selection of a high power, seventy five kilowatt klystron for the microwave generator. From his experience with power tubes operating in an Earth environment he was aware that even at the fifty kilowatt level it was necessary to water-cool the waveguide that was transferring power from the tube to a suitable microwave load. Therefore, in space where neither liquid or air cooling is available it would be difficult to transfer power at that level to the slotted waveguide array and distribute it within the array. This problem is apart from cooling the high power tube itself which would be even more difficult. Also, high efficiency in the klystron could only be achieved with an awkward recycling of unused kinetic energy at the beam collecting electrode back into DC power. It became quite obvious that the 75 kilowatt klystron proposed for the reference system was not a suitable choice but the tube still frequently appears in superficial studies of the SPS system by foreign countries and student projects. If the klystron was not suitable, then what kind of tube or solid state device might fidfill the requirements? Fortunately, Richard Dickinson of JPL had previously suggested the use of the microwave oven magnetron for some proposed experimental effort at JPL. In the ensuing evaluation of the magnetron for that purpose at Raytheon, it was found that the tube was amazingly quiet and so a relatively low power magnetron, operated with external hardware to convert it into an amplifier, was proposed to the DOE/NASA study as a good candidate to replace the high power klystron in the reference design. In response to NASA’s expressed concern for the life of such a tube in the SPS application, an evaluation of the life of the magnetron was made by the author at Raytheon and found to be in excess of twenty years. The evaluation was based upon well established theory for the life of thermionically emitting cathodes and collaborative experimental data on the life of high power negative grid tubes utilizing carburized thoriated tungsten filamentary cathodes. The microwave oven magnetron and the proposed SPS use such a cathode. In the following material we will review the application of the magnetron to the SPS in the context of the many requirements imposed upon the microwave generator for that application. The Necessity for High Conversion Efficiency of De Power Into Microwave Power to Alleviate the Heat Disposal Problem In Space The fundamental problem in the use of any microwave generator in space is getting rid of any dissipation caused by inefficiency of the device. Such dissipated power appears in the form of heat which must be radiated into the vacuum of space and space is not a very good heat sink. The problem can be solved by making the generator device as efficient as possible and then letting it run at a high temperature to let it and its attached radiator take advantage of the 4th power relationship between the amount of power that can be radiated from a given area and the absolute temperature (Kelvin scale) at which the device is operated. Figure 1 shows the ratio of microwave power radiated to that dissipated as a function of Figure 8-1 RF Output/Dissipated Power versus Tube Efficiency
RkJQdWJsaXNoZXIy MTU5NjU0Mg==