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High Electric Field Deuterium Ion Sources for Neutron Generators Birk Reichenbach

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High Electric Field Deuterium Ion Sources for Neutron Generators Birk Reichenbach University of New Mexico UNM Digital Repository Physics & Astronomy ETDs Electronic Theses and Dissertations 2-9-2010 High electric field deuterium ion sources for neutron generators Birk Reichenbach Follow this and additional works at: https://digitalrepository.unm.edu/phyc_etds Recommended Citation Reichenbach, Birk. "High electric field deuterium ion sources for neutron generators." (2010). https://digitalrepository.unm.edu/ phyc_etds/56 This Dissertation is brought to you for free and open access by the Electronic Theses and Dissertations at UNM Digital Repository. It has been accepted for inclusion in Physics & Astronomy ETDs by an authorized administrator of UNM Digital Repository. For more information, please contact [email protected]. High electric field deuterium ion sources for neutron generators by Birk Reichenbach Vordiplom, Julius Maximilans Universit¨atW¨urzburg,2002 DISSERTATION Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Physics The University of New Mexico Albuquerque, New Mexico December, 2009 °c 2009, Birk Reichenbach iii Dedication To my newborn son Bastian and the best thing that ever happened to me, my wife Iris. iv Acknowledgments I had many excellent physics teachers since I first started out in physics. However, most of all I want to thank Paul Schwoebel for advising me over the years. He has been a great mentor and has always given selflessly of his time and his knowledge. People who know me, know I ask a lot of questions and Paul never seemed tired of answering these at any time, often involving lengthly discussions. I also want to thank Sid Solano with whom I performed many of the experiments presented here. While and maybe because we often disagreed, this work doubtlessly would have taken longer without him. In addition I thank Ben Johnson for continuing the research that we started. Furthermore I want to thank John Panitz and David Dunlap for the valuable discussions and helpful suggestions. Last but not least I want to thank my wife Iris for her constant support and the fun we have talking about physics. v High electric field deuterium ion sources for neutron generators by Birk Reichenbach ABSTRACT OF DISSERTATION Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Physics The University of New Mexico Albuquerque, New Mexico December, 2009 High electric field deuterium ion sources for neutron generators by Birk Reichenbach Vordiplom, Julius Maximilans Universit¨atW¨urzburg,2002 Ph.D., Physics, University of New Mexico, 2009 Abstract Active interrogation systems for highly enriched uranium require improved fieldable neutron sources. The target technology for deuterium-tritium neutron generators is well understood and the most significant improvement can be achieved by improving the deuterium ion source through increased output and, in some cases, lifetime of the ion source. We are developing a new approach to a deuterium ion sources based upon the field desorption/evaporation of deuterium from the surfaces of metal tips. Electrostatic field desorption (EFD) desorbs previously adsorbed deuterium as ions under the influence of high electric fields (several V/A˚ ), without removing tip material. Single etched wire tip experiments have been performed and have shown that this is difficult but can be achieved with molybdenum and tungsten tips. Electrostatic field evaporation (EFE) evaporates ultra thin deuterated titanium films as ions. It has been shown that several 10s of atomic layers can be removed vii within a few nanoseconds from etched tungsten tips. In the course of these stud- ies titanium deposition and deuteration methods were studied and new detection methods developed. Space charge effects resulting from the large ion currents were identified to be the most likely cause of some unusual ion emission characteristics. In addition, on W < 110 > oriented substrates a surprising body-centered cubic crystal structure of the titanium film was found and studied. The ion currents required for neutron generator applications can be achieved by microfabrication of metal tip arrays. Field desorption studies of microfabricated field emitter tip arrays have been conducted for the first time. Maximum fields of 3 V/A˚ have been applied to the array tip surfaces to date, although fields of ∼ 2 V/A˚ to ∼ 2.5 V/A˚ are more typical. Desorption of atomic deuterium ions has been observed at fields of roughly 2 V/A˚ at room temperature. The desorption of common surface adsorbates, such as hydrogen, carbon, water, and carbon monoxide is observed at fields exceeding ∼ 1 V/A˚. In vacuo heating of the arrays to temperatures of the order of 800 ◦C can be effective in removing many of the surface contaminants observed. For both the field desorption and the field evaporation approaches further im- provements to array design and fabrication are required if arrays are to provide sufficient deuterium ion currents to produce 109 to 1010 n/cm2 of tip array area for the detection systems. viii Contents List of Figures xv List of Tables xxiii 1 Introduction 1 2 Theoretical background 7 2.1 Field ionization . 