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Studies of Electron Cyclotron Resonance Ion Source Plasma Physics

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Studies of Electron Cyclotron Resonance Ion Source Plasma Physics DEPARTMENT OF PHYSICS UNIVERSITY OF JYVÄSKYLÄ RESEARCH REPORT No. 8/2005 STUDIES OF ELECTRON CYCLOTRON RESONANCE ION SOURCE PLASMA PHYSICS BY OLLI TARVAINEN Academic Disser tation for the Degree of Doctor of Philosophy To be presented, by permission of the Faculty of Mathematics and Science of the University of Jyväskylä, for public examination in Auditorium FYS-1 of the University of Jyväskylä on Decenber 16, 2005 at 12 o’clock Jyväskylä, Finland December, 2005 PREFACE PREFACE The work presented in this thesis has been carried out at the JYFL Accelerator Laboratory in the University of Jyväskylä and at Argonne National Laboratory during the years 2003-2005. First, I would like to thank my supervisor Dr. Hannu Koivisto for giving me an opportunity to work in the ion source group of the JYFL Accelerator Laboratory and for his invaluable guidance and support in the course of this work. Another person who deserves my sincere gratitude is Mr. Pekka Suominen, without whom I could not have finished this thesis. I wish to thank him not only for professional help and fruitful comments but also for all the cheerful moments that we have spent together in work, free time and international conferences and summer schools. I extend my thanks to all the people working at the JYFL Accelerator Laboratory, especially my closest colleagues in cyclotron- and RADEF-groups. I am also indebted to the technical personnel of our laboratory. I have been privileged to work and share thoughts with colleagues from accelerator laboratories all over the world. I would like to thank especially the ion source people from Argonne National Laboratory and Lawrence Berkeley National Laboratory. I would also like to express my gratitude to the referees of this work, Dr. Sandor Biri and Dr. Hans Beijers, for valuable comments. I wish to thank Mr. Marcus Rinkiö for genuine friendship and unforgettable time that we have spent together studying physics and playing beach volley. I like to thank my family: mother, father and brother for their encouragement. Finally, I direct my most heartfelt thanks to my loving wife Paula for constant support and for providing a “physics-free environment” at home, which has helped me to separate work from leisure. Jyväskylä, November 2005 Olli Tarvainen 1 ABSTRACT ABSTRACT This thesis consists of an introduction to the plasma physics of electron cyclotron resonance ion sources (ECRIS) and a review of the results obtained by the author and co-workers including discussion of related work by others. The thesis begins with a theoretical discussion dealing with plasma physics relevant for the production of highly charged ions in ECR ion source plasmas. This is followed by an overview of different techniques, such as gas mixing and double frequency heating, that can be used to improve the performance of this type of ion source. The experimental part of the work consists of studies related to ECRIS plasma physics. The effect of the gas mixing technique on the production efficiency of different ion beams was studied with both gaseous and solid materials. It was observed that gas mixing improves the confinement of the heavier element while the confinement of the lighter element is reduced. When the effect of gas mixing on MIVOC-plasmas was studied with several mixing gases it was observed that applying this technique can reduce the inevitable carbon contamination by a significant factor. In order to understand the different plasma processes taking place in ECRIS plasmas, a series of plasma potential and emittance measurements was carried out. An instrument, which can be used to measure the plasma potential in a single measurement without disturbing the plasma, was developed for this work. Studying the plasma potential of ECR ion sources is important not only because it helps to understand different plasma processes, but also because the information can be used as an input parameter for beam transport simulations and ion source extraction design. The experiments performed have revealed clear dependencies of the plasma potential on certain source parameters such as the amount of carbon contamination accumulated on the walls of the plasma chamber during a MIVOC-run. It was also observed that gas mixing affects not only the production efficiency of the ion beams but also their energy distribution. This finding strongly supports the conclusion that ion cooling explains the beneficial effect of this technique. It was demonstrated with simulations that the momentum spread of the ion beam, which is partly due to the the plasma potential, affects the emittance of the ion beams through dispersive ion optical components. An important observation was that double frequency heating does not affect the emittance of the ion beams. The result confirms that this method can be used to improve the extracted beam currents of highly charged ions without adversely affecting the beam quality. 2 Table of contents TABLE OF CONTENTS PREFACE...........................................................................................................................................1 ABSTRACT........................................................................................................................................2 TABLE OF CONTENTS ..................................................................................................................3 1. INTRODUCTION..........................................................................................................................5 2. PLASMA PHYSICS OF ELECTRON CYCLOTRON RESONANCE ION SOURCES.......7 2.1. Definition of plasma and plasma conditions............................................................................................................ 7 2.2. The operation principle of electron cyclotron resonance ion sources...................................................................... 9 2.3. Electron cyclotron resonance heating .................................................................................................................... 10 2.3.1. The propagation of electromagnetic waves in magnetoplasmas .................................................................... 11 2.3.2. Electron energy in ECRIS plasmas................................................................................................................. 15 2.4. Collisions in ECRIS plasmas................................................................................................................................. 18 2.4.1. Ionizing collisions and charge exchange........................................................................................................ 18 2.4.2. Electron-electron and ion-ion collisions......................................................................................................... 21 2.5. Confinement of charged particles in ECRIS plasmas ............................................................................................ 24 2.5.1. Trapping of charged particles in the magnetic bottle..................................................................................... 25 2.5.2. Plasma potential ............................................................................................................................................. 27 2.5.3. Measurement of the plasma potential............................................................................................................. 31 2.5.4. Confinement of ions in ECRIS plasmas .......................................................................................................... 32 3. PRODUCTION OF HIGHLY CHARGED ION BEAMS WITH ECR ION SOURCES.....38 3.1. Methods to improve the performance of ECR ion sources .................................................................................... 38 3.1.1. Wall coating and plasma chamber material................................................................................................... 39 3.1.2. Biased electrode technique ............................................................................................................................. 40 3.1.3. Gas mixing...................................................................................................................................................... 41 3.1.4. Multiple frequency heating ............................................................................................................................. 42 3.1.5. Afterglow and long pulse operation modes..................................................................................................... 44 3.2. Methods for introduction of material into the ECRIS plasma ............................................................................... 45 3.2.1. Gaseous elements and compounds.................................................................................................................. 45 3.2.2. Evaporation oven............................................................................................................................................ 45 3.2.3. The MIVOC method........................................................................................................................................ 46 3.2.4. Sputtering........................................................................................................................................................ 47 3.3. Ion beam extraction and the quality of ion beams produced with ECR ion sources.............................................. 48 3.3.1. The plasma
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