Srivastava V, Ghosh TK, Sharma RK, Sinha AK, Singh VVP, Raju RS, Alaria MK, Imtiaz SM, Kiran V*

Microwave Tubes Area,

Central Electronics Engineering Research Institute Pilani (Raj.)-333 031 Fax: 01596-242294, phone: 01596-252228, e-mail: vs@ceeri.ernet.in *Microwave Tubes Division, Bharat Electronics Limited, Bangalore

Abstract – High efficiency space travelling wave tube (TWT) is essentially required for defense coherent radars and for satellite communication systems as a high power, high gain microwave amplifier. High efficiency (more than 60%) and high linearity (phase shift less than 30 degrees) of a space TWT are very important aspects. Indigenous efforts are being made at CEERI (jointly with BEL, Bangalore) for successful design and development of high efficiency space TWTs for space applications since 1997. Two projects have been sponsored by ISRO, Bangalore. A С-band 60W space TWT has been successfully designed and prototypes have been developed jointly with BEL. Desired RF and electrical specifications were achieved over an extended frequency range of 3.40GHz to 4.20GHz with an output power more than 60W, large-signal gain more than 50dB, overall efficiency more than 55%, AM/PM factor less than 4 degrees/dB, and harmonic components and inter-modulation (I/M) components more than 12dBc down. Agreements between the simulated and the experimental values ​​of the AM / PM factor, I / M components and the second harmonic power were achieved well within 10%. ‘Quality Assurance (QA)’ of the С-band 60W TWT has been initiated for space qualification. The design of radiation-cooled Ku-band MOW space TWT has recently been completed and design is transferred to BEL for joint development of this TWT in near future.

I.  Introduction

A space traveling wave tube (Fig. 1) is an ultra-high vacuum microwave device that is used as a high power microwave amplifier in communication satellites. Broad bandwidth, high gain, high efficiency and high linearity of a space TWT are highly desirable for handling a large number of downlink signals in satellite communication. Any loss of efficiency is unacceptable in amplifiers for space applications because it leads to increase in the size and weight of the amplifier and of the waste energy that must be dissipated as heat. The challenge for the space TWT designer is, therefore, to design a TWT that can be operated as close as possible to saturation with d.c. to r.f. conversion efficiency exceeding 60% and without the generation of unacceptable levels of phase shift (AM/PM factor) and inter-modulation (I/M) distortion. This task requires the ability to predict the overall performance of each design using computer simulation so that the task of designing a TWT with specified efficiency and linearity is achieved by simulation. Other important considerations for a space TWT are: high reliability, low weight and small size. Indigenous efforts are being made at CEERI (jointly with BEL, Bangalore) for successful design and development of high efficiency space TWTs for space applications since 1997. Two projects have been sponsored by ISRO, Bangalore for design and development of high efficiency helix TWTs for space applications [1,2]. At present the frequency distribution of globally used space TWTs in different frequency bands are as given in Table-I:

Table-I: Space TWTs in different frequencies

L

S

З

X

Ku

Ka

band

band

band

band

band

band

2 %

2 %

18 %

1 %

75 %

2 %

Within the last 10-20 years the share between the C- band & Ku-band helix TWTs has interchanged globally even at ISRO, making today the Ku-band TWTs as being used dominantly. Ka-band TWTs may also be needed in large quantity in near future for the multimedia applications and thus the requirement may shift to higher frequency TWTs. For the multimedia applications, inter satellite links operating around 60 GHz is needed in near future. ISRO is presently supporting two projects for indigenous design and development of space TWTs in C- band and in Ku-band. Both the projects are being carried out jointly by CEERI and BEL. CEERI has a responsibility to design the complete tube and to participate with BEL for its successful development. The С-band 60W helix TWT was designed by CEERI and was developed jointly with BEL. Four prototypes have been made and full electrical and RF specifications were achieved including overall efficiency, output power and gain over the required frequency band. Work is in progress for its space qualification in association with ISRO, Bangalore. The indigenous design of Ku-band 140W space TWT has also been completed in this year, and it has been transferred to BEL for its joint development.

II. Design of High Efficiency Helix TWT

Fig. 1 shows a schematic diagram of a typical space TWT. The major components of a space TWT are: electron gun, helix slow-wave structure (SWS) and integral- pole-piece (IPP) barrel assembly with samarium-cobalt periodic permanent magnets (PPM), input and output RF couplers, beam refocusing section (BRS) and multistage depressed collector (MDC) along with the base plate and isotropic fin-type radiator. In-house developed software packages were used for design of different components of the helix TWT [3-4] for meeting the major requirements of the space TWT, e.g., high efficiency, high linearity, high reliability, long life, low mass and small size.