7 2.1.1 Field ion microscopy . 8 2.2 Field desorption and field evaporation . 10 2.3 Time-of-flight experiments . 13 3 Experimental setup 15 3.1 Array experimental details . 19 3.2 Titanium evaporation: Experimental details . 20 3.2.1 Titanium deposition . 21 ix Contents 3.2.2 Ion imaging of the titanium film . 24 3.2.3 Annealing . 25 4 Field desorption studies of deuterium with single tips 26 4.1 Introduction to single tip field desorption . 26 4.2 Clean metal tips . 29 4.3 Tip Surface Cleanliness . 31 4.4 Hydrogen/deuterium desorption . 36 4.4.1 Field desorption vs. gas phase ionization . 37 4.4.2 Hydrogen promoted evaporation of the tips . 39 4.4.3 The desorption of hydrogen without metal . 42 4.4.4 Total quantity of desorbed deuterium ions . 46 4.5 Conclusion to the single tip desorption . 48 5 Microfabricated arrays 50 5.1 Introduction to microfabricated arrays . 50 5.2 Array operating field requirements . 51 5.3 Electric field calibration of array tips . 52 5.3.1 Electron emission onset fields . 53 5.3.2 Field ionization onset fields. 54 5.4 Array breakdown mechanisms . 54 x Contents 5.5 The design of ion desorption arrays . 56 5.6 Preliminary qualification of array performance . 58 5.7 Pulsing . 60 5.7.1 TOF and short tip-to-gate electrode distances . 60 5.7.2 TOF results . 61 5.8 Identification of Breakdown processes . 67 5.9 Conclusion . 72 5.9.1 Future work . 73 6 Titanium film evaporation 74 6.1 Introduction to titanium coated tips . 74 6.2 Results and discussion . 76 6.2.1 Titanium crystal structure . 76 6.2.2 Control and measurement of layer thickness . 80 6.2.3 Time of flight measurements with titanium films . 82 6.2.4 Slow evaporation of titanium films . 94 6.3 Space charge effects . 102 6.3.1 Introduction to space charge . 102 6.3.2 Space charge effects in the context of titanium desorption . 105 6.4 Titanium field evaporation from micro fabricated arrays . 115 6.4.1 Neutron production estimate with titanium coated tips . 119 xi Contents 6.5 Conclusion . 120 6.5.1 Future work . 121 7 Detector calibration 123 7.1 Detector gain calibration . 123 7.1.1 CEMA Calibration . 124 7.2 Absolute signal calibration . 126 7.2.1 Calibration using field evaporation of tungsten tips . 126 7.2.2 Absolute calibration with titanium . 127 8 Detector performance considerations 131 8.1 Detector gain settings . 131 8.1.1 Linear response range of the detector . 133 8.1.2 Relating detector studies with electrons to those with ions . 137 8.1.3 Noise considerations . 138 8.1.4 Optimum Detector settings . 141 8.2 CEMA efficiency . 141 8.3 The influence of the spatial size of the detector on TOF peak widths 143 8.3.1 Possible improvements to the detection system . 147 9 Conclusion 149 xii Contents Appendices 153 A Tip preparation and use 154 A.1 Tip polishing procedure . 155 A.2 Tip annealing . 158 B Titanium experimentation and additional explanation 160 B.1 Total amount of titanium deposited . 160 B.2 Evaporation of titanium films with a single pulse . 163 B.3 Experimentally eliminated mechanisms for low deuterium to Ti ratios 164 B.3.1 Dc holding voltage . 164 B.3.2 Tip heating during the deposition of titanium . 164 C Voltage pulse considerations 167 C.1 Pulse reflection . 167 C.2 Pulse-shape . 170 C.3 Electronics delay times . 173 C.4 Pulse Pickoff considerations . 173 D Temperature measurements 176 D.1 Temperature measurements with an optical pyrometer . 176 D.2 Temperature measurements with elemental resistance thermometry . 178 xiii Contents E Further figure explanation 181 References 186 xiv List of Figures 1.1 A schematic shows deuterium ion field desorption from metal tips. 3 1.2 A schematic showing field evaporation of deuterated titanium from tungsten tips. 4 1.3 Schematic diagram of the DT neutron generator using a field desorp- tion or field evaporation deuterium ion source. 5 2.1 Potential energy diagram for an electron in an H atom in the presence of an applied field near a metal surface. 8 2.2 Illustration of the image forming process of the FIM. 9 2.3 Potential energy curves illustrating field desorption. 12 3.1 Picture of the ultra high vacuum system used for the microfabricated arrays. 16 3.2 Schematic of the key components of the ultra high vacuum system. 17 3.3 Schematic of array mounted onto a TO-5 header. 19 3.4 Shows the assembly of arrays onto a coldfinger. 20 3.5 The titanium deposition coil and its surrounding is shown.
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