This work presents research on the one-microsecond, L-band relativistic klystron amplifier (RKA) project conducted at Los Alamos National Laboratory. A collection of theoretical analyses is presented for rf cavities, intense electron beams, beam-cavity interactions, and small-signal klystron bunching. Electromagnetic field simulations were done for three dimensional cavity structures using HFSS with very accurate results. Particle-in-cell simulations of the complete RKA were done using the two dimensional code ISIS. Extraction efficiency for intense modulated beams is discussed and verified in simulations. Designs for input and idler cavities are reviewed. Extremely low-Q, single-gap, output cavities are investigated for coupling rf power from very low-impedance, modulated, electron beams.
Output cavities with a Q less than 4 have been designed, measured, and tested. Methods were implemented for designing 2D equivalent output cavity structures to model 3D structures in 2D codes. A technique for ex-situ rf conditioning of the output cavity gap pieces is presented. A beam-pipe center conductor, intended to reduce the space-charge potential depression of the beam, is discussed. Diagnostics for intense-beam and high-power rf measurements are presented.
A coaxial directional coupler and load, capable of handling more than 500 MW at 1300 MHz, were designed. Mode conversion from coax to waveguide is discussed for $>$100 MW power levels. Methods for determining the gap voltage in an operating cavity are presented. Pulse-shortening of the rf in the RKA is also discussed.
A 650 kV, 5 kA, one-microsecond, annular beam has been produced from a stainless-steel, explosive-field-emission cathode. The beam current was modulated up to 70% $(I\sb1 /I\sb0$ = 70%) using a two-cavity bunching section operating at 1300 MHz. RKA structures simulated in ISIS have extracted up to 250 MW. This number was consistent with the extracted power actually measured in the equivalent experiment. Overall energy extraction was as high as 160 J per pulse. The average rf output power coupled into the 6-inch-diameter coax transmission line was approximately: 300 MW for 300 ns, 250 MW for 500 ns, and 100 MW for 1 $\mu$s. Peak power levels as high as 475 MW have also been produced.