Experimental studies of 7-cell dual axis asymmetric cavity for energy recovery linac

TL;DR

This paper presents the first experimental RF studies of a 7-cell dual axis asymmetric cavity designed for energy recovery linacs, demonstrating improved mode properties and validating a coupled mode theoretical model.

Contribution

It introduces and experimentally verifies a novel dual axis asymmetric cavity design with a coupled mode model for ERLs, enhancing BBU current limits.

Findings

Good agreement between experimental data and numerical predictions.

Validated coupled mode (Fano-like) model for cavity eigenmodes.

Measured mode field flatness and structure, confirming theoretical predictions.

Abstract

High average current, transportable energy recovery linacs (ERLs) can be very attractive tools for a number of applications including next generation high-luminosity, compact light sources. Conventional ERLs are based on an electron beam circulating through the same set of rf cavity cells. This leads to an accumulation of high-order modes inside the cavity cells, resulting in the development of a beam breakup (BBU) instability, unless the beam current is kept below the BBU start current. This limits the maximum current which can be transported through the ERL and hence the intensity of the photon beam generated. It has recently been proposed that splitting the accelerating and decelerating stages, tuning them separately and coupling them via a resonance coupler can increase the BBU start current. The paper presents the first experimental rf studies of a dual axis 7-cell asymmetric cavity and confirms the properties predicted by the theoretical model. The field structures of the symmetric and asymmetric modes are measured and good agreement with the numerical predictions is demonstrated. The operating mode field flatness was also measured and discussed. A novel approach based on the coupled mode (Fano-like) model has been developed for the description of the cavity Eigenmode spectrum and good agreement between analytical theory, numerical predictions and experimental data is shown. Numerical and experimental results observed are analyzed, discussed and a good agreement between theory and experiment is demonstrated.