SIMONS, SEDGWICK LEWIS, JR.
Doctor of Philosophy
The phototron, originally known as the photoklystron, is a vacuum tube device invented by Dr. John Freeman at Rice University for use on the solar power satellite. The design objective called for a device which would convert sunlight to microwaves in one simple direct step. Several test model tubes were constructed by ITT for concept evaluation at frequencies of 2 MHz. to 200 MHz. The operation of these test model phototrons has been found to be distinctly different from the operation of conventional klystron tubes. Past work by the author identified a multi-pass mode in which the electrons in the phototron exhibited a period of motion equal to a multiple of the RF period. In this mode improperly phased electrons are selectively removed from the tube by collision with the photocathode. Recent work has revealed a new and potentially more efficient mode of operation in which the electron period of motion is approximately half of the RF period. Computer plots of the electron trajectories reveal that unlike the multi-pass mode, this mode exhibits a genuine electron bunching process. Since these "half-cycle" modes operate at low bias voltages an investigation of space charge effects and electron emission energy has been made. Theoretical results from a computer program based on an analysis by Guernsey and Fu show that potentials within the phototron can be dramatically reshaped by space charge. These potentials will alter the electron motion in the phototron and also filter slow electrons out of the beam. Experimental measurements of the electron energy spectra have documented this filtering effect. Efficiency calculations based on a theoretical analysis and illustrated by experimental observations show that the greatest restriction on overall efficiency is due to the photocathode itself. The test model tubes show a cathode efficiency of only about 1%, but better materials may improve this value to about 10%. If the efficiency of the conversion of DC to RF in the phototron can also be improved an overall device efficiency of 1% for the conversion of sunlight into RF may be realized in the near future. Further improvements of the basic design may ultimately improve the overall efficiency to near 3%. In principle such a device could easily be scaled to produce microwaves instead of RF. At this efficiency the phototron could fulfill many applications, most notably in satellite communications. Its output is easily modulated in response to light or voltage, so it may also be useful in some transducer applications. If the efficiency can be improved the phototron may offer cost and weight advantages over other energy conversion systems.