Performance improvement of FEMs by prebunching of the electron beam

FEM performance enhancement achieved by use of r.f. prebunching of the e-beam in the FEM developed at TAU was investigated theoretically and experimentally. For FEM operation as an oscillator, use of e-beam prebunching enables stable, coherent, high output power throughout the r.f. output pulse at any selected oscillator eigenfrequency for which the net gain is above unity. Prebunching enables faster r.f. output power buildup. An eigenfrequency of maximum efficiency and power output can be selected by e-beam prebunching at or near that eigenfrequency. FEM efficiency is thus, considerably improved. By contrast, FEM operation without prebunching leads to saturation in the highest gain mode, giving a lower efficiency. Frequency "hopping" of the FEM r.f. output between various eigenfrequencies is also attainable via prebunching. FEM operation as a high gain amplifier between the premodulator input and the FEM output is reported for our FEM operating with an e-beam current of only 0.6 A. High FEM gain, broad bandwidth, high power operation possibilities at millimeter waves are also described.