Paper ID: MOP004
DESIGN AND STUDY OF CAVITY QUADRUPOLE MOMENT AND
ENERGY SPREAD MONITOR *1112
Q. Wang , Q. Y. Dong , L. T. Huang , Q. Luo

1Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China
2NSRL, University of Science and Technology of China, Hefei 230029, China

Abstract
A nondestructive method to measure beam energy spread using the quadrupole modes within a microwave cavity is proposed. Comparedwith a button beam position monitor (BBPM) or a stripline beam position monitor (SBPM), the cavity monitor is a narrow band pickup andtherefore has better signal-to-noise ratio (SNR) and resolution. In this study, a rectangular cavity monitor is designed. TM220 mode
operating at 4.76 GHz in the cavity reflects the quadrupole moment of the beam. The cavity plans to be installed behind a bending magnet in
Dalian Coherent Light Source (DCLS), an extreme ultraviolet FEL facility. In this position, the beam has a larger dispersion, which isbeneficial to measure the energy spread. A quadrupole magnet, a fluorescent screen, and a SBPM with eight electrodes is installed near thecavity for calibration and comparison. The systematic framework and simulation results are also discussed in this paper.

INTRODUCTION

In order to achieve the energy spread measurement with high-resolution, we proposed the method of RF resonant cavity. Similar toSBPMs, cavities are installed at positions with high value of dispersion function, allowing extraction of the beam’s quadrupole moment toobtain the energy spread.
Advantages of measuring energy spread using cavity-based method：➢ Compared to placing a fluorescent screen or OTR behind a bending magnet, the cavity is a nondestructive monitor➢ In contrast to an SBPM, which can extract quadrupole moments, the quadrupole mode of the cavity has higher SNR and resolution➢ The longitudinal dimension of the cavity is very short

Layout diagram

Systematic framework

Parameter

Value (mm)Parameter

Value (mm)

a86.2

l10py

61.8

d1.4

structure of the rectangular cavity

THEORETICAL BASISSIMULATION

For a rectangular cavity rotated 45° about the axis, its TM220
mode amplitude is proportional to the beam quadrupole moment

The cavity model is established in CST, and virtual beams (300MeV, 500 pC) are loaded for simulation.
The results show that there is a good linear relationshipbetween the amplitude of the TM220 mode and the quadrupolemoments.

Where x and y are the beam positions in the two direc-tions. σxand σy are the beam rms sizes in the two directions.

Electric field of TM220 mode in rectangular cavity. Warmer colors represent
stronger electric fields.

Assuming that the vertical dispersion, ηy, is zero, therelationship between energy spread and quadrupole moment can bewritten as
Where βx and βy are β functions, ηx is the horizontal dispersion,and εx is the horizontal emittance. g is the correction factor, which
can be determined by calibration.

The output signal of the cavity when beam position x= 10, y = 0, σx = 0, and σy = 0. (a) is the time-domain
signal. (b) is the frequency-domain signal.

Variation of TM220 mode amplitude with beamquadrupole moment. (a) is beam position xvarying from 1 to 10 mm when y = 0, section size
σx = 0, and σy = 0. (b) is σy varying from 0.03 to8.66 mm when x = 0, y = 0, and σx = 1 mm.

* Work supported by the National Key Research and Development Proram of China NO.2YFA1602201, the International Partnership Program of the Chinese
Academy of Sciences Grant NO. KYSB20200057 and the STCF Key Technology Research Project.