ELEKTROTECHNIEK RUIMTEV AART. IEEE Student Branch Eindhoven

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1 SYMPOSUM ELEKTROTECHNEK N DE RUMTEV AART Proceedings EEE Student Branch Eindhoven Uitgever: EEE Student Branch Eindhoven Post bus MB Eindhoven Aile rechten voorbehouden. Niets uit deze uitgave mag worden verveelvoudigd, opgeslagen in een geautomatiseerd gegevensbestand, of openbaar gemaakt, in enige vorm of op enige wijze, hettij elektronisch, mechanisch, door fotocopieen, opnamen, of op enige andere manier, zonder voorafgaande schriftelijke toestemming van de uitgever. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronically, mechanically, by photocopying, recording, or otherwise, without a prior written permission of the publisher.

2 nhoudsopgave bz. nhoudsopgave 0.1. nleiding Elektrotechniek in de Ruimtevaart C.H.F. Lennartz, Chairman EEE Symco' Hoogspanningsproblemen in satellieten dr.ir. J.M. Wetzer, Technische Universiteit Eindhoven 2.1. Satellietcommunicatiesystemen: techniek ell technologie ira R. Suttels, Alcatel Bell Space Systems 3.1. De tracking van de ERS-! satelliet en de verwerking van ERS-! radarhoogtemetingen ira B. Arrlbrosius, Technische Universiteit Delft 4.1. Satellite communication systems planning in an interference environment ira R. Hekmat, PTT Research 5.1. Space Power Electronics Design Drivers D. O'Sullivan, B.E., ESA ESTEC 6.1. De betekenis van robotica in de ruimtevaart ira W. de Peuter, ESA-ESTEC Z 1. Space-qualified optical memory for the Columbus Pressurized Module ira T. Algra, Nationaal Lucht- en Ruimtevaart Laboratorium 8.1. A modular instrumentation concept for experiments under microgravity ing. L.J. Aartman, N ationaal Lucht-en Ruimtevaart Laboratorium 9.1. Nawoord J.M.W.M. Janssen, Chairman EEE Student Branch Eindhoven Bestuur EEE Symco' Lijst van adverteerders Commissie van aanbeveling Woord van dank

3 Chris Lennartz: n/eiding Elektrotechniek in de Ruimtevaart nleiding Elektrotechniek in de Ruimtevaart C.H.F. Lennartz De Eindhoven Student Branch van the nstitute of Electrical and Electronics Engineers, nc. is een organisatie die haar leden onder andere in de gelegenbeid stelt onl toegang te hebben tot dezelfde informatiebronnen als degenen die de ingenieurstitel al verworven hebben. Een manier van informatieoverdracht die altijd tot de verbeelding heeft gesproken en dat waarschijnlijk ook altijd zal blijven doen is het symposium, zeker als het de verbeelding van de student betreft. De grootste kracht van een symposium ligt in het feit dat allerlei mensen van verschillend pluimage, zowel academisch als industrieel, met elkaar van gedachten kunnen wisselen over het onderwerp. Dit onderwerp en de invalshoek waaronder het tijdens het symposium belicht zal worden moet dus zeer bewust gekozen worden. Het criterium voor de onderwerpkeuze van afgelopen jaren was dat het onderwerp een "hot item" in de elektrotechniek behoorde te zijn en titels als Fuzzy Logic: Systems and Design, Multimedia, Telematics: Technology and Applications en Mobiele Communicatie waren afgelopen jaren dus niet van de lucht. Met deze onderwerpen is de voorraad hot items voorlopig uitgeput, en het nu houden van een symposium over nieuwe ontwikkelingen in een van deze gebieden zou hetzelfde (tegen)effect hebben dat een sequel, een geforceerd vervolg op een succesfilm, doorgaans sorteert. Vandaar dat dit jaar gekozen is voor een ander criterium: aspectintegratie. Dit houdt in dat een onderwerp gekozen wordt waarin vele aspecten van de elektrotechniek vertegenwoordigd zijn, zodat de specialisten op een bepaald gebied hun vakgebied in een bredere context geplaatst zien. Een ideaal onderwerp dat aan dit criterium voldoet is natuurlijk "Elektrotechniek in de Ruimtevaart". Bovendien heeft ruimtevaart altijd al tot de verbeelding van de. mens gesproken, vooral vanaf het begin van de bemande ruimtevaart met als "kleine stap voor een mens, maar grote stap voor de mensheid t, Neil Armstrong's eerste schreden op het maanoppervlak. Het is geen toeval dat de vakgebieden van de elektrotechniek en de ruimtevaart vanaf het begin der zestiger jaren een parallelle explosieve groei hebben doorgemaakt. De wisselwerking tussen beide disciplines he eft duidelijk aan deze groei bijgedragen. De meest in het oog springende toepassing van elektrotechniek in de ruimtevaart zag het levenslicht toen Arthur C. Clarke in 1945 in Wueless World een 0.1

