UNIVERSITATEA „POLITEHNICA” din BUCUREŞTI Ştiinţe Aplicate FACULTATEA ______________________ Fizică I CATEDRA _________________________ Nr. Decizie Senat …….. din ……… TEZĂ DE DOCTORAT TEHNICI DE PRODUCERE ŞI ACCELERARE A FASCICULELOR RADIOACTIVE ________________________________________________________________ TECHNIQUES TO PRODUCE AND ACCELERATE RADIOACTIVE ION BEAMS ________________________________________________________________ Liviu Constantin PENESCU Autor: Ing. ______________________________________________ 4 8 2 Gheorghe CĂTA-DANIL 9- Conducător de doctorat: Prof. dr. ing. ________________________ 0 0 2 - S I S E H T COMISIA DE DOCTORAT N-9 0 R0 E/2 Preşedinte Constantin UDRIŞTE de la Fac. Ştiinţe Aplicate C/ Conducător de doctorat Gheorghe CĂTA-DANIL de la Fac. Ştiinţe Aplicate Referent Alexandru JIPA de la Fac. de Fizică Referent Ion M. POPESCU de la Fac. Ştiinţe Aplicate Referent Şerban DOBRESCU de la IFIN-HH Referent Thierry STORA de la CERN BUCUREŞTI 2009 CONTENTS Page Contents − iii List of acronyms and abbreviations − vii Acknowledgements − x INTRODUCTION − 1 CHAPTER 1 Producing and accelerating Radioactive Ion Beams (RIB). The Isotope − 2 Separation On-Line (ISOL) method. 1.1. Motivation for RIBs. − 2 1.2. RIB production: ISOL versus “in-flight” methods. − 3 1.3. Present of ISOL. − 7 1.3.1. ISOLDE (CERN). − 7 1.3.2. SPIRAL (GANIL). − 9 1.3.3. ISAC (TRIUMF). − 11 1.3.4. HRIBF (Oak Ridge). − 12 1.3.5. CRC (Louvain-la-Neuve). − 13 1.3.6. IGISOL (Jyväskylä). − 13 1.4. Future of ISOL. Motivation for ion source development. − 15 CHAPTER 2 ISOL ion sources. − 16 2.1. ISOL requirements for the ion sources. − 16 2.2. Available ion sources. − 18 2.2.1. Surface ion sources. − 18 2.2.2. Arc discharge ion sources. − 18 2.2.3. ECR ion sources. − 19 2.2.4. EBIS ion sources. − 20 2.2.5. RILIS. − 20 2.3. Physics of arc discharge ion sources. − 21 2.3.1. Neutral feed of the ion source. − 21 2.3.2. Electron emission. − 22 2.3.3. Electron impact ionization. − 22 2.3.4. Charge exchange. − 22 2.3.5. Ion recombination. − 23 iii 2.3.6. Surface ionization. − 23 2.4. Overview. Motivation of the choices followed during this study. − 24   CHAPTER 3 Investigation tools employed in the present study. − 25 3.1. Experimental tools. − 25 3.1.1. The target-ion source unit. − 25 3.1.2. ISOLDE offline mass separator. − 26 3.1.3. ISOLDE online mass separator. − 28 3.1.4. Beam energy meter. − 28 3.2. Analytical calculations. − 30 3.3. Simulation tools. − 30 3.3.1. VORPAL (“Versatile Object-oRiented Plasma Analysis code with − 30 Lasers”). 3.3.2. CPO (“Charged Particle Optics”). − 31 CHAPTER 4 Modeling of the ionization efficiency. − 33 4.1. Existing models for electron impact ion sources and their − 33 limitations. 4.1.1. The Nielsen model. − 35 4.1.2. The Kirchner model. − 35 4.1.3. The Alton model. − 36 4.2. An operation oriented model for FEBIAD sources. − 38 4.2.1. Typical parameters of FEBIAD plasma and model assumptions. − 38 4.2.2. Influence of the operation parameters on the ionization efficiency. − 45 4.2.3. The inference of an ionization model based on the operation − 47 parameters.   CHAPTER 5 Experimental investigations of the ionization efficiency of ISOLDE − 50 FEBIAD ion sources 5.1. FEBIAD ion sources in use at CERN-ISOLDE. − 51 5.2. Dependence of the ionization efficiency on the internal magnetic − 52 field. 5.3. Dependence of the ionization efficiency on the ion source − 56 potentials. 5.4. Dependence of the ionization efficiency on temperature. − 61 5.5. Dependence of the ionization efficiency on the internal pressure − 65 5.6. Dependence of the ionization efficiency on gas composition and − 70 on impurities. iv 5.7. Overview. − 72 CHAPTER 6 Beam energy measurements. − 73 6.1. Dependence of the beam energy on the separator parameters. − 74 6.2. Dependence of the beam energy on the ionized element. − 75 6.3. Dependence of the beam energy on the operation pressure. − 76 6.4. Dependence of the beam energy on the operation temperature. − 77 6.5. Dependence of the beam energy on the anode potential. − 78 6.6. Dependence of the beam energy on the source magnet. − 79 6.7. Dependence of the beam energy on the ionization mechanism. − 79 6.8. Overview and conclusions. − 81   CHAPTER 7 Limitations acting on the ionization efficiency. − 82 7.1. Model limitations. Correction for the gas pumping. − 83 7.1.1. Experimentally observed limitations of the ionization efficiency − 83 7.1.2. Phenomena neglected in the employed model. − 85 7.1.3. Sources of errors (affecting the calculation of the f factor). − 87 7.1.4. Correction of the model for the gas pumping. − 88 7.2. Analysis of the limiting phenomena. − 93 7.2.1. The charge exchange from the 1+ ions to the neutral atoms. − 93 7.2.2. The minimum extraction time. − 95 7.2.3. The maximum ion and electron densities into the ion source − 97 volume. 