S.M. Sharma

20.6: Design and development of 100 GHz folded waveguide TWT

In the recent decade there has been a growing interest of the researchers all over the world in the design and development of the high frequency (in terahertz range), moderate power and efficient devices based on MEMS (semiconductor) and vacuum electronics technologies. Development of so-called vacuum microelectronic devices (VMDs) for the mm and sub-mm wave devices surmount the limitations of conventional microwave tubes (MWTs) as well as of solid-state devices. CEERI has started work towards the design and development of Folded wave guide TWT (FWTWT) initially for 100 GHz. This paper would report design and development work done for electron gun and RF structure of 100 GHz FWTWT.

Design and development of Ku-band 140W space TWT

A Ku-band 140 W space TWT of gain (>55 dB), electronic efficiency (>24%), collector efficiency (>75%), overall efficiency (>55%), I/M components (<-12 dBc) for frequency 10.9-11.7 GHz has been designed and developed for satellite communication system. This paper presents critical design issues and approach related to different assemblies like electron gun, PPM focusing, helix SWS, input & output couplers, and 4-stage depressed collector in order to meet the stringent requirement of space TWTs.

Dual anode electron gun & PPM focusing for Ka-Band TWT

A low perveance (0.14µp) convergent electron gun with dual anodes has been designed using codes EGUN (2D), TRAK (2D), OPERA (3D) and OMNITRAK (3D) for Ka-Band helix (TWT) for space applications. Pierce type electron gun with an electrically isolated beam forming electrode (BFE), control anode (A1) and an ion barrier anode (A2) has been opted. An M-type dispenser cathode with low cathode emission loading (∼1.0 Amp/cm2), has been selected to ensure the desired long life of more than 10 years and high reliability. The application of +100 to + 250 volts at the ion barrier anode with respect to ground anode (A1) protects the cathode surface from the ion bombardment and hence enhances the life and reliability of the tube. Potential at A1 can be used to regulate the current over the life of the tube without changing the accelerating potential; this also contributes towards the life and reliability of the tube. In order to further enhance the life and reliability, non-evaporating type SAES ST-175 getter, has been incorporated in the gun assembly. Application of BFE negative bias of the order of 10 to 20 volts with respect to the cathode is used to achieve the desired beam transmission in the CW tube.

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.

Electron gun design for multi-beam pulsed amplifier

This paper presents the design of an eight beam electron gun using OPERA 3D code for pulsed amplifier. The electron gun has to deliver total current 560 (70×8) mA at voltage 6 kV. Each beam has a perveance of 0.15 μP making a total gun perveance of 1.2 μP corresponding to a total beam power of more than 3 kW. The cathode radius is 13.5 mm and individual emitter radius is 1.55 mm having current density 0.92 A/cm2. Focusing has been accomplished through solenoid. The beam pulsing can be accomplished through beam forming electrode relatively at lower negative potential at BFE with respect to cathode. This design has an added feature of cathode protection from ion bombardment with the application of additional ion barrier anode.

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.

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.

P2-19: Design of dual anode electron gun and beam focusing for Ku-band 140W short length space TWT

A dual anode electron gun has been designed using EGUN and TRAK codes for Ku-band 140W short length space Traveling Wave Tube (TWT). Pierce type electron gun with an electrically isolated beam forming electrode (BFE), a control anode and an ion barrier anode (A0) in addition to ground has been opted. Due considerations have been given in the design of gun assembly to ensure the long life (>10 years) and high reliability. A PPM circuit with confined beam flow focusing has also been designed with Sm2Co17 magnets using in-house developed code MAGFLD. A good beam laminarity and beam ripples (<5%) have been obtained. An electron gun assembly has been designed and developed with due considerations for high voltage and thermal considerations from space TWT point of view

Carbon coated electrodes for multistage depressed collector of high efficiency helix TWTs

Efficiency of Space-TWT is of prime concern along with the long life and high reliability. The collector efficiency plays a major role in determining the desired overall tube efficiency. Various collector efficiency enhancement techniques are employed. This paper describes the use and development of technology of carbon coated OFHC copper electrodes in multi-stage depressed collector for the enhancement of collector efficiency in helix space TWT.

Design and Development of Electron Gun and PPM focusing for Vacuum Power Booster TWT of Broadband MPM

The electron gun and PPM focusing system have been designed and developed for above VPB-TWT. M-type dispenser cathode and electrically isolated BFE has been used. The gun adapter and the anode have been made of Cu-Ni and molybdenum respectively to get desired magnetic flux at the cathode. Compact electron gun assembly has been designed. High voltage analysis, theoretical as well as experimental, is carried out before finalization of assembly in order to meet the desired hold off voltages between different electrodes. Temperature and emission characteristics of the cathode are measured under the loaded conditions