4 Chris lamartz: nleiding Elektrotechniek in de Ruimlevaart artikel publiceerde waarin hij de geostationaire baan van een kunstmaan aantoonde; de basis voor wereldomvattende satellietcommunicatie was gelegd. Om de betrouwbaarheid van zulk een communicatielink te garanderen zal de satelliet zeer nauwkeurig genavigeerd moeten worden, wat geschiedt met behulp van geavanceerde regelsystemen. Het spreekt natuurlijk vanzelf dat de apparatuur aan boord van de satelliet voor het merendeel be staat uit electronica in de vorm van hoog-technologische telecommunicatieapparatuur die ervoor moet zorgen dat de "ruimtelijke ordening" zo min mogelijk voor communicatiestoringen zal zorgen. Om deze apparatuur te laten functioneren is een efficiente energievoorziening in de vorm van bijvoorbeeld zonnepanelen noodzakelijk. Het voeden van de Traveling Wave Tube Amplifiers brengt een aantal hoogspanningsaspecten met zich mee die noodzakelijk zijn voor een goede werking. Blijkbaar zijn er alleen al in de satellietcommunicatie genoeg aspecten van elektrotechniek om een symposium aan te wijden, maar in andere disciplines doet elektrotechniek evenzeer van zich spreken. Bij de verkenning en observatie van hemellichamen speelt remote sensing m.b.v. radar en/of radiometrie een vooraanstaande rol, voordat wordt overgegaan op meer fysieke verkenning waar de robotica van wezenlijk belang is. Verder is de radiocommunicatie in alle vormen van het bedrijven van ruimtevaart van essentieel belang, wat hopelijk geen uitleg behoeft. Deze voorbeelden geven blijk van het feit dat het gehele vakgebied der elektrotechniek onmisbaar is bij het bedrijven van ruimtevaart. Dit geldt echter ook vice versa: vele producten worden eerst in de ruimte getest om na te gaan of ze onder extreme atmosferische omstandigheden nog adequaat functioneren. Concluderend kan er dus gesteld worden dat een symposium over Elektrotechniek in de Ruimtevaart uitermate informatief kan zijn voor aldiegenen die vanuit welk oogpunt dan ook enigermate geinteresseerd zijn in ruimtevaart. k hoop dan ook dat deze proceedings zullen bijdragen aan het verwerken van die informatie. C.R.F. Lennartz Chairman Symposiumcommissie 0.2

5 Hoogspanningsproblemen in satellieten de.. W F:indh9ven. 11.J?84.. pr()tl1?y.e~rd~.. hijg)~ 1l:?n4rr~9. ~ : Aaa~..9ptische maanden. door. bij. 8ntaTjo. ydf()jtes~.a~fh.pivi~i?1.. ;i1/.t9ront();.. <?anada, werkt~ daaraang~geis()leerd~~~hakelsyst~fu~l1i :.... '.... en Zijhhaidig ~ef~~~~~~li~~~g~~~~i~~~ de~~~~~~~~~~~t1~dingen en..i~o(lti~,:...l1et~arn.~j~yacll~jll. ej.!n.g~s:y?rtl1ig~:ne<ii~i \Va~);tij.'. zich bezig. t~di~~~~~?!e~!m~~;ll~'~/lil~~~r~~f~f~r~=~l~~!~~ uitgevoerd in.sallellwerlting. niet::a~.e4rppes~. ~qil1tey~(lt.torg~nisati~esa..n.1992 ontvinghijvpor 41t.Werlc<le:. 'C.9#.tt~itQP AW~q"~tij~~1$ d tweejaar1ijkse co~erentie<>.v~r. ~pj~ H#rg s ri1d~!f ~~ica rrisuhiti6hliitvacuthp."....: :...:.. 1.1

