International Committee for Future Accelerators Sponsored by the Particles and Fields Commission of IUPAP Beam Dynamics Newsletter No. 39 Issue Editor: K. J. Kim Editor in Chief: W. Chou April 2006 3 Contents 1 FOREWORD..........................................................................................................9 1.1 FROM THE ICFA CHAIRMAN: GOING GLOBAL – A VIEW FROM ICFA ......................9 1.2 FROM THE BEAM DYNAMICS PANEL CHAIRMAN ....................................................11 1.3 FROM THE EDITOR ..................................................................................................12 2 INTERNATIONAL LINEAR COLLIDER (ILC)............................................12 2.1 INTERNATIONAL ACCELERATOR SCHOOL FOR LINEAR COLLIDERS.........................12 3 PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY..........................14 3.1 R&D PATHS TOWARDS ACHIEVING ULTIMATE CAPABILITIES ................................14 3.1.1 Introduction ..................................................................................................14 3.1.2 Fundamental Limits......................................................................................15 3.1.2.1 BCS Surface Resistance .................................................................15 3.1.2.2 Critical Magnetic Field...................................................................17 3.1.3 Issues…. .......................................................................................................17 3.1.3.1 Issue #1...........................................................................................17 3.1.3.2 Issue #2...........................................................................................18 3.1.3.3 Issue #3...........................................................................................18 3.1.3.4 Issue #4...........................................................................................18 3.1.4 Some Suggestions.........................................................................................18 3.1.5 Summary and Conclusions ...........................................................................22 3.1.6 Acknowledgement........................................................................................22 3.1.7 References ....................................................................................................22 3.2 OVERVIEW OF SURFACE MEASUREMENTS: WHAT DO SURFACE STUDIES TELL US ABOUT Q-SLOPE? .....................................................................................23 3.2.1 Abstract.........................................................................................................23 3.2.2 Introduction ..................................................................................................23 3.2.3 Surface Issues ...............................................................................................24 3.2.3.1 Experiments on Cavities.................................................................25 3.2.3.2 Diffusing Species...........................................................................25 3.2.3.3 What Surface Technique? ..............................................................26 3.2.3.3.1 Photoemission ................................................................26 3.2.3.3.2 X-ray Diffraction............................................................28 3.2.3.3.3 Chemical Polishing (BCP) versus Electropolishing (EP)….............................................................................29 3.2.3.3.4 The Interstitial Oxygen Scenario and Local Measurement of the Superconducting Gap Δ.................30 3.2.4 Surface Morphology.....................................................................................31 4 3.2.5 Conclusion ....................................................................................................32 3.2.6 References.....................................................................................................32 3.3 THERMAL RF BREAKDOWN OF SUPERCONDUCTING CAVITIES................................34 3.3.1 Abstract.........................................................................................................34 3.3.2 Introduction...................................................................................................34 3.3.3 Isothermal BCS Surface Resistance .............................................................35 3.3.3.1 Linear BCS Surface Resistance......................................................35 3.3.3.2 Nonlinear BCS Surface Resistance ................................................37 3.3.4 Thermal RF Breakdown ...............................................................................38 3.3.4.1 Linear Surface Resistance ..............................................................39 3.3.4.2 Nonlinear Surface Resistance.........................................................41 3.3.5 Discussion.....................................................................................................42 3.3.6 Acknowledgments ........................................................................................43 3.3.7 References.....................................................................................................43 3.4 A MAGNETO-OPTICAL INVESTIGATION OF NIOBIUM FOR SUPERCONDUCTING RF CAVITIES ...........................................................................44 3.4.1 Abstract.........................................................................................................44 3.4.2 Introduction...................................................................................................44 3.4.3 Description of the Samples...........................................................................45 3.4.4 Magnetization Measurements.......................................................................46 3.4.5 Magneto-Optical Measurements...................................................................47 3.4.6 Discussion.....................................................................................................52 3.4.7 References.....................................................................................................53 3.5 FIELD EMISSION OVERVIEW: CLEANLINESS AND PROCESSING................................53 3.5.1 Abstract.........................................................................................................53 3.5.2 Introduction...................................................................................................54 3.5.3 Present Picture of Field Emission.................................................................54 3.5.3.1 Instruments .....................................................................................55 3.5.3.2 Nature of Field Emitters .................................................................56 3.5.3.3 Processing of Field Emitters...........................................................57 3.5.4 Standard Preparation Procedures..................................................................58 3.5.4.1 Final Chemical/Electrochemical Treatment ...................................58 3.5.4.2 High Pressure Rinsing ....................................................................59 3.5.4.3 Assembly and Drying .....................................................................60 3.5.4.4 Pumping and Venting.....................................................................61 3.5.4.5 Risks of Contamination ..................................................................62 3.5.4.6 Practical Consequences and Open Questions (Personal View)......63 3.5.5 Alternative Cleaning Approaches.................................................................64 3.5.6 Processing in Accelerator Structures ............................................................65 3.5.6.1 RF Processing in LEP2...................................................................65 3.5.6.2 High Peak Power Processing on TESLA Cavities .........................65 3.5.6.3 High Peak Power Processing at Gradients above 30 MV/m on TESLA Cavities? ......................................................67 3.5.7 Summary.......................................................................................................67 3.5.8 Acknowledgments ........................................................................................67 3.5.9 References.....................................................................................................67 5 3.6 OVERVIEW ON HIGH FIELD Q-SLOPE ......................................................................69 3.6.1 Abstract.........................................................................................................69 3.6.2 Introduction ..................................................................................................69 3.6.3 The Description of the Q-slope ....................................................................70 3.6.3.1 RF Properties and Temperature Mappings.....................................70 3.6.3.2 Longtime Air Exposure ..................................................................72 3.6.3.3 Bake-out Temperature....................................................................73 3.6.3.4 Depth of the Bake-Affected Zone in the Niobium.........................74 3.6.3.5 Surface Studies ...............................................................................75 3.6.3.6 Susceptibility Measurements on Samples ......................................76 3.6.4 Summary.......................................................................................................78