavities,two mode convertors and a 3dB power divider.Its performance depends on the resonant cavities.

Fig.2.(Color online)Schematic of the SLED type pulse compression.

In the C-band microwave unit,the klystron outputs RF pulses of 2.5µs in width,and the power pulses are transmitted into the two SLED cavities.When electromagnetic fi eld is induced in the cavities,waves of increasing amplitude are emitted from the coupling holes of two resonant cavities.Two emitted waves of the same phase are combined at the accelerator-side port of the 3dB power divider,but are cancelled by 180°phase difference at the klystron-side port. There are two waves re fl ected from the coupling hole of the cavity,andwavesemittedfrominnercavity.There fl ectedand emitted waves are of 180°difference in phase,but they add together when the input wave is reversed to the same phase at the time of 2.0µs.

From this principle of SLED-I,the power for the resonant cavity is expressed by Eq.(1):

where,α=2β/(1+β),βis the coupling coeff i cient;Tc=2QL/ωis cavity f i lling time,QLis cavity load quality factor,ωis radian frequency;Eeis the emitted wave from the coupling aperture,andEKis the reverse wave of equal magnitude to the incident wave.Considering the principle of SLED operation,Eq.(2)is derived as[2]:

whereγ≡α(2−e−τ),τ=t/Tc,τ1=2.0µs andτ2=2.5µs.Fig.3 shows the prof i les of input wave and output wave of SLED.Although the energy gain factor is 2.5 in principle,as the LLRF system has power loss for modulation of f l at-top output pulse,the f i nal energy multiplication factor is about 1.8,and correspondingly,the power gain factor is 3.2.

Fig.3.(Color online)The prof i les of input power and output power.

III.SIMULATION AND RESULTS

Based on theoretical analysis,MATLAB is used for parameters optimization of the Q-value,coupling coeff i cient and power eff i ciency.CST MICROWAVE STUDIO[11],which is good at asymmetrical structure computation,is used for three-dimensional electromagnetic simulation.

A.Resonant cavity

As a crucial component for an RF pulse compressor,a resonant cavity stores the energy and dominates performance of the RF pulse compressor,such as the gain factor and power eff i ciency.By tuning the Q0and the coupling coeff i cient,the RF power eff i ciency and energy multiplication factor can be mapped,as shown in Fig.4.The optimal parameters are decided by the practical applications.In our design,for a multiply factor of 3.2,the coupling coeff i cient is f i xed at 8.5 and Q at 180000.

TE0.1.15is used for higher quality factor,with lower power loss on the copper surface.However,there are many modes that are close to TE0.1.15and are harmful for SLED-I performance,hence the need of cavity geometry optimization to suppress those modes is proposed.The frequency of different modes can be expressed as Eq.(3)by cavity geometry parameters[12].

where,f0is the TEm.n.p/TMm.n.pfrequency of the cavity,Dis the cavity diameter,lis the cavity length.Using Eq.(3), TE0.1.15and its adjacent modes are given in Fig.5,where each line indicates the relationship between the resonant frequency and the cavity parameters.The marked point is faraway from the adjacent modes,and the cavity geometry parameters are thus decided.

Fig.4.(Color online)The multiplication factor and power eff i ciency mapped with the Q0and coupling coeff i cient.

Fig.5.(Color online)The mode chart of the circular cavity.

According to previous analysis,CST MICROWAVE STUDIO is used for three-dimensional f i eld simulation,including frequency,Q0and coupling coeff i cient.The electromagnetic simulation results are shown in Fig.6.

Fig.6.(Color online)The electric(left)and magnetic(right)f i elds in the cavity of calculated by CST.

Based on the simulations,the frequency of resonant cavity is fi xed at 5712MHz by adjusting the position of the end plat to fi nd the exact coupling hole size for coupling coef fi cient of 8.5 and the fi nal Q-value of the resonant cavity of about 180000.

After the cavity fabrication,the operating frequency of TE0.1.15mode can be con fi rmed,so as to tune the cavity to 5712MHz.As reference for measurements,we simulated the fi eld distribution,Q-value and frequency sensitivities.Fig.7 shows the simulated longitudinal magnetic fi eld along the cavity axis and radius direction.For TE01mode,the fi eld distribution is axial symmetrical,magnetic fi eld comprises ofHrandHzcomponent,but onlyEϕexists for electrical fi eld. There are 15 fi eld peaks ofHzon axis based on the CST simulation(Fig.7(a))and the radial distribution ofEϕin the middle of the cavity(Fig.7(b)).

Fig.7.Simulated longitudinal magnetic f i eld along the cavity axis (a)and radius direction(b).

