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

I V Konoplev,R Ainsworth,G Burt,A J Lancaster,A Seryi,K Metodiev

doi:10.1103/physrevaccelbeams.20.103501

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.

Highlights

Transportable sources capable of efficient generation of high luminosity and intensity photon beams in THz, EUV and x-ray regions are attractive tools for a number of applications [1,2,3,4,5,6]
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
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

Summary

INTRODUCTION

Transportable sources capable of efficient generation of high luminosity and intensity photon beams in THz, EUV and x-ray regions are attractive tools for a number of applications [1,2,3,4,5,6]. To verify the results of the numerical studies a dual axis 7-cell rf cavity (Fig. 1) has been machined from aluminum and the experimental investigations have been carried out. There are a number of limitations associated with such machining and any TESLA like SCRF cavity will be built using the conventional pressing and electron beam welding technique [15] Discussion of these limitations is outside the scope of this paper, the prototype is relatively inexpensive and convenient for basic studies of rf properties. To test conventional manufacturing techniques (pressing, cleaning, welding and securing the position of the cells), the parts shown were built using copper sheets which have similar mechanical properties to niobium. The cavity will be cleaned and fully assembled using a conventional welding technique and the arms will be secured to assure that the axes are parallel and are on the same plane

EXPERIMENTAL MEASUREMENTS OF 7-CELLS RF CAVITY SPECTRUM

STUDY OF EIGENMODES FIELD DISTRIBUTION USING RF BEAD-PULL

CONCLUSION