ABSTRACTAt Teledyne e2v, a number of multi-octave and narrow band helix travelling wave tubes (TWTs) have been designed, developed and manufactured which are suitable for integration in microwave power modules (MPM) used in electronic warfare (EW) systems.

Other developments include phase matched broadband mini-TWTs and narrow band mini-TWTs. Some of these TWTs have been integrated with Teledyne e2v’s MPMs.

In this paper we demonstrate Teledyne e2v’s capabilities in the critical areas of design, development, test and manufacture of mini tubes meeting electrical and environmental specifications.

KEY WORDS• helix TWT• microwave power module

(MPM)• mini TWT• modeling and simulation • multi-octave band• phase matched• travelling wave tube (TWT• ultra-wide band (UWB)

Helix TWTRF Power

I. INTRODUCTIONDue to the implementation of fast and accurate design methodology and continuous improvement to the design and fabrication processes it is now possible to develop helix mini-TWTs meeting the stringent requirements of Electronic Warfare (EW) systems, electronic counter measure (ECM), phased array radar and microwave power modules (MPMs) for commercial and military communications and ground mapping radar.

At Teledyne e2v we have developed a number of broadband and ultra wide band helix mini-TWTs during the last decade; primarily used in EW systems. The ability to miniaturise the size while maintaining the capability to handle low to medium power positions helix tubes ahead of others in EW applications. It is possible to match the phase of some broadband tubes within ±15°. A number of narrow band tubes have also been developed for military satellite communication applications.

Since inception of mini-TWTs in 1996, significant efforts have been made to improve performance of the device. Use of modern software for better accuracy plus the application of a combination of analytical and numerical methods have made the complex design a reality. Practical implementation of the innovative designs was achieved through the improvement of existing processes and tight control of component tolerances.

To achieve a compromise between the electronic and mechanical design, thermal and structural analysis of all components were carried out. To increase the life of the device, the diameter of the cathode was increased so the emission current density is reduced.

The devices have gone through a series of environmental qualifications to ensure they meet the stringent requirements of the EW systems. Experimental performance of a number of devices designed using these techniques are presented here. The design steps and simulation techniques of different components are also discussed.

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II. DESIGN OPTIMISATION AND DEVELOPMENT All first generation mini-tubes at Teledyne e2v used a uniform helix pitch profile to avoid complexity in the manufacturing process. To improve the performance, through a beam wave synchronisation for a longer period, a tapered helix pitch profile was introduced [1]. A broadband TWT requires maintaining the dispersion characteristics over the entire operating band. Optimisation of the shape and size of the metallic vanes was carried out to control the dispersion characteristics in such way that the phase velocity is nearly flat throughout the operating band. This improves the performance at the band edges without which the output power and gain drops significantly at these frequencies.

A number of codes available both commercially, and developed in-house at Teledyne e2v, have been used to reach the desired design. To achieve improved match, a new technique was developed for design optimisation of co-axial couplers [2]. Sensitivity analysis of different components e.g., electron gun, slow wave structure (SWS), input and output coupler and multistage collector was carried out to finalize the tolerances of different dimensions.

Adaptation of modern optimization techniques has also enabled Teledyne e2v to develop a series of improved mini tubes based on its flagship TWT, a 4.5 to 18 GHz mini tube that delivers typically 25W, 130W and 75W at low-band, mid-band and high-band respectively. The second generations of the original mini tube are (i) N20160 - 6-18 GHz, 100 W minimum and (ii) N20154 - 13.75-14.5 GHz, 120 W minimum. Recent developments include (i) 2-18 GHz ultra-wide band mini-TWT (ii) gain and phase matched mini-TWTs and (iii) TWTs for X-band military satellite communication.

It is necessary to carry out sensitivity analysis of gun geometry to understand interdependencies of different dimensions for setting tolerances and desensitising purposes.

It can be observed from Fig. 2 that the confidence level of 95% TWT is within beam current variation ±2.5% which is close to the practical value achieved over a few thousand manufactured tubes.

Fig. 2: Sensitivity analysis of electron gun over 7500 iterations

The second major component in a TWT is SWS. Analytical equations are used to calculate initial dimensions of the geometry. These dimensions are used to create a model of a single turn helix with vanes and dielectric support rods in Microwave Studio to generate circuit parameters such as phase velocity, Pierce impedance as shown in Fig. 3.

