Thermal Impact on the Performance of Highly Efficient Multi-stage

DOI 10.1515/freq-2013-0095

Frequenz 2014; aop

Vishant Gahlaut*, A. Mercy Latha, Parvez Ahmad Alvi and Sanjay Kumar Ghosh

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. Keywords: travelling wave tube, multi-stage depressed collector, thermal analysis, structural analysis, collector performance PACS® (2010). 41.20.-q, 44.10.+i, 44.40.+a *Corresponding author: Vishant Gahlaut: CSIR-Central Electronics Engineering Research Institute (CEERI), Pilani 333031, Rajasthan, India. E-mail: [email protected] A. Mercy Latha, Sanjay Kumar Ghosh: CSIR-Central Electronics Engineering Research Institute (CEERI), Pilani 333031, Rajasthan, India Parvez Ahmad Alvi: Department of Physics, Banasthali University, Banasthali, Rajasthan, India

been designed with modification in the geometry to achieve high collector efficiency of ~90% [11]. The thermal analysis of the high efficient collector has been carried out in the worst-case condition (at un-modulated condition with maximum ambient temperature), which indicates the requirement for thermal management. Hence, choice of optimal thermal management has been made based on various criteria (discussed in detail later). After incorporation of the optimal thermal management, structural analysis has been accomplished. Thermal stresses and structural deformations at each electrode have been assessed for un-modulated and modulated conditions. The change in electrical performance of collector due to thermal deformations, in terms of collector efficiency, back-streaming electrons and body current, has also been evaluated. Further, the change in the high voltage withstanding capability of collector with the presence of beam (hot condition) has also been examined.

2 Thermal management A 3D axially symmetric model of collector has been developed in Solid Works [12] (Figure 1) and imported in ANSYS [13] for performing thermal and structural analysis. Thermal analysis has been done in the worst-case condition (at un-modulated condition with maximum ambient

1 Introduction Space travelling-wave tube (TWT) forms an integral part of microwave amplification system in most communication satellite transponder [1]. In a typical Ku-band space TWT, 30% of the consumed power is wasted as heat, most of which is in the collector region [2]. Hence, the amount of waste heat in the collector has to be reduced by any method. The amount of waste heat in the collector can be reduced by (i) making the collector highly efficient [3–7] and (ii) using proper thermal management [8–10]. A multistage depressed collector for Ka-band 40W space TWT has

Fig. 1: 3D model of the collector

Authenticated | [email protected] author's copy Download Date | 1/10/14 6:25 AM

2

V. Gahlaut et al., Highly Efficient Collector for Space TWT

temperature) without any cooling mechanism incorporated. The thermal input and boundary conditions considered for simulation are tabulated in Table 1. The data for heating power at each electrode has been obtained by particle solver of CST studio suite [14] including the effect of secondary electron emission. The temperature distribution in the worst-case con dition is shown in Fig. 2. From Fig. 2, it can be seen that a maximum temperature of 611 °C is developed at the fourth electrode. Obviously, this temperature is above the safe limit because the tube processing is done only till 400 °C. Any operation beyond 400 °C will cause excessive gassing thereby deteriorating the vacuum inside the tube drastically. Hence, proper thermal management has to be done to trap out the developed heat in a better way and dissipate it effectively. Table 1: Input and boundary conditions for thermal analysis Heat Power (for un-modulated condition) Ambient temperature Conduction Radiation Convection Thermal Resistance

1st electrode = 14 W 2nd electrode = 3.1 W 3rd electrode = 0 W 4th electrode = 86.9 W 80 °C According to the material property (thermal conductivity) At all open surfaces with emissivity according to material Natural convection on the outer surfaces 0.0001 (K m2 W-1) at brazed joints

Fig. 2: Temperature distribution in the worst-case condition without any cooling

Selection of optimal thermal management has been based on the following conditions: i. Maximum temperature should be under 400 °C for electrodes ii. Stresses developed within the collector should be within safe limits iii. Change in the electrical performance should be within tolerable limits a. Change in collector efficiency should not be > ±1% within the entire frequency band of operation b. Back-streaming electron current should not increase beyond 1 mA c. Body current should not increase beyond 1 mA iv. No high voltage breakdown problems should occur After several iterations, thermal management using forced convection cooling has been found optimum. Forced convection cooling is generally accomplished in space TWTs by liquid coolant circulation in the satellite panel, where the TWTs are placed. The results after incorporation of the optimal thermal management has been discussed in the following sections. Temperature distribution in collector after incorporation of the optimal thermal management is shown in Fig. 3 in the worst-case condition (unmodulated including the effect of secondary electron emission at 80 °C ambient temperature). It could be observed that the maximum temperature is ~365 °C at fourth electrode, which is within the safe limit.

