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d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 1 4DESIGN AND TEST OF A PROTOTYPE 324 MHz RF DEFLECTOR IN THE
BUNCH SHAPE MONITOR FOR CSNS-II LINAC UPGRADE*W.L. Huang†,1, X.Y. Liu, Y.F. Sui, X.J. Nie
Institute of High Energy Physics, CAS, Beijing, China
Q.R. Liu, M.Y. Liu, University of Chinese Academy of Sciences, Beijing, China
1also at China Spallation Neutron Source, Dongguan, ChinaAbstract
During the upgrade of linac in CSNS-II, the beam injection energy will increase from 80.1 MeV to 300 MeV
and the beam power from 100 kW to 500 kW. A combinedlayout of superconducting spoke cavities and elliptical cavities is adopted to accelerate H- beam to 300 MeV. Due toa ~10 ps short bunch width at the exit of the spoke SC section, the longitudinal beam density distribution will bemeasured by bunch shape monitors using low energy secondary emission electrons. As the most important part of abunch shape monitor, a prototype 324 MHz RF deflector isdesigned and tuned on the basis of a quasi-symmetriclambda/2 325 MHz coaxial resonator, which was fabricated for the C-ADS proton accelerator project. Preliminary parameters of the bunch shape monitor are presented.
Simulation of the RF deflector and test results in the laboratory are described and analysed.

INTRODUCTION
China Spallation Neutron Source (CSNS) is the first
pulsed neutron source built in China [1]. It consists ofan 80 MeV H- linac, a 1.6 GeV proton rapid cycling synchrotron (RCS), two beam transport lines and a target station. We achieved the design goal of phase I with protons
bombarding the target at a beam power of 100 kW in Feb.
2022, and now begins the upgrade project to 500 kW. During the linac upgrade, the DTL section will be followed bya section of 324 MHz double-spoke superconducting cavities and a section of 648 MHz elliptical superconducting
cavities [2], as shown in Fig. 1. The proton beam will be
accelerated to 300 MeV at the exit of the 8th cryomodule inthe second SC section.

Figure 1: Superconducting cavities in CSNS-II [2].
Longitudinal bunch density distribution in ion linac is
one of the main characteristics of accelerated beam. Bunch
shape information is extremely important for medium energy accelerators consisting as a rule of two main parts withdifferent rf frequencies. Results of bunch shape measurement after accelerating tank may be used to set rf phase and___________________________________________* Work supported by Natural Science Foundation of Guangdong Province,
2021A1515010269, National Natural Science Foundation, 11475204†huangwei@ihep.ac.cn

amplitude. What’s more, longitudinal bunch density maybe used to calculate energy spectrum and longitudinalbeam emittance [3-6]. In the CSNS-II linac upgrade plan,
two bunch shape monitors will be installed. The transverseand longitudinal beam parameters at the BSM installation
point are listed in Table 1.
Table 1: Micro Bunch Parameters in the Linac of CSNS-II
Micro Bunch
Energy (MeV)
RF Freq. (MHz)Φrms(°)
Xrms(mm)Yrms(mm)
Zrms(mm)Spoke 1
86.97
324
2.77
2.27
4.1
2.86

ELL7
300.1
648
1.05
2.38
2.28
1.75

Due to the ultrahigh bandwidth requirement and long cable attenuation, the normal phase detectors, such as fastcurrent transformers and wall current monitors, are notsuitable to measure the bunch shape in ion linacs. The technique of a coherent transformation of a temporal bunch
structure into a spatial charge distribution of secondary
electrons through RF-modulation was initially implemented by R. Witkover for BNL linac [7]. An energy (longitudinal) RF-modulation of low energy secondary electrons was used. In the Feschenko type Bunch Shape Monitor (BSM), developed in INR RAS, a transverse RF-scanning is used [8]. Thus a similar BSM is adopted in the linac
upgrade project of CSNS-II, as shown in Fig. 2.

Figure 2: Configuration of bunch shape monitors in CSNSII.
As the most important part of a bunch shape monitor, aprototype 325 MHz RF deflector was fabricated for thelongitudinal bunch shape measurement in C-ADS. Due tothe limitation of installing space, it was assumed to betested at the CSNS linac. This paper will illustrate its design parameters and test results in the laboratory.

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ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

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RF DEFLECTOR DESIGN
The prototype 325 MHz RF deflector is a quasi-symmetric coaxial λ/2 standing wave resonator. It is mainly composed of a resonating cavity, two deflecting plates, two RFports, two HV ends, two bellows and ceramic supports, as
shown in Fig. 3.