7.2.4. The ion space charge limitation at the extraction puller. − 101 7.2.5. The ion space charge limitation at the outlet plate. − 102 7.2.6. The electron space charge limitation at the accelerating grid. − 103   CHAPTER 8 FEBIAD prototypes based on the developed ion source model. − 105 8.1. Optimization of the ion source extraction. Improvement of the − 106 FEBIAD ionization efficiency for the noble gases. 8.1.1. Motivation. − 106 8.1.2. Original approach. − 107 8.1.3. Technical implementation. − 112 8.1.4. Experimental results. − 113 8.1.5. Overview. − 116 8.2. Optimization of the impurity level inside the source. − 116 Improvement of the FEBIAD ionization efficiency for all the elements. v 8.2.1. Motivation. − 116 8.2.2. Original approach. − 117 8.2.3. Technical implementation. − 118 8.2.4. Experimental results. − 118 8.2.5. Overview. − 122 8.3. Improvement of the RIB yields. Conclusions. − 123   CHAPTER 9 The VADIS concept. Applicability − 124 9.1. Extension of the proposed model. − 125 9.1.1. Element dependence of the FEBIAD ionization efficiency − 125 9.1.2. Influence of the element volatility. − 125 9.1.3. Influence of the isotope lifetime. − 127 9.2. The VADIS concept. − 128 9.2.1. Customization of the source design for specific ISOL − 128 requirements. 9.2.2. Diagnose of the source performance. − 128 9.3. Examples of VADIS applicability. − 130 9.3.1. Noble gases. − 130 9.3.2. Refractory elements. − 130 9.3.3. Enabling different chemistry approaches. Reduction of impurity − 130 level. 9.3.4. Short isotope lifetimes. − 132 9.3.5. Higher gas loads. − 132 9.3.6. Light elements. − 133 9.3.7. Laser ionization. − 133   CONCLUSIONS C.1. General conclusions. − 134 C.2. Original contributions. − 134 C.3. Future perspectives. − 135   ANEXES A.1. List of publications. − 136 A.1. Results dissemination at Conferences and Workshops. − 137   REFERENCES  − 138 vi List of acronyms and abbreviations CERN – European Organization for Nuclear and Particle Physics Research CPO – Charged Particle Optics (commercial simulation code) CRC-UCL – Cyclotron Research Center at UCL (Université Catholique de Louvain), Louvain la Neuve, Belgium EBIS – Electron Beam Ion Source (ion source type) EBGP – Electron Beam Generated Plasma (ion source type) ECR – Electron Cyclotron Resonance (ion source type) ECRIS – Electron Cyclotron Resonance Ion Source EURISOL – EURopean ISOL radioactive beam facility FC – Faraday Cup FEBIAD – Forced Electron Beam Induced Arc Discharge (ion source type) GANIL – Grand Accélérateur National d’Ions Lourds (Caen, France) GPS – General Purpose Separator (located at ISOLDE) HIE ISOLDE – High Intensity and Energy ISOLDE (facility upgrade) HIGHINT – Marie Curie Project at CERN, aimed to find solutions (targets, ion sources and beam transport) for the future ISOL facilities dealing with HIGH INTensity primary beams HRIBF – Holifield Radioactive Ion Beam Facility (located at ORNL) HRS – High Resolution Separator (located at ISOLDE) IGISOL – Ion Guide ISOL IGUN – Commercially available code for the simulation of ion beam extraction from a plasma ion source ISAC – Isotope Separator and ACcelerator ISOL – Isotope Separation On Line (method) ISOLDE – Experimental facility at CERN; acronym for Isotope Separator On Line KENIS – Kinetic Ejection Negative Ion Source KOBRA – Commercially available code for the simulation of ion beam extraction from a plasma ion source LINAC – LINear ACcelerator MK3,MK5,MK7 – Acronyms used for the FEBIAD subtypes used at ISOLDE before the present work ORNL – Oak Ridge National Laboratory PSB – Proton-Synchrotron Booster (at CERN) REX ISOLDE – Radioactive beam EXperiment at ISOLDE REXTRAP – Radioactive beam EXperiment cooling TRAP RF – Radio Frequency RFQ – Radio Frequency Quadrupole RIA – Rare Isotope Accelerator RIB – Radioactive Ion Beam RILIS – Resonant Ionization Laser Ion Source vii SCALA – Commercially available code for the simulation of ion beam extraction and transport from a plasma ion source SPIRAL – Système de Production d’Ions Radioactifs Accélérées en Ligne (facility at GANIL) TIS – Target-Ion Source (compact unit employed in the ISOL method) TrapCAD – Code for the simulation of the electron dynamics in an ECRIS TRIUMF – TRI-University Meson Facility (Vancouver, Canada) VADIS – Versatile Arc Discharge Ion Source (series of FEBIAD-type ion sources, implemented in 2009 at ISOLDE) VD3, VD5, VD7 – New acronyms used for the ISOLDE FEBIAD subtypes, employing the developments introduced by the present work (VADIS series) VORPAL – Versatile Object oRiented Plasma Analysis code with Lasers (commercial simulation code) viii