174 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 1, JANUARY 2001

Briefs___________________________________________________________________________________________

Advances in Space TWT Efficiencies

David S. Komm, Robert T. Benton, Helen C. Limburg,William L. Menninger, and Xiaoling Zhai

Abstract—Overall dc-to-rf conversion efficiency continues to be thesingle most significant figure of merit for commercial space traveling wavetube amplifiers (TWTAs). Improvements in TWTA efficiency immediatelytranslate into potential revenue increases for satellite operators, since ad-ditional transponders can be carried on board the spacecraft for the sameprime power and waste heat handling capacity. The revenue implicationsof increased efficiency have spurred TWT manufacturers to continuewringing out every possible increase in efficiency. By optimizing both rfcircuit and collector designs, overall efficiencies 70% have now becomeroutine. This paper will discuss the results of several experiments toimprove TWT efficiency demonstrating how these impressive efficienciesmay be achieved.

Index Terms—Efficiency, space, traveling wave tube, traveling wave tubeamplifiers.

I. INTRODUCTION

Traveling wave tube amplifiers (TWTAs) remain the high power am-plifier of choice for general commercial communications spacecraftapplications. A TWTA consists of two main parts, the TWT itself andits associated electronic power conditioner (EPC), which converts thesatellite bus voltage (typically 50 V to 100 V dc) to the necessary elec-trode voltages (kVdc) for the TWT. This paper will discuss improve-ments in TWT efficiency only. The typical space EPC already operatesat an efficiency>93%, so the bulk of the TWTAs waste heat is gener-ated by the tube. A simple economic analysis will now show why evena single percentage point increase of overall efficiency is very signifi-cant to a space TWT.

Let us consider the case of a commercial communications satellitewhose payload consists of a group of 120 W rf output transponders.These transponders will be leased out to users who will relay their sig-nals through the various channels (transponders). In this class of ser-vice, the typical annual lease is approximately $2M per year. Now as-sume that the spacecraft has 10 kW of prime power available to op-erate transponders. If the EPC and the TWT have efficiencies of 93%and 69%, respectively, the TWTA will consume 187.0 W of dc powerfrom the spacecraft bus. In this case the spacecraft manufacturer canfit 53 transponders on board. If, however, the spacecraft manufacturercan obtain 70% efficient TWTs, each transponder will only draw 184.3W from the bus, and the manufacturer can then fit 54 transponderson board. (The extra mass of the transponder and associated equip-ment does not significantly enter the economic analysis because satel-lite launch costs are effectively quantized by launch vehicle consider-ations.) Currently, a typical communications satellite has an expecteduseful life of�15 years. Over the life of the spacecraft, that one extratransponder has the potential to earn $30M. The present cost of a typ-ical space TWT is $65K, so the cost of the flight complement of TWTson board is about $3M. Consequently, even if the 69% efficient tubes

Manuscript received August 16, 2000; revised September 10, 2000. The re-view of this brief was arranged by Editor D. Goebel.

The authors are with Boeing Electron Dynamic Devices, Inc., Torrance, CA90509 USA (e-mail: david.komm@boeing.com).

Publisher Item Identifier S 0018-9383(01)00321-5.

Fig. 1. TWT power balance.

cost the spacecraft manufacturer nothing and full price had to be paidfor the 70% efficient tubes, the cost of the more efficient tubes wouldbe amortized in just 18 months. This is the powerful economic incen-tive that forces TWT manufacturers to continuously strive to produceever more efficient TWTs.

II. A PPROACHES TOINCREASINGEFFICIENCY

In order to raise efficiency, a careful analysis of where waste heatis generated in a TWT is required. Then, the most beneficial areas forimprovements can be identified and a development program generated.Fig. 1 shows the power balance for a typical Ku-band space TWT op-erated at saturation. The largest amount of the total consumed poweris converted to rf energy, some 70%. We are concerned with the 30%of consumed power converted to waste heat which breaks down as fol-lows: About 2% of the total power is needed to heat the cathode, anequal amount is wasted due to beam current intercepted on the helix,Iw, and 7% is lost due toI2R heating on the rf circuit and in the outputcoupler. The largest fraction, by far, of waste heat is generated in thecollector, and so the primary focus of improving TWT efficiency hasbeen in this area.

It should be noted that the above analysis is for a TWT operated atsaturation. Most devices currently deployed are operated in this fashion(or at least within a few tenths of a dB of saturation) whether the mod-ulation is analog or digital. However, current industry trends to moreexotic digital modulation methods require the TWT to operate backedoff from saturation by several dB. The power balance for a TWT op-erated in this way simply shifts to less consumed power converted intouseful rf energy and more converted into waste heat in the collector.Consequently, regardless of whether the TWT is operated at saturationor backed off, the primary emphasis for efficiency improvements al-ways resides in reducing the waste heat deposited in the collector.

