The electrostatic-accelerator free-electron laser (EA-FEL) operating at mm wavelength is considered as a source for energy transfer through the atmosphere to a high altitude platform. The high average power and high efficiency attainable from appropriately designed EA-FEL make it a suitable candidate as an efficient source of mm-waves for power beaming from a ground station. Various aspects of the FEL as a high power oscillator (operating voltage, c-beam current, gain and efficiency) are reviewed; design tradeoffs are described. The study includes consideration of typical requirements of power beaming to a high altitude platform such as atmospheric absorption versus frequency and transmitting and receiving antenna requirements. A conceptual design of a compact, moderate voltage (0.5-3 MeV), high current (1-10 Amp) EA-FEM operating in the mm-wavelength band is presented as an efficient power source for space beaming. The FEM design parameters are presented based on analytical and numerical models. Expected performance parameters of an FEL (gain, energy conversion efficiency, average power) are discussed as related to the proposed application.