S. K. Ghosh

Performance improvement of helix TWT using metamaterial helix-support structure

An analysis to determine performance parameters for dispersion characteristics and interaction impedance of helix traveling-wave tube using metamaterial is presented. The conventional positive dielectric supports of the helix slow-wave structure have been replaced by an equivalent homogeneous metamaterial medium. The analysis has been carried out in tape helix model, to include the effects of space harmonics. Structure characteristics, namely, dispersion and interaction impedance have been studied, as the former predicts the phase velocity of the RF wave and the latter has direct relevance to the device gain. It has been observed that, with metamaterial supported helix, both phase velocity and interaction impedance increase as compared to conventional positive dielectric supports. The results have been presented for fundamental space harmonic mode.

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.

A model for quick thermal prediction of multi-stage depressed collector

Abstract Multi-stage depressed collectors (MDCs) aid in efficiency enhancement of travelling-wave tubes (TWTs) by recovering the unused energy from the spent electron beam. Evaluation of the thermal performance of a MDC is crucial for an optimal design. Although several commercial software tools are readily available for performing the thermal analysis, (1) they are time-consuming and (2) the final results depend largely on few critical parameters (like thermal contact resistances at various joints, convection heat transfer coefficient, etc.). Hence, here a simple analytical model has been developed for quick prediction of the worst-case thermal behaviour of the MDC. Brazing non-uniformities that arise in any practical scenario have been taken into consideration in terms of their thermal contact resistances. Moreover, any cooling mechanism used, either convection or radiation, can also be correspondingly incorporated. Analytical results obtained from the model have been validated with simulated and experimental results. This model also aids in accurate calculation of few vital input parameters required for performing simulation with commercial software tools.

Dynamic analysis of a multi-stage depressed collector for high-efficiency travelling wave tubes

ABSTRACT High-efficiency helix travelling wave tubes (TWTs) use multi-stage depressed collectors (MDC) for efficiency enhancement. They are subjected to various and rigorous dynamic loads if the TWTs are to be used for space applications. The MDC is composed of several metal–metal, metal–ceramic joints and become considerably bulkier and robust than any other constituent parts of a TWT; hence, before integrating the MDC with the TWT, perhaps it could be important to subject the MDC for dynamic load tests. In this paper, we have presented a dynamic analysis of an MDC, to study the ruggedness of the structure before integrating it into the actual tube, under different stringent environmental conditions for space applications. The study was conducted, with respect to modal, static, sine and random vibration analyses in the software ANSYS to estimate the resonance frequency, structural deformations, stresses and so on.

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 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.

Multi stage depressed collector with reduced size and weight for space applications

Multi-stage depressed collector plays a major role in determining the overall efficiency of high efficiency space TWTs particularly for space applications. This paper presents the dependency of collector efficiency on the radial dimensions of the multi-stage depressed collector. In addition, an approach to reduce the radial dimensions of the collector without significantly compromising the collector efficiency using external magnetic field in the collector region would also be presented.

The analytical model for multi-stage depressed collectors

Traveling-wave tubes (TWTs), used in civil and defence applications, require high power and multi-octave bandwidth rather than high efficiency and hence just a single stage depressed collector is sufficient. However, TWT used in space applications necessarily has to have very high efficiency rather than wide bandwidth. A user friendly multi-stage depressed (MDC) collector design code in MATLAB for high collector efficiency is in progress of development.

Design of coaxial transformer for ku-band short length TWT

To extract rf power from a traveling wave tube (TWT), proper impedance transformation of the helix slow-wave structure (SWS) has to be done through coaxial couplers to a standard 50 ohm connector. Mismatch of impedance gives poor return loss (RL) behavior and leads to reflection of rf power causing oscillation and destruction of the TWT. In this paper authors have presented design of a co-axial coupler for high power space TWT using CST Microwave Studio and ANSOFT HFSS.

Selection of helix tape parameters for Ku-band 140 W SL-TWT

This paper presents the optimum selection of helix tape parameters for the design of Ku-band 140W short length TWT for space communication application. CST MWS code and in-house developed SUNRAY-3D codes have been used to design helical slow wave structure. In this model, helix is considered as perfectly conducting tape. The dependency of gain flatness and gain per unit length on tape thickness, tape width and helix pitch have been studied by maintaining the propagation constant(β) fixed at central frequency(i.e. 11.3GHz). Dependency of interaction impedance on helix tape width and tape thickness has also been studied.