6 .M. Wetzer: Hoogspanningsprob/emen in satellielen Hoogspanningsprohlemen in satellieten Dr.ir. J.M. Wetzer Vakgroep Hoogspanningstechniek en EMC Technische Universiteit Eindhoven Satellieten herbergen een veelheid aan elektrische en elektronische apparatuur. Soms spelen hoge spanningen daarbij een belangrijke rol. Hoge spanningen zijn bijvoorbeeld essentieel voor het functioneren van lopende-golf zendbuizen (travelingwave tubes of TWTs). Deze buizen worden gebruikt om hoogfrequente signalen, continu dan we gepulst, te versterken. Versterking van CW (continuous wave) signalen is van belang voor satelliet-communicatie, versterking van gepulste signalen speelt onder andere een rol bij radar (denk aan de European Remote Sensing satelliet). Bij CW -buizen gaat het om frequenties in de range van GHz en vermogens van W, bij gepulste buizen liggen de frequenties in de orde van 5 GHz en bedraagt het puls-vermogen ongeveer 5 kw. De hierbij gehanteerde spanningen varieren van 1 tot 25 kv, corresponderend met veldsterkten in de range van kv cm. De genoemde spanningen en velden moeten betrouwbaar geproduceerd en gehanteerd worden in vacuum, hetzij het vacuum in de zendbuis (ongeveer 10-6 mbar), hetzij het satelliet-vacuum (ordegrootte 10 4 mbar). Vacuumisolatie wordt overigens niet alleen in de ruimtevaart bedreven, maar ook op aarde. Hierbij kunnen we bijvoorbeeld denken aan beeldbuizen, rontgenbuizen en vacuumschakelaars. Toepassing in satellieten stelt echter hogere eisen aan de isolatie vanwege: 1. de gewenste betrouwbaarheid (in verband met de continuiteit van transmissie en de hoge "voorrijdkosten"), 2. de agressieve omgeving (hoogenergetische fotonen), en 3. de variatie in de actuele druk; satellieten worden op aarde getest bij atmosferische druk maar moe ten, na lancering, opereren in het satelliet -vacuum; bovendien is dit satelliet-vacuum veranderlijk van samenstelling en druk. De vakgroep Hoogspanningstechniek en EMC van de TU Eindhoven verricht sinds 1983 onderzoek aan hoogspanningsproblemen in satellieten, in nauwe samenwerking met het onderzoekinstituut ESTEC van de Europese Ruimtevaart Organisatie (ESA). Dit werk is zowel fundamenteel als toegepast van aard. Het fundamentele onderzoek houdt zich bezig met de vraag welke mechanismen en processen verantwoordelijk zijn voor doorslag. Het toegepaste werk heeft tot doel deze inzichten te vertalen naar richtlijnen voor het ontwerp en bedrijf van toekomstige componenten, en deze richtlijnen te verifieren. Deze ontwerpregels zijn onder meer met succes toegepast bij het ontwerp van de ERS-l radar-zendbuis. n deze presentatie wordt ingegaan op de geschetste problematiek en worden de bevindingen besproken van een in 1991 afgerond onderzoek. Als leidraad bij deze presentatie is de "Executive Summary" van het eindrapport bijgevoegd. 1.2

