Exact cancellation of emittance growth due to coupled transverse dynamics in solenoids and rf couplers

Weak, rotated magnetic and radio frequency quadrupole fields in electron guns and injectors can couple the beam’s horizontal with vertical motion, introduce correlations between otherwise orthogonal transverse momenta, and reduce the beam brightness. This paper discusses two important sources of cou...

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Bibliographic Details
Published in:Physical review. Accelerators and beams 2018-01, Vol.21 (1), p.010101, Article 010101
Main Authors: Dowell, David H., Zhou, Feng, Schmerge, John
Format: Article
Language:English
Subjects:
Couplers
Couplings
Electron guns
Electrons
Emittance
Injectors
PARTICLE ACCELERATORS
Quadrupoles
Radio frequency
Solenoids
Vertical motion
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Summary:Weak, rotated magnetic and radio frequency quadrupole fields in electron guns and injectors can couple the beam’s horizontal with vertical motion, introduce correlations between otherwise orthogonal transverse momenta, and reduce the beam brightness. This paper discusses two important sources of coupled transverse dynamics common to most electron injectors. The first is quadrupole focusing followed by beam rotation in a solenoid, and the second coupling comes from a skewed high-power rf coupler or cavity port which has a rotated rf quadrupole field. It is shown that a dc quadrupole field can correct for both types of couplings and exactly cancel their emittance growths. The degree of cancellation of the rf skew quadrupole emittance is limited by the electron bunch length. Analytic expressions are derived and compared with emittance simulations and measurements.
ISSN:2469-9888
2469-9888
DOI:10.1103/PhysRevAccelBeams.21.010101