Q. Wang, Q. Luo, B. G. Sun, Y. L. Yang, Z. R. Zhou, P. Lu, F. F. Wu, L. L. Tang, T. Y. Zhou, X. Y. Liu, J. H. Wei

University of Science and Technology of China

The 60th ICFA Advanced Beam Dynamics Workshop on Future Light Sources

Outline

Research StatusTheoretical BasisStudies and Improvements

The design of a cavity bunch length monitor (CBLM) with high order mode for the NSRL IR-FEL (FELiChEM)The design of the single cavity bunch length monitorThe influence of beam position offset

Conclusion

Outline

Research StatusTheoretical BasisStudies and Improvements

The design of a cavity bunch length monitor (CBLM) with high order mode for the NSRL IR-FEL (FELiChEM)The design of the single cavity bunch length monitorThe influence of beam position offset

Conclusion

• Bunch length measurement methods and devices for Linac and FEL.

Research Status

4

– Coherent Radiation – Electro-optic Sampling– Transverse RF Deflecting Cavity– Harmonic Method

Research Status

5

• Advantage of cavity monitors– Simple structure, wide application rage, and high signal

to noise ratio

T

1/2 0

0=1/T

Research Status

6

• Advantage of cavity monitors– Compared with the Transverse RF Deflecting Cavity, the

resonant cavity is nondestructive

Research Status

7

• Advantage of cavity monitors– Compared with the Transverse RF Deflecting Cavity, the

resonant cavity is nondestructive

Research Status

8

• Advantage of cavity monitors– The eigenmodes of cavities are used in combined

measurement of bunch length, beam intensity,position and quadrupole moment so that the wholediagnostic system is simplified and compact.

Outline

Research StatusTheoretical BasisStudies and Improvements

The design of a cavity bunch length monitor (CBLM) with high order mode for the NSRL IR-FEL (FELiChEM)The design of the single cavity bunch length monitorThe influence of beam position offset

Conclusion

Theoretical Basis

10

TM010 mode

Theoretical Basis

11

TM010 mode

Theoretical Basis

12

• Assume a Gaussian distribution in beamdirection with beam size στ, RF field isestablished when the bunch passes through acavity. The power of the RF field can beexpressed as

Where I0 is pulse current, ω is resonancefrequency of the mode, and R/Q is shuntimpedance.

2 22

0[ exp( )]2

P I R

Theoretical Basis

13

• We need two cavities at least and the bunchlength can be calculated by solving equation

2 221

1 0

2 222

2 0

[ exp( )]2

[ exp( )]2

P I R

P I R

Both BEPCII and CERN utilize two cavities to determine the bunch length.

Outline

Research StatusTheoretical BasisStudies and Improvements

The design of a cavity bunch length monitor (CBLM) with high order mode for the NSRL IR-FEL (FELiChEM)The design of the single cavity bunch length monitorThe influence of beam position offset

Conclusion

Studies and Improvements

• System Chart

15

Cavity 2 (TM020)

Cavity 1 (TM010)

Coupling

Probes

Microwave

Filters

ADC Module

FPGA Module

LO

Beam Pipe

Studies and Improvements

• System Chart

16

Cavity 2 (TM020)

Cavity 1 (TM010)

Coupling

Probes

Microwave

Filters

ADC Module

FPGA Module

LO

Beam Pipe

RF Pick-up

Studies and Improvements

• System Chart

17

Cavity 2 (TM020)

Cavity 1 (TM010)

Coupling

Probes

Microwave

Filters

ADC Module

FPGA Module

LO

Beam Pipe

RF Pick-up

Superheterodyne Receiver

Studies and Improvements

• System Chart

18

Cavity 2 (TM020)

Cavity 1 (TM010)

Coupling

Probes

Microwave

Filters

ADC Module

FPGA Module

LO

Beam Pipe

RF Pick-up

Superheterodyne Receiver

Post-processing Module

Studies and Improvements

19

• The expression of the cavity bunch lengthmonitor theoretical resolution equation

• In traditional cavity bunch length monitor, boththe two cavities' work modes are TM010. Thedisadvantage of this type is that the beam piperadius restricts working frequency of the cavity.

