[en] The purpose of this program was to study the various approaches to building an amplifier to produce 100 kW or more cw power at 120 GHz to decide on an optimum approach, and to perform design calculations. The study has led to the conclusion that a cyclotron resonance amplifier (gyrotron) is the optimum approach for 100 kW and that this type of interaction offers the best possibilities for going to still higher power levels. This report includes discussion in depth of all the topics and approaches considered in the study. Sections 2 and 3 deal with output waveguide and window designs. The fourth section discusses linear-beam amplifiers. The fifth section considers periodic-beam (other than cyclotron resonance) devices. Cyclotron resonance devices are considered in detail. Arguments are given leading to the choice of the gyrotron as the optimum approach in section 7. Design calculations for a 100 kW gyrotron amplifier are given based on the background and theory presented in section 6