Coupler induced monopole component and its minimization in deflecting cavities

Abstract

Deflecting cavities are used in particle accelerators for the manipulation of charged particles by deflecting or crabbing (rotating) them. For short deflectors, the effect of the power coupler on the deflecting field can become significant. The particular power coupler type can introduce multipole rf field components and coupler-specific wakefields. Coupler types that would normally be considered like standard on-cell coupler, waveguide coupler, or mode-launcher coupler could have one or two rf feeds. The major advantage of a dual-feed coupler is the absence of monopole and quadrupole rf field components in the deflecting structure. However, a dual-feed coupler is mechanically more complex than a typical single-feed coupler and needs a splitter. For most applications, deflecting structures are placed in regions where there is small space hence reducing the size of the structure is very desirable. This paper investigates the multipole field components of the deflecting mode in single-feed couplers and ways to overcome the effect of the monopole component on the beam. Significant advances in performance have been demonstrated. Additionally, a novel coupler design is introduced which has no monopole field component to the deflecting mode and is more compact than the conventional dual-feed coupler.

Design of the Compact Linear Collider main linac accelerating structure made from two halves

“…Three-dimensional field simulations on the structure cell were carried out to gather field data and to calculate high-order multipolar components [37][38][39][40][41]. The transverse strength for the beam moving off-axis can be calculated from the field data by integrating Lorentz force equation [Eq.…”

Design of high gradient, high repetition rate dampedC-band rf structures

“…The input and output coupler cells have been designed with a racetrack shape to suppress the quadrupole field components induced by the presence of the waveguide hole apertures [33,34]. It is easy to verify by HFSS simulations that the quadrupole field component in the coupler is completely dominated by the magnetic field asymmetry (see Appendix C).…”

Design and application of multimegawattX-band deflectors for femtosecond electron beam diagnostics

“…Expansion of the fields into waveguide modes propagating along the deflector axis does not contain either monopole or quadrupole components. For detailed study of multipole fields in deflector couplers see [9]. We note that quadrupole fields in the couplers could be excited by the beam or by asymmetry of the circuit feeding the waveguide.…”

“…The preliminary electrical design of a 1-m-long traveling wave deflector that satisfies these requirements is presented in [8]. The structure described in [8] uses a power coupler never before considered for rf deflectors in order to reduce multipole components of the field [9] and improve high-power performance.…”

“…Requirements for that deflector are different from ours as follows: total kick is smaller; multibunch wakefields have to be suppressed; and nondeflecting fields in couplers have to be minimized. To satisfy the last requirement a new type of coupler was recently introduced for the CLIC deflector [9].…”