/ 202 22 1

1 (10 )( )

SNRr

Studies and Improvements

20

• ImprovementTM010 mode TM020 mode

• This design overcome the difficulty of workingfrequency restriction caused by beam piperadius and get higher resolution.

Studies and Improvements

21

• Simulation Results6000 bunches

The output signals in time domain and infrequency domain of the second harmoniccavity (0.952GHz).

The output signals in time domain and infrequency domain of the thirtieth harmoniccavity (6.188GHz).

/ext outQ U P

Studies and Improvements

22

• Simulation Results

Outline

Research StatusTheoretical BasisStudies and Improvements

The design of a cavity bunch length monitor (CBLM) with high order mode for the NSRL IR-FEL (FELiChEM)The design of the single cavity bunch length monitorThe influence of beam position offset

Conclusion

Studies and Improvements

24

– Compare with the traditional way, the new methoddo not need reference cavity. Two eigenmodes of arectangular cavity are utilized to measure beamcurrent and bunch length so that the promotedmonitor is simplified and compact.

– TM310 mode (2.856GHz) and TM130 mode(7.616GHz) are used to measure the bunch length.

Studies and Improvements

25

• Technical Challenge– Couple independently.

• Solutions– Two coaxial antennas are penetrated at special

points to couple out the signals.

Improved CBLM

26

• Probe 1TM310

TM130

Improved CBLM

27

• Probe 2TM310

TM130

Studies and Improvements

28

• Simulation Results

According to the results obtained from the CST, thismonitor shows good performance in bunch lengthmeasurements.

Outline

Research StatusTheoretical BasisStudies and Improvements

The design of a cavity bunch length monitor (CBLM) with high order mode for the NSRL IR-FEL (FELiChEM)The design of the single cavity bunch length monitorThe influence of beam position offset

Conclusion

Studies and Improvements

30

• When passing through the cavity with a position offset, thebunch will excite dipole modes such as TM110. Thesemodes may make an impact on the output signals.

Studies and Improvements

31

• The red line is the output signal when the bunch moves onaxis of the cavity, and the green line is the output signalwhen the beam offset is seven millimeters.

Studies and Improvements

32

• The experimental device

• The experimental device consists of two cavities. The firstone is the reference cavity that works at TM010 mode with0.476 GHz. The second one is the main cavity that worksat TM010 mode as well.

Studies and Improvements

33

• The father away the beam sets from the axis of the cavity,the greater the deviation is. It follows that the SNR and theresolution decline.

Studies and Improvements

34

• The same beam position will introduce greater noise ifworking frequency is higher.

/ 202 22 1

1 (10 )( )

SNRr

Studies and Improvements

35

• Comparing with TM010 mode, TM020 mode can be excitedin larger cavity whose quality factor Q is higher. UsingTM020 mode is able to obtain the high SNR and the highresolution compared with the traditional cavity with TM010mode.

Outline

Research StatusTheoretical BasisStudies and Improvements

The design of a cavity bunch length monitor (CBLM) with high order mode for the NSRL IR-FEL (FELiChEM)The design of the single cavity bunch length monitorThe influence of beam position offset

Conclusion

Conclusion

37

• The framework of the whole diagnostic system is designed.• To remove the limitation that working frequency and

resolution are restricted by the radius of beam pipe, thebunch length monitor based on high order mode cavity isproposed.

• A kind of new method to measure bunch length of FEL withsingle cavity is presented.

• The laws of resolution change caused by some decisivefactors such as working frequency and electromagneticmode are analyzed.

• Offers the theoretical support for the design and applicationof bunch length monitor in the future light sources.

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