Vishant Gahlaut

Multistage Depressed Collector With Improved Thermal Management for High Efficiency Travelling Wave Tubes

Multistage depressed collectors (MDCs) are used to enhance the overall efficiency of travelling-wave tubes (TWTs). Thermal management is one of the critical concerns when such TWTs are used for space application. In this paper, authors have presented an innovative technological concept to develop a typical four stage MDC in a single ceramic for better thermal management. Here, multiple brazing joints have been reduced, which, in turn, reduces thermal contact resistances (TCRs) and thereby improves the thermal management better than its conventional counterpart, where brazing joints are more. A 1-D analytical model has been developed for better understanding of the effect of TCRs at various braze joints on heat dissipation from the MDC. Results obtained from this analytical model have been compared with 3-D simulations and validated experimentally.

Thermal Impact on the Performance of Highly Efficient Multi-stage Depressed Collector for Space TWT

Abstract In a travelling wave tube, much of the waste power is dumped into the collector. If the waste heat is not properly managed, it might pose a serious problem causing even failure of tube. In this paper, the optimal choice of thermal management of a highly efficient multistage depressed collector designed for a space TWT has been made based on several criteria. The structural deformations and stresses developed due to thermal impact have been evaluated. The influence of thermal deformations on the collector electrical performance and high voltage withstanding capability has been studied during hot condition.

A non-conventional multi-stage depressed collector for high efficiency applications

Non-conventional 3-stage collector geometry has been designed using EGUN code for high efficiency applications deviating from typical 4-stage depressed collector. Efficiency of this collector is very much similar to 4-stage collector with reduced dimensions, weight, etc, which are very important if such collector is to be used for space applications. Such collector also reduces the complexity of electronic power conditioner (EPC) for the TWT. Practical feasibility for fabrication has also been studied by developing the model in solid-works.

Transmission Line Theory and Thermal Analysis of Coaxial Coupler for TWTs

This paper presents the transmission line analysis, thermal and structural analysis of a multi-section coaxial coupler (MSCC) used in traveling-wave tubes (TWTs) to handle high average power over a wide frequency range. Power transmission through coupler from the device demands very good matching between load and source impedances such that low voltage standing wave ratio (VSWR) is achieved. Modeling of the MSCC for wideband TWTs in commercially software packages takes very long iteration time even in high-end computers. An analytical approach has been developed to model MSCC which takes less iteration time even in ordinary computer. Analytical results have been compared to those obtained from CST microwave studio. Finally, thermal and structural analysis has been carried out to study the thermal aspects for handling high average RF power.

Electrostatic and thermal analysis of a dual-anode electron gun for space traveling-wave tubes

We propose the design of a dual-anode electron gun, so named due to the presence of an additional electrode, namely, the control anode in the vicinity of the cathode in addition to the ion-barrier anode and the ground anode. We found the potential of the control anode to play an important role in enhancing the life of a space traveling-wave tube. By a telemetric command, one could control the control anode potential with a view to enhancing the cathode current that would otherwise deteriorate with the progress of time. Further, due care was taken in the thermal design by providing a heat shielding of the cathode of the proposed dual-anode gun to ensure that the cathode, which is operated at a lower temperature for a longer cathode life, does not further suffer a decrease in temperatures due to heat dissipation. Furthermore, to compensate for the deterioration of the cathode current with the progress of time, we propose the application of a nominal change in the control anode potential by a telemetric command. The electrostatic and thermal simulations of the proposed dual-anode gun were carried out by the commercial simulation codes EGUN and ANSYS, respectively. The paper presents the effect of heat shield thickness and material property on the cathode temperature as well as the effect of the structural deformation caused by the axial and radial expansion of the cathode on electron beam optics, a demountable dual-anode gun was set up for validating the cathode temperature against simulation.

Thermal and Structural Analysis of Co-axial Coupler used in High Power Helix Traveling-Wave Tube

Abstract In traveling-wave tubes (TWTs), coaxial couplers or window assemblies are used for coupling milliwatt (mW) to hundreds of watts of average power. For the proper transformation of impedance of interaction structure to the standard connector, coaxial couplers are suitably modeled as multi-section coaxial transformer. Due to high average power propagation, center conductor of coupler gets heated and the dimensions of multi-section coupler get deformed from its cold condition which causes impedance mismatch and increase of thermal load. Due to impedance mismatch, reflection of RF signal occurs from the couplers causing oscillation and finally leads of destruction of the TWT. This paper presents the thermal and structural analysis of coaxial coupler to quantify the temperature distribution at different regions, deformation of cold dimensions and stress due to material property of window disc.

Thermal management of helix used in high-power high-efficiency traveling- wave tubes

In this paper, an approach has been presented to estimate thermal contact resistance (TCR) between helix slow-wave structure to support rod and support rod to metal envelope to study the effective heat dissipation from the helix. Based on this method, TCRs are optimized by comparing analytically and experimentally estimated helix temperature with those obtained from ANSYS simulation. Helix temperature, at a given power loss in the helix, is obtained by varying the fraction of effective resistance between helix and support rod for a given fraction of effective resistance between support rod to barrel envelope and vice versa. This helix temperature has been compared with the measured temperature to predict the TCRs.

Heat dissipation from helical periodic structure in vacuum

Two dissimilar metals, not brazed together, give high thermal contact resistance which further enhanced in vacuum if surface finishing is not good enough. Here, the effect of thermal contact resistance of a periodic structure supported with another dielectric material in vacuum has been studied and presented.

Prediction of thermal contact resistances and temperature distribution at different joints in helix traveling-wave tubes

Improper fitting of helix bundle to rod and rod to barren envelope leads to high thermal resistances and hence restricts average power handling capability of a TWT. In this paper, authors have calculated thermal resistances at different joints of a helix traveling-wave tube (TWT), analytically and validated with COSMOS soft ware.