The electron gun was designed using M-type tungsten dispenser cathode with efficient heater for long life. Efforts were made to achieve high laminar flow of the electron beam with beam ripples less than 10% at required operating voltages and beam current for the designed PPM focusing field. The helix slow-wave structure (SWS) which supports the propagation of the RF signal and controls the interaction of the modulated electron beam with the RF signal, primarily determines the overall performance of a TWT. The helix SWS [5] was designed for better stability of the tube with no loss in efficiency. Dimension of tape helix and APBN support rods of stepped-in-rectangular shape were chosen optimally for achieving the highest possible interaction impedance of the helix SWS. The matching of the high impedance helix SWS with coaxial coupler at the terminations was achieved having better than 15dB return loss over the desired band by optimizing either the helix pitch at the input/output ends or using tuning stubs at the input and at output. The effects of the induced backward voltage and the reflected voltage components were simulated. For inter-modulation (I/M) calculation, the performance of the TWT was simulated for two simultaneously excited input carriers with 3rd order I/M components. These results were simulated using in-house developed SUNRAY-1D code for one-dimensional multisignal large signal analysis [6, 7]. SUNRAY-1D model was extended to SUNRAY-3D model [8] for more accurate analysis of the helix TWT. The I/M performance simulated from SUNRAY-1D code was found comparable with the experimental performance of developed TWTs [9, 10].

The input and output couplers were designed with low loss coaxial ceramic window of minimum possible thickness and of material low loss alumina-99.5%. Beryl- lia is not so far used for RF window because of toxicity. Both ANSOFT HFSS and in-house developed PEACOCK software packages were used for design of couplers with helix SWS to achieve return loss better than 15dB over the desired band. The BRS was designed to reduce turbulence in the electron beam coming out from the high efficiency SWS before it enters into the collector for enhancing the collector efficiency. The 4-stage depressed collector was used that was designed to recover maximum possible kinetic energy of the spent electron beam before landing the spent beam electrons at different stages of the collector. The simulated collector efficiency was achieved better than 80% for both DC and saturated RF output power. The collector design includes the effect of secondary electrons.

ANSYS code was used at CEERI and NASTRAN & I- DEAS at ISAC, Bangalore for detailed thermal and structural analysis of electron gun, SWS and collector assemblies including complete packaged TWT. Base plate and cover were designed for effective conduction cooling and well support of all assemblies with minimum size and weight. Isotropic fin-type radiator was designed for the collector for efficient radiation cooling.

III. Design of С-band 60W Space TWT

The С-band 60W helix TWT was designed with an extended frequency range of 3.40GHz to 4.20GHz and large-signal gain more than 50dB, conversion efficiency better than 55%, AM / PM factor less than 5 degrees / dB and 3rd order I/M components more than 12dBc down.

The electron gun was designed for 80mA cathode current at anode voltage 3.0kV with ion-barrier anode at + 100Volts with respect to the body. M-type tungsten dispenser cathode was used with heater power around 3.0Watt.The mounting arrangement of cathode inside the gun assembly was optimized with proper heat shield for minimizing heat loss from the cathode. All axial and radial spacing of the cathode with respect to the other electrodes were precisely maintained within tolerance of +0.01 mm. The gun was tested for cathode surface temperature and for cathode emission current before integrating the electron gun with the tube.

Helix SWS was fabricated with the tungsten tape using precise helix winding machine with pitch tolerance within +0.005mm. The helix was supported with three APBN stepped-in-rectangular shaped rods and this assembly was inserted in the IPP barrel assembly with optimum precision.

The copper surface of the collector electrodes was treated with sputter deposition of TiC for suppression of secondary electrons. Four electrodes of the collector were housed in a ground metallic shell with proper shaped alumina insulators and four feed-through. All assemblies (electron gun, SWS, collector) were integrated with optimum precision. The TWT was vacuum processed and it was evacuated both from the gun end and the collector end. Appendage pump of capacity 2l/s was used at gun end and at collector end during ageing and RF testing of the tube. Magnetic shunts were used for optimizing DC beam transmission better than 99%.

