Emittance of single-gate field emitter arraysEmittance is a key cathode parameter for the high brightness applications. However, experimental study is rare in literature, even for single-gate FEAs. We therefore measured array size dependence  and the uniformity dependence  of the single-gate FEA emittance for the first time. These measurements were done in part using the combined dioide-RF cavity accelerator, as well as in the dc gun teststand  previously established in the SwissFEL project. We observed that the single-gate FEA normalized emittance is ~2 mm-mrad per 1 mm-diameter. Combined with the double-gate result (an order of magnitude reduction of the rms transverse electron velocity), the normalized emittance in the order of ~0.2 mm-mrad per 1 mm-diameter cathode is foreseen. The number is compatible with the previous PIC calculation [4,5]. The in-situ uniformity control  of the field emission beam was achieved by using the noble gas conditioning method reported in Ref. .
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