Stacked double-gate field emitter arrays with large collimation gate aperturesDouble-gate field emitter array equipped with a collimation gate electrode for the individual emitters in addition to an electron extraction gate electrode can reduce the rms transverse electron veloecy of the field emission beamlet and the emittance of the FEA beam. We have shown recently that an order of magnitude reduction of the rms transverse electron velocity can be achieved with our structure with minimal reduction of the emission current [1,2,3,4]. Further effort to match the cathode spec [5,6] for the SwissFEL X-ray free electron laser  is under way.
Although high current FEAs have been intensively studied to realize a compact vacuum electronic amplifier, such as traveling wave tubes (TWTs) [8,9,10], and working FEA-TWT in 5 GHz range with 100 W output have been already demonstrated , double-gate FEAs with low normalized emittance will be essential to fully take advantage of the high current density characteristic of FEAs, in particular for the compact THz amplifiers [11,12] and THz power modules (a the vacuum electronic amplifiers seeded by solid-state low-power oscillators) [e.g. 13]. These FEAs may also be applicable for the massively parallel electron beam lithography tools [e.g. 14].
FIeld emission beam generated by double-gate FEA. (Left) Uncollimated beam. (Right) Collimated beam (small spots show x10 collimated beamlets from separate emitters
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