According to Fig. 5 b), we modified the (L_tune_in,
L_tune_out) as (6.2, 6.2) for the RF deflector, and had aresonant frequency of 324 MHz, the same as the RF frequency of DTLs in CSNS. Fig. 6 a) and b) show the electricfield distribution in the RF deflector at the resonant frequency of 324 MHz and the calculation of S-parameters. It
indicated that this prototype RF deflector could be used todeflect the secondary emission electrons from a wire scanner bombarded by ion beam, and works at 324 MHz, synchronizing to the RF frequency of DTL and SC sections.
a)Figure 3: Structure of the C-ADS 325 MHz RF deflector.

CST Simulations
The prototype RF deflector was simulated in MW studioof CST to get a proper resonant frequency f0 and to optimize the s-parameters. The geometry parameters are listedin Fig. 4. By changing the gaps between the tuner ends ofthe inner core and the cavity ends moved via bellows, thatis, (L_tune_in, L_tune_out) within (5~20 mm), the resonant frequency was calculated in the CST eigenmodesolver.
Finally, the combination of (L_tune_in, L_tune_out)
were swept and verified to get the f_0 of the RF deflector
and a tuning range of ±5 MHz, as shown in Fig. 5 a) andb). The S21 is lower than -30 dB at 325 MHz.

b)

Figure 6: E-field distribution in the prototype RF deflector
at 324 MHz and the S-parameters.

MEASUREMENTS IN LAB
Figure 4: Geometry parameters of the prototype RF deflector.
a)

The prototype RF deflector was fabricated at a domestic
manufacturer. Figure 7 gives the two parts before assembly.
It’s a pity that there was no locating pin in the initial design,
which brought the difficult of alignment during assembly.
The s-parameters were measured via a Keysight E5061B
network analyser, shown in Fig. 8. The S11@324 MHz is
only -8 dB, which is much bigger than the simulation resultand means more power should be fed in to establish an RF
field for deflecting the secondary electrons. A mechanicaloptimization in the second version of RF deflector designis undergoing.

b)

Figure 5: S21 calculation in CST by changing the combination of (L_tune_in, L_tune_out) to get the tuning range.
Figure 7: Two parts of the prototype RF deflector.
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ACKNOWLEDGEMENTS
The authors are grateful to Ms. Yuan Jingchun, Mr. Li
Rong for the good discussion of this work.

REFERENCES
Figure 8: S-parameters of the prototype RF deflector measured by Keysight E5061B.
Figure 9 shows the high power test to the deflector witha 1 kW power amplifier, which is excited by an R&S
SMA100B RF signal source. The resonator worked welland there is no waveform distortion observed.

[1] H.-S. Chen, and X.-L. Wang, “China's first pulsed neutron
source”, Nat. Mater., vol. 15, pp. 689-691, 2016.
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at the LINAC'22, Liverpool, UK, Aug.-Sep. 2022, paper
FR1AA02, unpublished.
[3] A.V. Feschenko and P. N. Ostroumov, “Bunch Shape
Measuring Technique and its Application for an Ion LinacTuning”, in Proc. LINAC'86, Stanford, CA, USA, Jun. 1986,
paper WE3-30, pp. 323-327.
[4] A. Feschenko, “Technique and Instrumentation For Bunch
Shape Measurements”, in Proc. RuPAC'12, Saint Petersburg,Russia, Sep. 2012, paper FRXOR01, pp. 181-185.
[5] A. V. Feschenko, “Bunch Shape Monitors Using Low Energy
Secondary Electron Emission”, AIP Conf. Proc., vol. 281,
p. 185, 1992.
doi:10.1007/978-1-4020-5724-3_43

Figure 9: Tested by a 324 MHz power amplifier(Pmax=1 kW).
A test bench with a Kimball electron gun will be established at the end of this year. Then the secondary electron
beam path in the deflector could be verified by tuning theamplitude and the phase of the RF field, and the high voltages on the plates.

[6] Y.V. Bylinsky, A.V. Feschenko, and P.N. Ostroumov,“Longitudinal emittance measurement of the 100 MeV protonbeam”, in Proc. PAC'91, San Framcisco, CA, USA, 1991,
pp. 3062-3063, vol. 5, 1991.
https://jacow.org/p91/PDF/PAC1991_3062.PDF[7] R.L. Witkover, “A non-destructive bunch length monitor fora proton linear accelator”, Nucl. Instrum. Methods Phys.
Res., Sect. A, vol. 137, pp. 203-211, 1976.
doi:10.1016/0029-554X(76)90330-X[8] A.V. Feschenko and P.N. Ostroumov, “Bunch shapemeasurements at the INR Linac”, in Proc. Workshop Adv.
Beam Instrum., pp. 236-245, 1991.
doi:10.1109/PAC.1993.309344

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CONCLUSION
A prototype of RF deflector was design as a 325 MHz
λ/2 quasi-symmetric resonator for C-ADS project. It is also
suitable to work at 324 MHz by adjusting the gap between
the bellow end and the core end. The S21 parameter got
unsatisfactory due to the difficulty of alignment.