7 1M. Wetter: Hoogspanningsproblemen in satellieten EUROPEAN SPACE AGENCY CONTRACT REPORT High-Voltage and EMC Group Department of Electrical Engineering Eindhoven University of Technology ESTEC CONTRACT 7186/87/NL/JG(SC) 1.3

8 .M. Weller: Hoogspanningsproblemen in satellieten EUROPEAN SPACE AGENCY CONTRACT REPORT The work described in this report was done under ESA contract. Responsibility for the contents resides in the authors or organisation that prepared it. November 1991 EXECUTNE SUMMARY OF THE STUDY ON HV-DESGN ASPECT'S OF MCROWAVE TUBES EHC/JW /MV /RAP91010 ESA Technical Management Dr. S.J. Feltham Project team EUT Dr.ir. J.M. Wetzer Dr. P.AA.F. Wouters ng. A.J.M. Pemen Dr. M.G. Danikas Prof.dr.ir. P.C.T. van der Laan A.J. Aldenhoven High-Voltage and EMC Group Eindhoven University of Technology P.O. Box MB Eindhoven The Netherlands ESTEC/Contract no. 7186/87/NL/JG(SC) Front cover: Example of optimized insulator design for concentric cylindrical conductors 1.4

9 .M. Wetter: Hoogspanningsprob/emen in sateuieten NTRODUCTON The present generation of traveling wave tubes (TWf's) suffers from spurious discharges and tube failure as a result of high voltage breakdown. Future TWf's for use in spacecraft will operate at increased frequency and power. As a consequence the designs should be made more compact, and yet able to withstand higher voltages. Various microwave tube designs typical of future application at frequencies above 11 GHz have been studied within the frame work of the European Space Agency's "Advanced Space Technology Programme". t was reported that spurious switch offs can occur on a seemingly random basis. nsulation breakdown in the region of the high voltage feedthroughs was mentioned as the prime suspect as the source of these SSO's. t was concluded that a detailed investigation of high voltage design aspects was required, in order to achieve a dramatic improvement of the high-voltage performance. n this study the high voltage (HV) design concepts used in microwave tube technology are investigated. The objective of this work is to provide guidelines for the improvement of the HV performance in order to meet future requirements. APPROACH Means to improve the high-voltage performance of microwave tubes involve the choice of materials and material treatments, field control techniques and conditioning. The present state of the art with regard to materials and material treatments does not offer opportunities for significant inlprovement, mainly due to the lack of information on long term behavior and stability. Much better opportunities for improvement are given by field control and conditioning techniques. Based on a literature survey, on an evaluation of existing designs and on earlier work, problem areas are defined. These problems areas are implemented in a number of test geometries that are subjected to various tests. The complete set of insulator shapes is shown in Figure 1. The tests include DC-current measurement, partial discharge measurement, breakdown voltage measurement, and time-resolved measurement of current and optical emission during breakdown. Also different conditioning procedures are studied experimentally. A theoretical study on surface charging is performed to support the evaluation of the experimental results. From the theoretical and experimental results guidelines are derived for the design and operation of vacuum high-voltage components. Specifically, guidelines are formulated regarding the design of insulators, feedthroughs, high-voltage cables and tube assemblies, and regarding procedures for electrode treatment, conditioning, potting and pre-flight testing at ambient pressure. 1.5

10 .M. Wetter: Hoogspanningsproblemen in sajeuieten J.J_ 2 3 * s" * * _ _.._---_..._ NSULATOR GEOMETRES FiguTf! 1 nsulator shapes used in the present study. For the asymmetric shapes denoted (4, 5, 6 and 10), extension "a" indicates that the cathode is at the right side (smallest contact surface), extension "b" indicates that the cathode is at the left side (largest contact surface)... _.. - PROGRAM DESCRPTON The contract was divided into two phases. Phase was split up into two workpackages. The following program was adopted: PHASE 1, Workpackage 1 update of the literature survey performed in 1983 under ESA contract 5419/83/NL/GM(SC); evaluation of existing HV tube sub-assembly designs with respect to geometry and materials (electrodes/insulators); reduction of the complex HV design configurations into basic design elements; investigation of available insulator and electrode materials. PHASE 1, Workpackage 2 design and procurement of an experimental set-up to investigate the HV performance of the designs analysed in workpackage 1; definition and procurement of test samples to be investigated. 1.6