The prototypes were tested with APCON power supply with cathode HT supply and four collector supplies. Desired specifications were achieved with output power more than 60W, large-signal gain more than 50dB, overall efficiency more than 55%, AM/PM factor less than 4 degrees/dB, and harmonic components and I/M components more than 12dBc down, over the required frequency range of 3.60GHz to 4.20GHz [11]. A good comparison (within 10%) between the experimental results and the simulated performance for gain, power and efficiency response over the operating band were found from the developed tubes [12]. Experimentally achieved results from the developed prototypes are summarized in Tables IMA and NIB. Table-IIIA shows the RF specifications as proposed (before the start of the project) along with the experimentally achieved specifications. Table-IIIB shows input electrical values as used for testing of these prototypes along with their proposed values.

IV.  Design of Ku-band 140W TWT

The design of Ku-band 140W space TWT has been completed at CEERI in this year with RF and Electrical parameters as given in Tables IVA & IVB. Table-ll-A shows RF parameters that have to be achieved by simulation from CEERI design for the input electrical parameters as shown in Table ll-B. Some safety margin is taken for the designed values. The design of electron gun assembly, helix SWS assembly with PPM field, input and output couplers, and four-stage depressed collector with base plate and the radiator has been completed. The differences in the design features of the C-band TWT and the Ku-band TWT are as in Table-ll:

Table-ll: Differences in Design of С-band TWT & Ku-band TWT:

Design

aspects

For C-band 60W TWT

For Ku-band 140WTWT

Electron Gun Assembly

Single anode

Double Anodes with isolated BFE

Helix SWS

Three sections

Two sections with beam shaver

Input

coupler

Coaxial with TNC

Coaxial with SMA connector

Output

coupler

Coaxial TNC

Coaxial window with WR-75 W/G

4-stage

depressed

Collector

Copper electrodes with TiC sputter coating

Copper electrodes with carbon sputter coating

Base plate

AI-6061 alloy

Mg alloy

Cooling

Conduction

Conduction and Radiation

For Ku-band TWT, double anodes with isolated BFE in gun assembly provide better control of beam optics and therefore it is helpful to reduce helix interception. The design of helix SWS in two sections enhances tube efficiency with reduction in circuit dissipation. SMA connector for the input coupler will reduce rf loss, size and weight whereas WR-75 Waveguide output will be helpful to handle increased output power with no problem of increase in temperature for prolonged continuous operation of TWT at high power output. Carbon sputter coating on copper electrodes of the depressed collector will further suppress the secondary emission coefficient and will therefore enhance the collector efficiency and thereby the overall efficiency of the TWT. Mg-alloy base plate will reduce the weight of the packaged TWT. This TWT will be developed jointly with BEL in near future.

V.   Conclusion

ISRO, Bangalore is supporting two projects for indigenous design and development of space TWTs in C- band and in Ku-band since 1997. Both the projects are being carried out jointly by CEERI and BEL. CEERI has a responsibility to design the complete tube and to participate with BEL for its successful development. Four prototypes with two Quality Models (QMs) of C-band 60W TWT have so far been fabricated by BEL based on CEERI design. Full electrical and RF specifications were achieved including overall efficiency, output power and gain over the required frequency band. Work is in progress for its space qualification in association with ISRO, Bangalore. The design of radiation-cooled Ku-band 140W space TWT has recently been completed and the design was transferred to BEL for its joint development. The indigenous design and development of Ka-band 50W/100W space TWT will be taken up in near future.

VI.  Acknowledgments

Thanks are due to ISAC/SAC/ISRO, Bangalore for sponsoring the space TWT projects to CEERI; to Dr Chandrashekhar, Director, CEERI and Sh SN Joshi, CEERI for their supports; and to BEL, Bangalore for joint development of space TWTs.

]

VII.  References

[1]  ‘Indigenisation of critical technologies for communication satellites’, KN Shankara, IETE Tech. Review, vol. 17, no.6, Nov.-Dec. 2000, pp.325-333.

[2]  ’60-GHz space TWT to address future market’, G.F. Korn- feld, et.al.., IEEE Trans., vol. ED-48, No.1, Jan. 2001, pp. 66-71.

[3]  ‘Design of High Efficiency Space TWT’, V Srivastava, et.al., IETE Tech Review, vol.16, no.2, March-April 1999, pp.249- 254.

[4]  ‘Design document of High Efficiency C-band 60W Space TWT’, V Srivastava, et.al., CEERI, Pilani, August 2001.

