Heat transfer and power capabilities of EHF helix TWT's

The heat transfer capability of the helix is the primary factor which determines average output power capability of a TWT. Thus, for maximum power capability in an EHF helix TWT, it is necessary to maximize thermal conduction from the helix to the external enclosure and to minimize the beam interception and rf loss. It is assumed that the latter two quantities will be minimized in the design of an EHF helix TWT; and, therefore, the maximization of the thermal conduction is the major concern in designing a TWT for high average output power. It has been shown previously that the helix heat transfer capability can be maximized by using helix support insulators which are made of high thermal conduction material such as diamond.(') Recent results have shown that in an EHF PPM focussed TWT, nearly as high power capability can be achieved by the use of a tungsten helix held in compression by anisotropic boron nitride (APBN) support rods, with an external wire wrap to ensure that uniform pressure is applied to all portions of the helix. Diamond has a thermal conductivity twice that of copper, while APBN has about 40 percent the thermal conductivity of copper; but the use of diamond helix supports in a PPM focussed TWT does not increase the heat dissipation capability by the ratio of the thermal conductivities. This is because the temperature across the insulator is only one of several approximately equal temperature differences. Thus, changing the temperature difference across the helix support insulator by a factor of five typically causes only thirty percent change in the total temperature difference between the helix and the external shell. The important point is that it is necessary to choose materials for the helix assembly which have high thermal conductivity; and it is necessary to construct the assembly so that the interfaces between the various materials will have high thermal conduction. Figure 1 shows for reference the thermal conductivities of several of the more important materials used in TWT helix assemblies. Note that the thermal conductivities of several of these materials vary considerably with temperature. Thus, in calculating the thermal dissipation capabilities of helix assemblies, it is very important that the variation of thermal conductivity with temperature be included in the calculations.