  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.
