Evaluating Effects of Geometry and Material Composition on Production of Transversely Shaped Beams from Diamond Field Emission Array Cathodes

TL;DR

This study compares two diamond field emitter array cathodes with identical geometry but different materials, demonstrating that material properties like ballast resistance influence the ability to produce transversely shaped electron beams, not tip sharpness.

Contribution

It provides experimental evidence that material properties, especially ballast resistance, are crucial for patterned beam production in diamond FECs, challenging the emphasis on tip sharpness.

Findings

Both cathodes sustained 44 MV/m gradients and produced 0.5 nC per RF pulse.

One cathode produced a patterned beam, the other did not.

Material characteristics, not tip sharpness, determine patterned beam capability.

Abstract

Field emission cathodes (FECs) are attractive for the next generation of injectors due to their ability to provide high current density bright beams with low intrinsic emittance. One application of FECs worthy of special attention is to provide transversely shaped electron beams for emittance exchange that translates a transverse electron beam pattern into a longitudinal pattern. FECs can be fabricated in a desired pattern and produce transversely shaped beams without the need for complex masking or laser schemes. However, reliable and consistent production of transversely shaped beams is affected by material properties of the FEC. This paper reports the results of testing two diamond field emitter array (DFEA) FECs with the same lithography pattern and emitter geometry but different material and tip characteristics. Although both cathodes were able to sustain gradients of 44 MV/m and produce maximum output integral charge of 0.5 nC per radiofrequency (rf) pulse, their emission patterns were quite different. One cathode did not produce a patterned beam while the other one did. Differences in field emission characteristics and patterned beam production were explained by the differences in the tip geometry and the cathode material properties. The main practical takeaway was found to be that the tip sharpness was not a prerequisite for good patterned beam production. Instead, other material characteristics, such as the ballast resistance, determined cathode performance.