Development of a high average current rf linac thermionic injector

S H Gold,V Jabotinski,A Ting,B Zhou,P Sprangle

doi:10.1103/physrevstab.16.083401

Abstract

Thermionic electron guns are capable of operating at high average currents in a variety of vacuum electronic applications, including conventional microwave tubes, but have been replaced by laser photocathode injectors for most applications requiring high-brightness electron beams. However, while laser photocathode guns are capable of providing the very high-brightness beams, they provide an increased level of system complexity and do not extrapolate well to injectors for high average current applications requiring high beam quality. We are developing a 714 MHz injector based on a gridded thermionic electron gun for these applications. This paper presents an experimental study, computer simulations, and analysis of the performance of an existing gridded thermionic electron gun as an injector prototype, and a design concept for an improved injector configuration based on these results.

Highlights

Radio-frequency linear accelerators are used to accelerate short bunches of electrons to high energy using the rf fields excited in standing-wave or traveling-wave structures
EXPERIMENTAL SETUP The goal of the experiment is to characterize the performance of a gridded thermionic electron gun that is similar to the electron gun used in the production of inductive output tube amplifiers (IOTs) [27]
The experimental measurements presented earlier in this paper demonstrate the capabilities of a gridded thermionic cathode as the first stage of an electron injector, and showed that the existing electron gun, which was designed for an entirely different application, falls somewhat short of the typical emittance and bunch length parameters that are required for some rf linac application

Summary

Introduction

Radio-frequency linear accelerators are used to accelerate short bunches (microbunches) of electrons to high energy using the rf fields excited in standing-wave or traveling-wave structures. Electron injectors can use several different types of cathodes to generate the electrons that are accelerated in the first rf cavity [3,4]. One approach is to use a thermionic cathode located on the axis of the upstream wall of the first accelerating cell. This approach is capable of producing high average currents, since such cathodes can operate at high current density (up to $10 A cmÀ2) while filling every rf bucket of the linac. Injectors using a simple thermionic cathode exposed to the rf fields of the first accelerating cavity do not provide a means to gate the electron emission. One result is that there is a large energy spread in the electrons that exit the first rf cell, as well as a large

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