The physics of photoconductive spark gap switching : pushing the frontiers Citation for published version (APA): Hendriks, J. (2006). The physics of photoconductive spark gap switching : pushing the frontiers. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR608818 DOI: 10.6100/IR608818 Document status and date: Published: 01/01/2006 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 02. Oct. 2021 The physics of photoconductive spark gap switching: Pushing the frontiers PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magni¯cus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op maandag 3 juli 2006 om 16.00 uur door Jimi Hendriks geboren te Hoensbroek Dit proefschrift is goedgekeurd door de promotoren: prof.dr. M.J. van der Wiel en prof.dr.ir. J.H. Blom Copromotor: dr.ir. G.J.H. Brussaard This research was ¯nancially supported by the Dutch Technology Foundation STW (ETF.6485) CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Hendriks, Jimi The physics of photoconductive spark gap switching : Pushing the frontiers / by Jimi Hendriks. - Eindhoven : Technische Universiteit Eindhoven, 2006. - Proefschrift. ISBN-10: 90-386-2471-9 ISBN-13: 978-90-386-2471-6 NUR 926 Trefwoorden: hoogspanningsschakelaars / lasers / hoogspanningspulsen / plasmafysica / elektrodynamica / fotogeleiding Subject headings: spark gaps / high voltage switches / lasers / high voltage pulses / plasma physics / plasma switches / electrodynamics / photoconduction Copyright °c 2006 J. Hendriks All rights reserved. No part of this book may be reproduced, stored in a database or retrieval system, or published, in any form or in any way, electronically, mechanically, by print, photoprint, micro¯lm or any other means without prior written permission of the author. Printed by Printservice Technische Universiteit Eindhoven, Eindhoven, The Netherlands Cover design by Jimi Hendriks and Paul Verspaget Contents 1 Introduction 1 1.1 History . 2 1.2 Switching . 3 1.2.1 Semiconductor photoconductive switch . 3 1.2.2 Gas-¯lled laser-triggered spark gap switch . 5 1.2.3 Photoconductive switching of an atmospheric gas-¯lled spark gap . 6 1.3 Applications . 6 1.4 Scope of this thesis . 8 2 Experimental setup 11 2.1 Laser setup . 11 2.1.1 Oscillator . 12 2.1.2 Ampli¯ers . 13 2.1.3 Switching optics . 16 2.2 High-voltage spark gap setup . 16 3 First demonstration of photoconductive switching 21 4 Parameter study of photoconductive switching 27 4.1 Introduction . 28 4.2 Experimental setup . 29 4.2.1 High-voltage spark gap setup . 29 4.2.2 Femtosecond Ti:Sapphire laser system . 31 4.3 Results and discussion . 32 4.4 Conclusions . 38 4.5 Acknowledgements . 39 5 Feasibility study of high-voltage and plasma diagnostics 41 5.1 Interferometer . 42 5.2 Electro-optic high-voltage pulse detection . 46 iii iv CONTENTS 5.2.1 Setup . 48 5.2.2 Pulse transmission simulations . 50 5.2.3 Measurements . 51 5.3 Conclusions . 51 6 Plasma simulations 53 6.1 Introduction . 54 6.2 Setup . 55 6.3 Cathode fall . 56 6.3.1 Cathode fall voltage . 56 6.3.2 Cathode fall formation time . 59 6.4 Arc plasma . 59 6.4.1 Analytical description of the switching plasma . 60 6.4.2 Simulated voltage drop for various switching currents . 62 6.5 Conclusions . 63 7 Electrodynamic simulations 67 7.1 Introduction . 68 7.2 Di®erent models . 69 7.2.1 Inductive lumped element model . 70 7.2.2 Transmission line model . 71 7.2.3 Electrodynamic model . 72 7.2.4 Comparison of the di®erent models . 76 7.3 Three-dimensional electrodynamic simulation of a spark gap setup with dis- continuities in the outer conductor . 77 7.4 Conclusions . 80 7.5 Acknowledgements . 81 8 Spark gap optimization by electrodynamic simulations 83 8.1 Introduction . 84 8.2 Three-dimensional electrodynamic spark gap model . 85 8.3 Spark gap optimization according to the literature . 86 8.4 Electrodynamic optimization of the spark gap con¯guration . 88 8.4.1 Optimization procedure . 90 8.4.2 The ideal spark gap con¯guration . 