Fig. 3: Temperature distribution in the worst-case condition with forced convection cooling



3

Table 3: Maximum deformations for un-modulated condition at 80 °C after thermal management Electrodes

Max radial deformation (µm)

Max axial deformation (µm)

First Second Third Fourth

19.2 21.8 27.1 26.6

34.6 10.7 22.5 76.9

Table 4: Maximum stress at each electrode-ceramic and ceramiccylinder joint for both modulated and un-modulated condition at 80 °C ambient temperature Region

Fig. 4: Temperature distribution for modulated condition with forced convection cooling

Temperature distribution in the collector for corresponding modulated condition (including the effect of secondary emission) at 80 °C ambient temperature, with forced convection cooling is shown in Fig. 4. A maximum temperature of ~185 °C is obtained in first electrode, which is well below the worst-case operational condition.

3 Structural analysis Analysis has been carried out to study the structural deformations and thermal stress developed due to temperature distribution in collector. All these studies are carried out in the worst-case condition with maximum ambient temperature of 80 °C. Dimensional deformations in both radial and axial directions, for modulated and un-modulated conditions, are tabulated in Table 2 and Table 3 respectively. As the deformation of electrodes is in the range of few tens of microns, they are within the safe limits, even under the worst condition. Table 2: Maximum deformations for modulated condition at 80 °C after thermal management Electrodes

Max radial deformation (µm)

Max axial deformation (µm)


3.8 15.9 13.6 15.0

21.3 7.1 14.2 36.2


Max stress in un-modulated condition (MPa)

Max stress in modulated condition (MPa)

Electrode– ceramic joint

Ceramic– cylinder joint

Electrode– ceramic joint

Ceramic– cylinder joint

15.138 2.233 26.621 31.733

24.058 13.632 122.450 86.646

9.114 5.794 1.616 5.604

31.271 8.433 6.0491 16.427

Stresses at critical joints between electrodes to ce ramics and ceramics to outer metal cylinder have also been estimated and are tabulated in Table 4. Results of stress analysis are within the margin of safety (MoS), which is calculated by

= MoS

Sa -1 Sl × SF

(1)

where Sa is the allowable stress given by (2), Sl is the stress induced by loads and SF is the safety factor (= 1.5).

Sa =

UltimateTensileStrength SF

(2)

A value of MoS = 0 signifies that the collector would pass in margin with an additional safety factor of 1.5. Any value of MoS > 0 implies that the collector assembly is safe. Table 4 shows the maximum stresses developed at each critical joint. However, the calculation of MoS reveals that even the minimum MoS is > 1 and hence, the collector is safe.


4


Table 5: Impact of deformation on electrical performance of the collector for un-modulated (DC) condition at 80 °C ambient temperature Parameter

Efficiency Back-streaming current (mA) Body current (mA)

Before deformation

After deformation

Only primaries

With secondaries

Only primaries

With secondaries

81.2% 0.0 0.0

78% 0.68 0.2

81.4% 0.0 0.0

77.1% 0.997 0.4

Table 6: Impact of deformation on electrical performance of the collector for modulated (RF ) condition at 80 °C ambient temperature Parameter

Efficiency Back-streaming current (mA) Body current (mA)

Before deformation

After deformation

Only primaries

With secondaries

Only primaries

With secondaries

90% 0.0 0.0

86.1% 0.31 0.4

89% 0.0 0.7

85.5% 0.221 0.8

4 Impact on collector performance 4.1 Electrical performance Due to thermal impact, the performance of collector changes under hot condition (in the presence of electron beam). The deformed collector electrodes exhibit change in its electrical performance which has been studied. The change in the electrical performance in terms of collector efficiency, back-streaming current and body current has been evaluated at 80 °C ambient temperature using par ticle tracking code – EGUN [15]. Tables 5 and 6 are for un-modulated and modulated condition showing the performance respectively (both using only primary electrons (primary) and including the effect of secondary electrons (secondary)). The variation of collector efficiency over the entire frequency of operation, before and after deformation, ignoring the effect of secondary electrons at ambient 80 °C temperature has been shown in Fig. 5. Fig. 6 shows variation of collector efficiency over the entire frequency of operation including the effect of secondary electrons at 80 °C. From both Figs. 5 and 6, it is obvious that the change in collector efficiency is within ±1% over the entire frequency band of operation.