[5]  ‘Design of helix SWS for high efficiency TWTs’, V Srivastava, et.al., IEEE Trans., vol.ED-47, no.12, Dec.2000, pp.2438-44.

[6]  ‘improved non-linear model for multi-signal analysis of helix TWT’,V Srivastava & SN Joshi, IEE Proceedings (UK), vol.139, Pt.H, no.2, April 1992, pp.129-134.

[7]  ‘Software packages for design of TWTs with large signal analysis’, V Srivastava, IETE Technical Review (India), vol.18, no.6. Nov.-Dec. 2001, pp.475-482.

[8]  ‘improved 2.5-Dimensional large signal model of helix TWTs’, V Srivastava, IETE Journal of Research (India), vol. 49, No.4, July-Aug 2003, pp239-246.

[9]   ‘ Computer simulation of inter-modulation distortion in TWTA’, RG Carter, W Bosch, V Srivastava & G Gatti, IEEE Trans, on Electron Devices, vol.48, no.1, Jan. 2001, pp.178-180.

[10]  ‘Simulation of Inter-modulation distortion of a high Efficiency high gain TWT for Space radars’, V Srivastava, International Radar Symposium India (IRSI-2001), Bangalore, December 11-14, 2001.

[11]  ’60 Watt space qualified helix traveling wave tube’, PV Bhaskar, KS Prasad, S Ghosh, V Kiran (BEL), and V Srivastava (CEERI), IRSI-2001, Bangalore, December, 2001.

[12]  ‘High efficiency space TWTs – indigenous design and development’, V Srivastava, National Conference on Microwaves Antennas and Propagation, MICROWAVE-2001, November 2001, Jaipur.

Table-IIIA: RF Parameters for C-band TWT

RF Parameters

Unit

Propose

value

Expt value

Frequency

GHz

3.6-4.2

3.4-4.2

Power output

watt

55-60

60-70

Gain at 60W

dB

47

>50

Efficiency

%

50

>50

coll efficiency

%

75

75-80

AM/PM factor

deg/dB

5

< 4.0

IM level

dBc

-10

<-12

2na harmonic

dBc

-10

<-12

Noise figure

dB

30

24

Table-IIIB: Electrical Parameters for C-band TWT

Electrical

Parameters

Unit

Propose

value

Expt value

Heater volt.

volt

5.0

5.50

Heater curr.

mA

700

550

Cathode volt.

kV

-3.0

-3.20

Cathode curr.

mA

80

75

Anode volt.

volt

+200

+ 150

Helix current

mA

2.0

2.5

Collector-I

Volts\m

-850

-1000

Volt./curr.

A

20

12

Collector – II

Volts/m

-1400

-1300

A

25

22

Collector – III

Volts/m

-1800

-2200

A

25

14

Collector – IV

Volts/m

-2500

-2700

A

10

26

Table-IVA: RF Parameters for Ku-band TWT

RF Parameters

Unit

Propose

value

Design

parameters

Frequency

GHz

10.9-11.7

10.9-11.7

Power output

watts

140

>150

Gain at(60W)

dB

>50

>55

Eff. at saturation

%

>55

>60

Coll efficiency

%

>80

>85

AM/PM factor

deg/dB

< 5.0

<3.0

IM level

dBc

<-10

<-14

2na harmonic

dBc

<-10

<-14

Noise figure

dB

<30

20

Table-IV-B: Electrical Parameters for Ku-band TWT

Electrical

Parameters

Unit

Propose

value

Used during simulation

Heater voltage

Volts

5.0

5.0

Heater current

MA

700

600

Cathode voltage

KV

-6.0

-5.8

Cathode current

MA

90

90

Control Anode

KV

-1.2

-1.5

Anode-1 voltage

Volts

+200

+ 100

BFE voltage

Volts

-15

0

Helix current

MA

2.0

0

Collector -I

Vo Its/m A

-2500 / 30

-2500 / 2

Collector – II

Vo Its/m A

-3100/30

-3050 / 45

Collector – III

Vo Its/m A

-3800 /18

-3800 / 28

Collector – IV

Vo Its/m A

-5200 /10

-5200 /15

(all voltages except heater voltage are with respect to the helix/body)

Fig. 1. Schematic diagram of Ku-band 140W space TWT

Джерело: Матеріали Міжнародної Кримської конференції «СВЧ-техніка і телекомунікаційні технології»