Cancellation of RF Coupler-Induced Emittance Due to Astigmatism

TL;DR

This paper analyzes how RF coupler fields cause beam emittance growth in superconducting linacs and demonstrates that a static quadrupole lens can effectively cancel this emittance without modifying cavity design.

Contribution

It reveals that transverse field tilt, not time dependence, mainly causes emittance and proposes a simple static quadrupole solution for mitigation.

Findings

Emittance growth is mainly due to transverse field tilt.

A static quadrupole lens can cancel the emittance.

Cavity design modifications are not necessary.

Abstract

It is well-known that the electron beam quality required for applications such as FELs and ultra-fast electron diffraction can be degraded by the asymmetric fields introduced by the RF couplers of superconducting linacs. This effect is especially troublesome in the injector where the low energy beam from the gun is captured into the first high gradient accelerator section. Unfortunately modifying the established cavity design is expensive and time consuming, especially considering that only one or two sections are needed for an injector. Instead, it is important to analyze the coupler fields to understand their characteristics and help find less costly solutions for their cancellation and mitigation. This paper finds the RF coupler-induced emittance for short bunches is mostly due to the transverse spatial sloping or tilt of the field, rather than the field's time-dependence. It is shown that the distorting effects of the coupler can be canceled with a static (DC) quadrupole lens rotated about the z-axis.