4.2 High voltage withstanding capability Under hot condition, due to thermal impact, gap between the electrodes changes, leading to change in high voltage

Fig. 5: Impact of deformations on collector efficiency over the entire frequency of operation ignoring the effect of secondary emission (primary condition)

withstanding capability. Sometimes, the maximum electric field between electrodes exceed the breakdown limits due to larger deformations and might lead to high voltage breakdown issues. Hence, it is vital to examine the high voltage withstanding capability at hot condition. Table 7 summarizes the change in maximum electric fields between electrodes after deformations. It could be observed that the inter-electrode distance between electrode 1 (E1) and electrode 2 (E2) has been reduced from 0.5 mm to 0.484 mm which causes the electric field to increase from 1.920 MV/m to 1.967 MV/m. Whereas, the inter-electrode distance between E2–E3 and E3–E4 increases and correspondingly the electric field decreases.



5

Acknowledgments: The authors are thankful to Director of CSIR-CEERI, Pilani for his kind permission to publish this work. They are also grateful to Mr. P.C. Panda and other team members for their cooperation and suggestions from time to time for improvement of the work. Received: July 12, 2013.

References

Fig. 6: Impact of deformations on collector efficiency over the entire frequency of operation including the effect of secondary emission (secondary condition)

The breakdown limit in vacuum is 30 MV/m [16]. Hence, at hot conditions there are no threats of breakdown problems.

5 Conclusions Choice of the optimal thermal management for a highly efficient collector has been made, considering (i) maximum tolerable temperature; (ii) maximum allowable stress; (iii) change in the collector performance and (iv) HV breakdown problems. After incorporation of optimal thermal management, structural deformations and thermal stresses have been found to be within the tolerable limits. The electrical performance and high voltage withstanding capability of the collector under hot conditions have been evaluated and confirmed to be within tolerable limits.

[1] M G Bodmer, J P Laico, E G Olsen, and A T Ross, The Bell System Technical Journal, 1703 (1963). [2] D S Komm, R T Benton, H C Limburg, W L Menninger, and X Zhai, IEEE Trans. on Electron Devices, Vol. 48, 174 (2001). [3] H G Kosmahl, IEEE Trans. on Electron Devices, Vol. 70, 1325 (1982). [4] F Sterzer, IRE Trans. on Electron Devices, Vol. ED-5, 300 (1958). [5] T S Chen, H J Wolkstein, and R W McMurrough, IEEE Trans. on Electron Devices, Vol. 11, 243 (1963). [6] P Ramins and B T Ebihara, IEEE Trans. Electron Devices, Vol. 33, 1915 (1986). [7] A N Curren and P Ramins, International Electron Devices Meeting, Vol. 31, 361 (1985). [8] Henry H Fong and David J Hamel, International Electron Devices Meeting, Vol. 25, 295 (1979). [9] M F Rose et al., IEEE Trans. on Electron Devices, Vol. 38, 2252 (1991). [10] L Yao et al., Proc. of International Vacuum Electronics Conference, 2006. [11] A M Latha et al., J. Infrared Milli Terahz Waves, Vol. 34, 53 (2013). [12] SolidWorks Manual, version 9.0 SW 3.0. (2009). [13] ANSYS Help Guide, version 10.1, ANSYS Inc. (2010). [14] CST Studio Suite 2010, Operating Manual (Darmstadt, Germany), licensed 2010. [15] W B Herrmannsfeldt, SLAC (1988). [16] A S Gilmour, Micro-wave Tubes, Artech House Microwave Library (1986).

Table 7: Impact of deformation on the electric field between various electrodes of the collector at 80 °C ambient temperature Region

E1–E2 E2–E3 E3–E4

Potential difference (V)

750 540 680

Before deformation

After deformation

Inter-electrode distance (mm)

Max electric field (×106 V/m)

Inter-electrode distance (mm)

Max electric field (×106 V/m)

0.500 0.500 0.500

1.921 1.470 1.822

0.484 0.554 0.560

1.967 1.080 1.225

