THE PHOTOKLYSTRON JOHN W. FREEMAN, SEDGWICK SIMONS WILLIAM B. COLSON*, FRANZ R. BROTZENt and JEFF HESTER Department of Space Physics and Astronomy and Space Solar Power Research Program 'Center for Space Physics and Space Solar Power Reserach Program tDepartment of Mechanical Engineering and Material Science Rice University Houston, Texas 77001, U.S.A. Abstract — This paper discusses a new device which oscillates at radio frequencies when illuminated by light. It was originally conceived as a reflex klystron with the thermionic electron source replaced by a photoemitter. In practice, the photoklystroni has been found to have different properties from what might be expected by simply scaling a reflex klystron to lower electron energies and oscillation frequencies. These include electron energy exchange with the rf field on multiple oscillations and plasma effects. The device can be made to “self-oscillate;” that is, no external accelerating bias voltage is necessary. The energy to sustain oscillation is derived solely from the photoelectrons. An electrical efficiency of 1% has been demonstrated for the first test model photoklystron. An ultimate efficiency of 10% appears possible. A solar power satellite configured with photoklystrons might be weight and cost competitive with solar cell designs. INTRODUCTION The Solar Power Satellite is basically a system for converting broadband, incoherent electromagnetic radiation (sunlight) into narrowband, coherent, ultra-high frequency electromagnetic radiation. In the conventional SPS concept this is accomplished by the conversion of sunlight to de high voltage electricity which is then converted to microwaves via an array of high power klystrons. A solid state system is also being studied in which the solar cells and micro wave amplifiers are an integral module. It occurred to us that some increase in efficiency might be possible if photoelectrons could be used to generate the rf directly, possibly overcoming bandgap energy limitations inherent in solar cells (1). The reflex klystron converts a monoenergetic electron beam to rf by passing the beam through a pair of grids on which an rf signal already exists. The rf field velocity modulates the electrons so that, upon reflection by a repelling electrode, the electrons may be bunched together instead of randomly distributed in their return arrival times at the grids. If the return arrival time of the bunch corresponds to a point in time at which the field between the grids is of such a polarity as to decelerate the electrons, the electrons give up some of their kinetic energy to the electric field thus t Patent applied for.
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