11 .M. Wetter: Hoogspanningsproblemen in satellieten PHASE 2, Workpackage 3 experimental program; evaluation of the results in terms of the mechanisms determining the voltage holdoff capability; formulation of guidelines for the High-Voltage design of future tubes and for the conditioning procedures to be applied; continuous update of the literature survey; preparation of the final report. BASC FNDNGS The two key mechanisms, important for the design of vacuum high-voltage components, are primary electron emission and surface charging. These mechanisms drastically affect both the holdoff performance and the conditioning process. Primary electron emission from negative electrodes can be reduced by field control, in particular by concentrating the field at the positive electrode. Surface charging should either be avoided or controlled: properly shaped insulators can trap charges such as to reduce the cathode field, proper conditioning can produce a beneficial charge distribution. The choice of conditioning procedure affects the choice of insulator geometry. Conditioning can drastically improve the voltage holdoff capability. Effective conditioning requires a number of breakdowns. t is therefore important that future designs permit breakdowns, with limited energy in order to avoid damage. The number of breakdowns required to establish a high breakdown voltage depends on the insulator geometry. Because conditioning is related to the deposition of surface charge, the conditioning effect may be lost as a result of charge leakage or exposure to gases. CONCLUSONS Materials and material treatments 1. A literature survey of new, extensively studied, electrode and insulator materials does not reveal alternatives which meet all requirements and perform significantly better than the materials used in practical designs. 2. New materials and material treatments such as laminated electrodes, and insulators with surface coating and bulk doping, have not been sufficiently studied with respect to their stability and can therefore at present not be recommended as reliable alternatives. Such materials and treatments could form the basis for future study. 3. Mechanical polishing of electrode surfaces is not recommended because impregnated particles can serve as electron emission sites which might initiate 1.7

12 JM. Wetter: Hoogspanningsproblemen in satellieten a breakdown. 4. The grinding of a sharp recessed corner into an alumina spacer can cause the formation of small cracks, which may lead to electrical breakdown. Recessed corners should preferably be rounded off in order to mimimize mechanical stress concentrations. 5. Next to the choice of insulating material, the quality of the manufacturing process is of crucial importance for the insulator hold-off performance. Field control 6. Field control techniques offer good opportunities for the improvement of the HV -performance of TWTs. The first objective should be the prevention of primary electron emission from negative electrodes. 7. The Finite Element Method (FEM) is a powerful tool to locate high field regions and to study design modifications. For DC, the applicability is sometimes limited by leakage of the insulator and by prebreakdown currents. Both processes result in charges on the insulator surface. Diagnostics 8. Sensitive pre-breakdown current measurements, such as (pa-range) DC current and (pc-range) partial discharge measurements, provide insight into the mechanisms responsible for insulator flashover. They give a reliable indication of the voltage hold off performance of an insulator or component only at voltages close to the breakdown voltage. 9. Breakdown voltage measurements reveal important information on the conditioning process and on the role of surface charge. The effect of surface charge can be observed very directly by performing breakdown measurements before and after an insulator has been exposed to low pressure dry N The time-resolved measurement of the breakdown current shows that the electrodes are fully discharged upon a flashover. More charge may be involved, depending on the external circuit. Further, the time-resolved measurement of current and optical emission during breakdown shows how the breakdown process is influenced by the insulator shape. Surface charging 11. Surface charging determines to a large extent both the voltageholdoff performance 1.8