94 8.5 Conclusions . 94 8.6 Acknowledgments . 96 CONTENTS v 9 General discussion 99 9.1 Introduction . 99 9.2 Conclusions and future research . 100 9.3 Applicability for electron acceleration . 102 Summary 105 Samenvatting 107 Publications 111 Dankwoord 113 Curriculum Vitae 115 vi CONTENTS Chapter 1 Introduction This thesis describes the development of a photoconductively switched atmospheric gas- ¯lled high-voltage spark gap in the framework of a Technology Foundation program to develop a compact, MV short-pulse transformer and use it to develop a novel compact electron accelerator and its diagnostics. 1 2 Introduction 1.1 History Breakdown in the form of lightning has fascinated mankind all through history. Actual research on breakdown has been done since electricity was discovered. With the work of Paschen and Townsend at the end of the 19th century, beginning 20th century (spon- teneaous) breakdown became a fairly well-understood phenomenon. Paschen described the relationship between breakdown voltage, pressure and dimensions of the breakdown medium and Townsend described breakdown in a low-pressure environment as an ionization process caused by avalanches of electrons. From about 1940 deviations from Townsend's theory were seen in high (atmospheric) pressure experiments. Breakdown occurred much faster than could be deduced from Townsend's avalanche theory. Streamers were discov- ered and the fundamentals for the theory of breakdown by streamers were developed by Loeb, Meek and Raether [1{3]. The introduction of the laser, especially the high-power Q-switched laser, has rapidly in- creased the interest in breakdown research. By simply focusing the laser beam a spark in air could be made. It did not take long before the influence of the laser on a high-voltage environment was tested. This research ¯eld expanded by the military demand for fast switching of high voltages. These fast switched high-voltage pulses were used to simulate the EMC-e®ects of nuclear explosions and for (broadband) radar purposes. Pendleton and Guenther [4] were among the ¯rst to implement a laser in a spark-gap setup. The laser- triggered spark gap was able to switch high currents and high voltages with nanosecond precision. The laser did not only cause the breakdown to occur faster than spontaneous breakdown, it seemed also possible to use the laser to trigger an under-volted spark gap to switch voltages below the self-breakdown voltage of the gap. In many di®erent laboratories di®erent spark gap geometries and positions and energies of the laser focus were investi- gated and all kinds of gases and liquids were inserted in the spark gaps. A good review of these research activities up to 1978 is given by Guenther and Bettis [5]. First measure- ments were done in a simple (two sphere) spark gap that was triggered by a perpendicular incident laser. Voltages up to 100 kV were switched and the e®ects of di®erent gases as switching medium were investigated. Later the spark gaps became more complicated in structure and spark gaps were developed that were able to switch Megavolts. When new lasers were developed (CO2, UV, Nd:YAG, Ti:Sapphire etc.), they were also tried in spark gaps [6{9]. Detailed studies of the switching plasma were made [10{12] in order to get a better understanding of the switching process. With the availability of short-pulse lasers (ps-range) a di®erent ¯eld, that of optoelectronic switching, took a flight. By using a short pulse laser, it was now possible to create enough charge carriers in a high resistance semiconductor to produce a conductivity of quasimetal- 1.2. Switching 3 lic properties. In 1975, Auston [13] made the ¯rst photoconductive switch by inserting a silicon substrate in a transmission line structure. By the absorption of a picosecond optical pulse with an energy of a few microjoules, he was able to open and close the switch in a few picoseconds and switch voltages up to 100 V.