Materials and surface aspects in the development of SRF Niobium cavities

Foreword from author; When I joined the CEA Saclay SRF group in 1989, my initial background was physical chemistry and surface science, which I completed later on with solid state physics and metallurgy. Most accelerator physicists at that time had training in RF, plasma physics, nuclear or particle physics. We were very few with a background in material science. Working with people with a different background than yours reveals to be both challenging and funny: you can impress them with things you consider basic while they simply do not believe you for other things you consider so well admitted that you do not even remember where it comes from. At the end it obliges you to reconsider your basics and re-question many results, which opens many new and sometimes unexpected paths. Like usual in science, answering one question rises many new ones, and trying to improve cavities performance led to fascinating physics problems. Exploring some of these problems often requires techniques and expertise that are far beyond the reach of one sole SRF lab, even well-funded. I have always worked in collaboration with laboratories with established expertise. It is a way of working I consider to be the only way to access new techniques. Indeed many today’s techniques, even complex ones, can deliver experimental results by merely “pushing a button”. Correct interpretation of the results requires advanced knowledge of the physics underneath as well as knowing the apparatus limitations or possible artifacts. Collaboration with experienced scientists is the only way to protect oneself against fallacious interpretation. The body of this monograph is based on my “habilitation” (HDR) thesis held in 2009 to join the University Paris-Sud faculty. The initial text was mostly based on my personal work, but since I had touched nearly all material and surface aspects in SRF over the past 22 years, I had also an introduction chapter on each topic with a lot of references to others’ work. In the present text I have tried to develop this introductory part and support my descriptions with more external references. A difficult task since lately the domain has been evolving very quickly and plenty of new interesting results are published every month. At some point I had to accept that whatever I could write, it will only be a partial picture of the SRF activity in the world at some date (let’s say 2011). This text is destined to accelerator physicists who are not familiar with material and surface science and students who enter the field. Its level is introductory, but I have also tried to develop prospective views, generally at the end of each chapter. For this I benefited a lot from continuous exchange with physicists all around the world. I want to thank here W. and X. Singer from DESY, G. Ciovati and J. Mammosser from JLab, A. Gurevich now at ODU, A. Romanenko, now at FNAL or T. Proslier at ANL and many others… It is a pleasure to see that a whole new generation of physicist is now trained in surface science, solid state physics or superconductivity theory, and that technical developments for SRF cavities led to new physics exploration that in turn allowed pushing the cavities limits. Some of the interpretations I put forward here are still debated. I deliberately decided to present things as I see them, through my own personal “material scientist” prism. In science, I do not think there is such thing as being wrong or right; we are simply describing the same thing with a different lighting. I hope that the lighting presented here will help the reader to reach a new understanding of SRF materials. I also wish to thank the reviewers to have helped me to improve this manuscript, which, I am sure, suffers a lot from the fact that English is not my native language. I apologize in advance if some of my sentences sound a little weird… Saclay 30.01.2012 Dr. Claire Antoine