13 .M. Wetzer: Hoogspanningsproblemen in satellieten and the conditioning process. t should therefore be incorporated in the insulator design. Conditioning 12. With an effective conditioning procedure it is possible to attain a dramatically improved and reproducible breakdown voltage. 13. For most insulator geometries, effective conditioning requires at least a few breakdowns. t is therefore important that future components are designed in such a way that breakdown conditioning is permissible. This requires a limitation of the breakdown energy. A value of around 30 mj appeared to be safe and effective in this work, but is not necessarily the optimum value. 14. Of the conditioning procedures tested, step-conditioning provides the fastest rise of breakdown voltage, and the smallest spread. The step-conditioning procedure is illustrated in Figure 2. 50~ ~ o eo 100 VOLTAGE (kv) VERSUS TME (minutes) (BD = breakdown) Figrue 2: Applied voltage versus time for step-conditioning procedure. 15. The choice of conditioning procedure should influence the choice of insulator geometry, and vice versa. 16. With a sufficient number of breakdowns, critically designed insulators may still attain 1.9

14 J.M. Wetter: Hoogspanningsproblemen in salellieten a high breakdown voltage. 17. The conditioning effect may be largely lost when insulators loose their surface charge. The loss of charge can be the result of charge leakage (when the voltage is switched off for long periods of time) or of exposure to low pressure N 2, or any other gas at low pressure. 18. For space applications, conditioning should be required only once, before launch, preferably at the pumpstand before sealing. This requirement can only be met for well-chosen geometries. Operation in vacuum and air 19. The design requirements with respect to operation in air and in vacuum are different and sometimes conflicting. Pre-flight tests in air are not representative for the behavior in vacuum, and may even cause insulation degradation. GUDELNES General guidelines 1. a. Reduce the field at the negative electrode (the "cathode") by shielding, and by eliminating protrusions as well as contamination by polishing (see guideline 2). b. Reduce in particular the field at the cathode triple junction by an appropriate shaping of electrodes and insulators eliminating voids and imperfect joints at the triple junction. As an alternative, voids or imperfect joints can be made field free by metallization techniques enhancing the field at the anode 2. Mechanical polishing of electrode surfaces is not recommended because impregnated particles can serve as electron emission sites which might initiate a breakdown. 3. t is not justified, and even hazardous, to quote safe local cathode-field values. The macroscopic (design) field depends on materials and geometry whereas the actual field also depends on microscopic enhancements and contamination. A better approach is to quote the average field (voltage over electrode distance) for a given geometry (see also Table 2). 1.10

15 1.M. Wetter: Hoogspanningsproblemen in satellieten nsulator shape 4. The choice of an insulator geometry depends on the conditioning process (with or without breakdowns, number of breakdowns), and on the operating conditions (regular exposure to gases, regularly switched off for long periods of time). S. mportant insulator design parameters are: the minimum breakdown voltage the conditioning speed the sensitivity to charge loss by gas exposure or leakage. For each parameter, or combination of parameters, a ranking of insulator shapes can be derived. As an example, Table 1 gives the ranking with respect to the combination of minimum breakdown voltage and conditioning speed. Table 1 Ranking of insulator shapes, on a scale from 0 to 10, with respect to the combination of minimum breakdown voltage and conditioning speed (DC voltage). The upper side of the samples shown is the cathode side. Sample Score Judgement Remarks! 6b -:7 10 EXCELLENT AE,SS lob =:J 9.2 AE,NS Sb 8.S AE,SS 4b 8.0 AE,NS Sa ~ 7.3 REASONABLE CE,SS 2 => 7.1 BE,SS 6a 6.4 CE,SS BE,NS loa ~ S.2 NSUFFCENT CE,NS 9 ~ S.O BE,NS 1 =:=J 4.7 NS 4a 3.2 BAD CE,NS BE,NS 3 =s 1.5 1) AE = anode enhanced CE = cathode enhanced BE = both enhanced SS = stepped shape NS = non-stepped shape 6. n terms of breakdown voltage, conditioning speed and sensitivity to charge loss, geometries with field enhancements at the anode are superior. 1.11