The waste heat in the collector is determined by two major consider-ations: The first is that for a given electron distribution function leavingthe rf interaction region of the TWT, only a certain fraction of the ki-netic energy remaining in the spent beam can be recovered with a finitenumber of electrode voltages. This fraction is easily calculated for anyfinite number of available stages. The second consideration is to obtainan electron optics design that comes as close as possible to realizing thismaximum theoretical recoverable power. These two considerations areobviously interlinked such that the design of both the rf circuit and thecollector optics are closely coupled.

Several strategies to increaseefficiencymaybe chosen. Oneapproachis to simply minimize the power entering the collector by designingan rf circuit with higher basic electronic conversion efficiency. This

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 1, JANUARY 2001 175

Fig. 2. Spent beam “collectability” examples.

approach may, however, be in conflict with maintaining a spent beamwith “good collectability” properties (defined later), or it may degradekey rf characteristics such as bandwidth or phase distortion. Anotherapproach which will always improve TWT efficiency is to simplyincrease the number of stages in the collector as exemplified by theswitch from three to four stages which occurred in the industry duringthe mid-1990s. The marginal improvement in efficiency does decreasewith each stage added, however. Nevertheless, all other aspects of theTWT being the same, this remains the final strategy for increasingefficiency. Regardless of the number of stages used, it is obviouslynecessary to obtain an electron optics design that comes as close aspossible to recovering the remaining kinetic energy in the spent beam.

As a final strategy for increasing TWT efficiency, we may also try toimprove the “collectability” of the beam without degrading the basicefficiency. The “collectability” of the beam refers to how much of theresidual kinetic energy the electrons retain after the rf interaction canbe recovered by a finite number of collector stages (electrodes). Fig. 2shows three spent beam distribution functions to illustrate this con-cept of “collectability.” The distribution functions are presented in in-tegral form because that is how they are traditionally measured andanalyzed. The plotted value is100[1� E

0ne(E

0)dE0], wherene(E)is the probability density of the beam electrons in energy space, and1

0ne(E

0)dE0 = 1. Electrons are always collected on electrodeswith lower voltage than the equivalent electron energy. Consequently,the greater the spread of total energy that must be covered by a set ofelectrodes, the greater the amount of residual kinetic energy the elec-trons possess on average as they impinge on the collecting stages. Thedistribution function labeled “low collectability” has a broad energyspread wherene(E) ranges from as low as 22% of cathode voltage(Ek), the so-called “knee” of the spent beam, to greater than 140%of Ek. A large amount of waste heat will be generated even with fourcollecting stages due to the great spread in energy. The same numberof electrodes will generate less heat when collecting the beam labeled“high collectability” because the spread in electron energies is less. Thebest “collectability” is exhibited by the third distribution function. Eventhough this latter distribution function has a greater spread in electronenergies than the former, because of its “stair-step” characteristic (i.e.,the electrons come out in several essentially mono-energetic groups),this beam can be efficiently collected with just three stages. Designingthe spent beam for “collectability” is the most subtle method of raisingefficiency.

The ability to develop TWTs with ever-increasing efficiencies has re-quired continual improvements in the design codes used to mathemat-ically model the physics of TWT operation. Because of the rapid im-

TABLE I85 100 H CHARACTERISTICS(CA. LATE 1999)

TABLE II88 125H CHARACTERISTICS

TABLE III9130H CHARACTERISTICS

TABLE IV85 100H CHARACTERISTICS(CA. EARLY 2000)

provement in computational hardware, numerical simulation method-ologies previously regarded as intractable are now quite feasible. Theresulting improvements in TWT modeling have allowed dramatic in-creases in the efficiency of space TWTs. Specific examples of these im-provements follow. Of necessity, these examples are confined to thosetube types produced at the authors’ facility.

III. EXAMPLES OF RECENT IMPROVEMENTS

The first example is the C-band 85 100H TWT. Typical electricalcharacteristics are shown in Table I. As recently as late 1998, a typ-ical C-band TWT such as the 8560H had an efficiency of just 60%. Byimproving the collectability characteristics of the spent beam distribu-tion function and switching to another existing collector design, theefficiency was raised a full five percentage points compared to the pre-vious design. The effect of reoptimizing the collector is shown below.This dramatic increase is a direct result of improvements in analyticalmodeling.

