An important limitation for rf sources such as klystrons and magnicons, designed for 10's of MW power outputs at cm-wavelengths, is the transverse electron beam size. Cathode current density limits require high beam area compressions to obtain small transverse beam areas for fixed currents. This paper discusses the limitations to high beam area compression, namely geometrical aberrations and thermal spread in transverse velocities. Compensation can oftentimes be introduced for aberrations, but thermal velocity spread presents a fundamental limitation. Examples where subtle compensation strategies were employed are discuxxed for three guns with 100 MW beam power, and area compressions greater than 2000:1. Two of these guns have already been built. A clear determination of effective cathode temperature has yet to be found experimentally, but possible means to measure it are discussed.
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