These parameters are used as input to a large signal analysis code to optimise the circuit geometry, length of each section, saturated gain etc. Choice of helix pitch profile and its optimisation is carried out at this stage to achieve highest efficiency. Different pitch profiles are shown in Fig. 4. It requires a number of iterations to optimise the geometry using Microwave Studio and large signal analysis code.

To design an electron gun, initial dimensions are calculated using the synthesis approach used to create 2D models in EGUN and CIELAS. These codes are used for design optimisation of the gun geometry as the time taken to simulate the geometry is significantly less than complex 2D and 3D codes. In a similar way, a periodic permanent magnet (PPM) focusing system is designed by analytical calculations and optimised through numerical modelling using OPERA 2D and Microwave Studio 3D. Finally, these codes are used to optimise a PPM focused electron gun as shown in Fig. 1.

Fig. 1: 3D model of PPM focused electron gun using Microwave Studio

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The impact of vane radius on phase velocity is shown in Fig. 5.Fig. 3: Simulated circuit parameters

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Fig. 4: Varying helix pitch profile techniques

Fig. 5: Effect of vane radius change on phase velocity Fig. 6: Output coupler with window

To maximise the power transfer and minimise reflection, the input and output should have good match with the outside interface. Generally input is SMA direct to the TWT or through phase matched solid state amplifier /lineariser etc. Output may be through SMA, TNC, N-type, or waveguide depending upon the application. A model is created using a 3D time domain solver as shown in Fig. 6 and the design is optimised through an iterative process.

Fig. 7: Output power of N20160 TWTs (qualified for fighter aircraft)

III. EXPERIMENTAL EVALUATIONIn this section experimental results of a number of different tube types have been presented.

One of them is a 6-18GHz mini-TWT which has been integrated in Teledyne e2v’s MPM and passed all qualification tests for use in fighter aircraft.

The experimental output power of 25 tubes is shown in Fig.7. It should be noted the minimum out power is 100W for maximum prime power 500W.

In Fig. 8, the output power is shown for a number of phase matched TWTs where the minimum output power is 62W for prime power 375W. The phase match between tubes is within ±15° as shown Fig. 9.

A recent development is an X-band 120W TWT for military satellite communication application. Saturated gain for this tube is 44dB. The experimental output power and saturated gain are shown in Fig. 10. Prime power for this tube is 400W.

Fig. 9: Swept phase of phase matched N20156 TWTs Fig. 10: Output power and saturated gain of N20188 TWTs (X-band military satellite communication application)

Fig. 8: Output power of phase matched N20156 TWTs

IV. CONCLUSIONWe have demonstrated Teledyne e2v’s capabilities in design, development and manufacture of both broadband and narrowband mini-TWTs. These devices have been integrated with Teledyne e2v’s MPMs and passed qualification for use in EW systems.

REFERENCES[1]. T. K. Ghosh, A. J. Challis, A. Jacob, and D. Bowler, “Design of helix pitch profile for broadband traveling wave tubes,” IEEE Trans. ED, vol. 56, no. 5, pp. 1135-1140, May 2009.

[2]. T. K. Ghosh, R. G. Carter, A. J. Challis, K. G. Rushbrook, and D. Bowler, “Optimization of co-axial couplers,” IEEE Trans. Electron Devices, vol. 54, no. 7, pp. 1753-1759, July 2007

Tushar K. Ghosh received Ph.D. degree in engineering from the University of Lancaster, U.K. He is currently working at Teledyne e2v technologies Ltd., U.K. Dr. Ghosh was awarded a Commonwealth Scholarship by the Association of Commonwealth Universities from 1999 to 2002 to pursue his PhD studies. He was awarded the first IEEE-EDS Graduate Student Fellowship in 2001. Dr. Ghosh is Life Member of Indian Physics Association, a Fellow of VEDA Society, a Fellow of the IET and a Senior Member of the IEEE. Dr. Ghosh is registered by the Engineering Council (UK) as a Chartered Engineer (CEng) and an International Professional Engineer (IntPE(UK)).

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