The frequency sensitivities against the cavity length and radius were calculated,too,as the reference for cavity tuning, with the frequency sensitivity of Δf/Δl=10.5MHz/mm for the cavity length(Fig.8(a))and Δf/ΔD=6.6MHz/mmfor the cavity radius(Fig.8(b)).

Fig.8.(Color online)The frequency change with the cavity length (a)and the cavity radius(b).

B.3dBpower divider

The 3dB power divider is used to direct RF power into two cavities of the RF pulse compressor(Fig.9).The power of the two cavities are added at the accelerator-side port and cancelled at the klystron-side port.

Fig.9.(Color online)Simulation model of the 3-dB power divider.

Based on principle of the 3dB power divider,the lengthLof coupling region can be tuned for matching,the input power can be equally divided into the two parts in Fig.9.The analytical lengthLof matched status is expressed as Eq.(4), whereλg10andλg20are the waveguide wavelength of the modes TE10and TE20 in the rectangular waveguide respectively[12].

Ladder type hybrid is used in the connection region for obtaining better performance and avoiding RF breakdown, while it is easier to input higher power.The CST simulation of the S-parameters of the 3dB coupler divider is shown in Fig.10,in which Port 1 is for the input power,Ports 2 and 3 are connected to two cavities,and Port 4 is used for the power output.From the f i gure,S11 and S41 are less than−30dB, S21 and S31 are about−3dB,reaching the design goal of power transmission and isolation.

Fig.10.(Color online)S-parameter of the 3dB power divider by CST simulation.

C.Mode convertor

Mode convertor,located between the 3dB power divider and resonant cavities,is used to obtain high purity of TE01mode and convert TE10of rectangular mode to TE01of circular mode.Four coupling holes are designed for mode converter for stable operation at 80MW(Fig.11).Fig.12 shows the simulated S-parameters for the mode converter.S21 and S11 are about 0dB and−50dB,respectively,well reaching the design goal.

Typical parameters of the C-band RF pulse compressor, comprising of two cavities,two mode convertors and a 3dB power divider,are summarized in Table 1.

Fig.11.(Color online)Schematics of the mode converter.

Fig.12.(Color online)Schematics of the mode converter.

TABLE 1.Typical parameters of the C-band pulse compressor

IV.COLD TEST RESULTS OF RESONANT CAVITY

An experimental model of resonant cavity(Fig.13(a)), with an input coupler,a tuner and a cavity,was fabricated to verify the design.The Q-value,frequency and f i eld distribution along the axis were measured,and tuning experiments were carried out.As shown in Fig.13(b),the measured frequency was 5711.63MHz,and the Q-value was 159640,a little lower than the design value,but this can be improved after brazing.

Fig.13.(Color online)The measurement table of the C-band pulse compression cavity(a)and its Q-value and frequency measured(b).

Longitudinal and radial distributions ofHzwere measured by resonant perturbation(Fig.14).The measured magnetic fi eld distribution along the cavity axis(Fig.14(a)),with 15 peaks,agrees well with the simulation result in Fig.7(a),but the radial distribution(Fig.14(b))differs quite a little from the data in Fig.7(b).This was caused by misalignment of measured line.In Fig.7(b),the pro fi le is located at the middle of cavity exactly;while in Fig.14(b),the measured pro fi le location was drifted by tuning process,so the test results consisted of both magnetic and electric fi elds.

Fig.14.(Color online)The measurement table of the C-band pulse compression cavity(a)and its Q-value and frequency measured.

The measured frequency sensitivity of cavity length is Δf/Δl=10.5MHz/mm,agreeing well with the simulation data of Δf/Δl=10.4MHz/mm.

V.CONCLUSION

The C-band RF pulse compressor,as a crucial component for the SXFEL test facility,has been designed and simulated. A resonant cavity model was fabricated and tested under low RF power.To a large extent,the low power experiment results agree well with the design goals.This accumulates much data and experience for further development of the C-band RF pulse compressor.The experimental cavity design and mode measurement provide an integrated and systematic method for the R&D of C-band RF pulse compressor.The integral C-band RF pulse compressor will be fabricated,and high power RF tests will be processed soon.

ACKNOWLEDGMENTS

The authors are grateful to Dr.WANG Ju-Wen of the SLAC National Accelerator Laboratory for valuable suggestions and fruitful discussions.We would like to thank Professor CHEN Huai-Bi and coworkers at the Accelerator Laboratory of Tsinghua University for experiment supports.

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10.13538/j.1001-8042/nst.25.020101

(Received February 19,2013;accepted in revised form March 22,2014;published online April 20,2014)

∗Corresponding author,zhaozt@sinap.ac.cn