In contrast to the 85 100H, the next example is a case where justthe electron optics design of the collector was improved thus raisingcollector efficiency. This is a Ku-band tube, the 88 125H. In late 1998,this tube had an efficiency of about 70%. By using improved analyticalmethods to redesign the collector electron optics, the TWT efficiencywas raised three percentage points shown in Table II. The greater effi-

176 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 1, JANUARY 2001

Fig. 3. Peak laboratory Ku-band TWT efficiency by year.

ciency of the Ku-band tube as compared to the C-band tube reflectsboth the narrower percentage bandwidth at Ku-band as well as sizeand mass compromises required at C-band. (Optimum efficiency wouldrequire a physically longer tube.)

Table III shows another example of both improving the spent beamcharacteristics as well as improving the collector efficiency. This ex-ample is the 9130H, a 125 W K-band TWT. Previous EDD K-bandTWTs as typified by the 968H produced efficiencies around 60%. Byimproving both the circuit and collector, four percentage points weregained in overall efficiency.

The last example is a newer version of the 85 100H. The data inTable I reflect the effects of improving the “collectability” of the spentbeam as discussed above and using an existing collector. We then re-designed the electron optics to optimize collector efficiency. The resultsare shown in Table IV. The efficiency of this model is now a full tenpercentage points greater than the original 8560H from which the de-sign improvements started. This significant increase in TWT efficiencyis directly due to the improvements in modeling techniques developedin the last few years.

IV. CONCLUSION

While TWT design is a mature field, significant gains in overall ef-ficiency will continue to occur. Future satellites will carry in excess of100 transponders per spacecraft. These large spacecraft will only in-crease the financial incentives to raise TWT efficiency. Fig. 3 showsthe peak efficiency of laboratory devices at Ku-band as a function ofcalendar year. We fully expect to see TWT designs at this frequencypass 75% efficiency in the next year or so with comparable increasesin the other bands. Given the continuing financial incentives to raiseefficiency, we believe>80% efficiency is definitely attainable.

First Pass TWT Design Success

Robert T. Benton, C. K. Chong, William L. Menninger,Charles B. Thorington, Xiaoling Zhai, David S. Komm, and

James A. Dayton, Jr.

Abstract—Making use of an ensemble of computer codes, Hughes Elec-tron Dynamics (HED) has achieved first pass success in the design of slowwave circuits for several traveling wave tubes (TWTs) during the last year.By first pass design success we mean that when tested, the first device fabri-cated achieved the design goals for RF output power, basic efficiency, gainand phase shift. The design goals are derived from the predictions of a com-puter model, based on the exact dimensions of the TWT. The establishmentof accurate computer models for the TWT slow wave circuit has enabledthe optimization of a new design on the computer, eliminating the need forseveral experimental iterations in the development process. This accom-plishment has enabled HED to significantly reduce the time required todevelop new devices and demonstrate new design concepts.

Index Terms—Design automation, traveling wave tubes.

I. INTRODUCTION

Large signal computer codes have been widely used for decades inthe analysis and design of TWT slow wave circuits. The precision ofthese computational procedures has always been dependent primarilyon the accuracy of the dispersion and interaction impedance data onwhich the computations are based. These quantities have previouslybeen obtained with limited accuracy either experimentally or analyti-cally using a number of approximations.

Although computational facilities and software have steadily im-proved, until recently it was quite common to require three to fiveexperimental iterations to achieve design success. The slow wave cir-cuit design procedure described here is based in part on a computermodel that produces accurate values of the dispersion and interactionimpedance from the exact dimensions and electrical properties of thematerials that make up the slow wave circuit [1]. The authors of this ear-lier work demonstrated that this procedure produces an accurate anal-ysis of existing circuits; the material presented here describes the suc-cessful application of this procedure to TWT design.

When the computational model of the slow wave circuit structureis coupled with computer codes for the analysis of the electron gun,magnetic circuit, collector and slow wave circuit interaction, an accu-rate description of TWT performance is obtained. The model has beendemonstrated to be accurate enough to permit an optimization of thedesign on the computer, which has enabled a significant increase inTWT performance.

II. COMPUTATIONAL PROCEDURES

A suite of proprietary computer codes is used to analyze TWTperformance and achieve first pass design success. The interactionof the slow wave circuit and the electron beam is modeled using thecode QHELIX, the electron gun is described by the CTHERMGUNcode, and the collector is modeled using the codes TCOLLECTORand CCOLLECTOR.

The values of dispersion and interaction impedance make up part ofthe data entered into the HED large signal code, QHELIX. QHELIX

Manuscript received August 16, 2000; revised September 8, 2000. The reviewof this brief was arranged by Editor D. Goebel.

The authors are with Boeing Electron Dynamic Devices, Inc., Torrance, CA90509 USA (e-mail: james.dayton@hughesed.com).

Publisher Item Identifier S 0018-9383(01)00322-7.

0018–9383/01$10.00 © 2001 IEEE