It was not possible to make long term tests lasting several days on the cooling fin, but it seems to be evident that the fin is suitable for demonstration tests such as those being proposed for the shuttle which are of short duration. However, a continuous test of nearly four days (ninety hours) was made on the same anodized surface at a temperature of 115 degrees Celsius without any noted deterioration of the emissivity. These tests were made in the context of cooling a diode rectifier in a rectenna array and involved a radiating fin that was 5 cm in diameter and 0.05 cm thick. Two magnetron radiating fins of the same design were tested in vacuum. Both of these fins were subjected to a large number of transient temperature buildups, and total operation in both cases amounted to several hours. There was no indication that the anode temperature on successive runs was increasing, thereby also indicating that the emissivity was not changing. A calculation of the emissivity was made, assuming that (1) all of the input power of 240 watts to the magnetron was conducted to the fin and radiated from it, (2) that the effective area of the radiating fin consisted of an area consisting of both sides of the fin and a radial fin dimension extending from the magnetron anode radius to the outer edge of the fin, (3) the average operating temperature of the fin is 235 degrees Celsius, or about 10 degrees hotter than the rim temperature, and (4) that the surrounding temperature environment was 27 degrees Celsius. Putting these values into expression (1), the emissivity is calculated to be 0.785. This is a high value of emissivity for a surface, and indicates that a black dyed anodized aluminum surface may be an excellent radiating surface at high temperatures in space if pyrographite is not available. Aluminum material is also an excellent choice if the mass is to be minimized. Aluminum has a heat conductivity of 0.55 calories per second per cubic centimeter per degree Celsius at 200 degrees Celsius compared with 0.97 for copper, but its density is 2.7 as compared with 8.9 for copper. Therefore, for a fin of equal conductivity the thickness needs to be increased in the ratio of 1.76 for aluminum. However the total mass of the radiator will be down in the ratio of 1.87, in favor of aluminum. Designing and Acquiring Suitable Permanent Magnets for High Temperature Operation The conventional microwave oven magnetron uses ceramic type permanent magnets that adequately fulfill their mission if the magnetrons are operated at reasonable temperatures. However, they will not be at all satisfactory at high temperatures. Alnico type magnets would easily withstand the high operating temperature but their use would greatly complicate the design because they would have to be added as a "C" or "E" type magnet which would penetrate through large circumferential sections of the radiating fin. Alnico magnets would also represent a large amount of mass which would be objectionable for space use. Replacing the ceramic magnets with Rare Earth Cobalt ring magnets of the same thickness, but of smaller diameter, has helped, but there is little margin left for operation of the
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