16 1M. Wetter: Hoogspanningsproblemen in sateuieten 7. f cathode field enhancements are unavoidable, stepped shapes are recommended. 8. The insulator shape dramatically affects the allowed averaged fieldstrength (see Table 2). The minimum breakdown fields observed are, of course, not safe design values but are subject to normal derating for space applications. n our opinion the insulator shapes at the bottom of the list ought to be derated more than those at the top of the list. Table 2. Averaged breakdown field (breakdown voltage over distance), the number of breakdowns required to reach 10 kv/mm, and the breakdown energy used in this work. Note that in the design the maximum rated fields are subject to normal derating for space applications. The upper side of the samples shown is the cathode side. Averaged Averaged Number of Breakdown Sample breakdown field breakdown field breakdowns to energy (kvjmm) (kv/mm) reach 10 (mj) MNMUM MAXMUM kv/mm 6b ~ 10.2 > lob =:J 9.0 > b 8.2 > b J 7.4 > Sa =s 7.2 > => 7.0 > a 6.6 > > loa 5.4 > > =:=J 5.0 > a ~ 4.0 > => 3.8 > > Especially for space applications, insulator shapes with enhanced anode fields (6b, lob, 5b,4b), which do not rely on surface charge, are advised because: hardly any breakdown is required in the conditioning procedure, and conditioning is required only once; repeated conditioning after long time switch-off is not necessary. 1.12

17 .M. Wetter: Hoogspanningsproblemen in satellieten 10. A proposed optimized insulator shape for DC voltages is shown in Figure 3. The advantages include breakdown voltage, conditioning speed and insensitivity to charge loss. Further, it is taken into account that triple junctions are never perfect and should be made harmless by surface charging. Other design examples based on the guidelines presented are discussed in the Final Report. 11. Based on the first breakdown voltage, a preliminary guideline would be to use stepped insulator shapes for AC applications. Before a definite guideline can be given, the adverse effects of surface charge at AC voltage should be further investigated. 8 Fi8f.W 3:.An example of an optimized insulator design for DC Feed through 12. Cathode field concentrations can be minimized by the shape of electrodes or insulators metallization of the inside of the tubular insulator a larger clearance between central conductor and insulator 13. Feedthroughs should be specifically designed either as a vacuum feed through or as a potted feedthrough. A well-designed vacuum feedthrough looses quality if potted. 14. Feedthrough connections should be well shielded (e.g. by a stress cone) in order to 1.13

18 .M. Wetter: Hoogspanningsproblemen in satellieten avoid field concentrations. High-voltage cable 15. n order to avoid partial discharge activity, and subsequent damage, it is advised to: surround each insulated wire with a (semi-) conductive layer, and use extruded, rather than tapewound and sintered, dielectrics. Such cables do not yet exist and should be a candidate for future development. 16. Cable ends should preferably not be potted, because potting impedes cable outgassing and introduces field enhancements. Potting 17. Potting can give reasonable results if done properly (clean surfaces, potting under vacuum and outgassing of potting material). mproper potting causes partial discharge activity in voids (at the interfaces or in the bulk). Literature usually reports on bulk properties of potting materials, and not on applications and manufacturing techniques, whereas these are essential for the quality. Potting may prevent proper outgassing of the potted component and thereby makes the inside pressure uncontrolled (see high-voltage cable). Potting techniques must follow a strictly controlled and qualified procedure. Tube assemblies 18. Because an effective (i.e. breakdown) conditioning procedure results in a dramatic improvement of the breakdown voltage, future components should be designed in such a way that breakdown conditioning is permissible. This requires control of the breakdown energy (or capacitance), and can be realised by using subdivided assemblies (if the energy is too large) or additional capacitance (if the energy is too small). Subassemblies should be decoupled only during the (high frequency) breakdown event, for example by means of inductances. An example of such a subdivided assembly is shown in Figure

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21 ALe '.T E: L' BELL TELEPHONE ADVANCED TECHNOLOGES N SATELLTE TELECOMMUNCATONS... TECHNOLOGCAL DRVERS N TERMS OF : - MSSON COMPLEXTY; -AFFORDABLE PRCE; - ON-BOARD NTELLGENCE; - HGHER FREQUENCES.... SPACE APPLCATON SPECFC TECHNOLOGES : - LAUNCHER; - PLATFORM; - PAYLOAD.