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Superconducting Radiofrequency Accelerating Structures

Superconducting Radiofrequency Accelerating Structures. Lutz Lilje DESY -MPY- ‘Physics at the Terascale’ 406. WE-Heraeus-Seminar Bad Honnef 30.4.2008. Acknowledgement. Several people were supporting with viewgraphs etc. H. Hayano KEK H. Padamsee Cornell

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Superconducting Radiofrequency Accelerating Structures

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  1. Superconducting Radiofrequency Accelerating Structures Lutz Lilje DESY -MPY- ‘Physics at the Terascale’ 406. WE-Heraeus-Seminar Bad Honnef 30.4.2008

  2. Acknowledgement • Several people were supporting with viewgraphs etc. • H. Hayano KEK • H. Padamsee Cornell • J. Sekutowicz, D. Reschke, W. Singer DESY Lutz Lilje DESY -MPY-

  3. References • Superconducting RF • RF Superconductivity for Accelerators, H. Padamsee, J. Knobloch, and T. Hays, John Wiley & Sons, 1998. • The Superconducting TESLA Cavities, B. Aune et al., PRST-AB, 3, September 2000, 092001. • SRF Workshop Tutorials • http://www.lns.cornell.edu/public/SRF2005/program.html • http://www.pku.edu.cn/academic/srf2007/program.html#tutorial • Accelerators • XFEL: http://xfel.desy.de/tdr/tdr/index_eng.html • ILC: http://www.linearcollider.org/cms/ Lutz Lilje DESY -MPY-

  4. Outline • Superconducting Radiofrequency Accelerating Sturctures (‘SRF Cavities‘) • History • Properties • Technology • Example for Accelerator Systems • XFEL and ILC Lutz Lilje DESY -MPY-

  5. 1. Introduction and History Milestones that led to accelerators based on SRF Superconductivity RF Acceleration 1924: Gustaf Ising (Sweden) The First Publication on the RF Acceleration, Arkiv för Matematik, Astronomi och Fysik,18 (1924) 1-4. 1908: Heike Kamerlingh Onnes (Holland) Liquefied Helium. 1911: Heike Kamerlingh Onnes Discovered Superconductivity. 1928: Rolf Wideröe (Norway, Germany) Built the first RF Accelerator. Arch. für Elektrotechnik 21 (1928),387-406. 1928-34: Walther Meissner (Germany) Discovered Superconductivity of Ta, V, Ti and Nb. 1930: Russell and Sigurd Varian (USA) Invented Klystron. 1947: Luis Alvarez (USA) Built first DTL (32 MeV protons). 1947: W. Hansen (USA) Built first 6 MeV e-accelerator, Mark I (TW-structure). J. Sekutowicz, SRF2007 Beijing Lutz Lilje DESY -MPY-

  6. 1. Introduction and History Superconductivity RF Acceleration 1961: Bill Fairbank (Stanford Univ.) presented the first proposal for a superconducting accelerator. 1964: Bill Fairbank, Alan Schwettman and Perry Wilson (Stanford University) First acceleration of electrons with sc lead cavity. 1967: John Turneaure (Stanford University) Epeak =70 MV/m and Q~1010 in 8.5 GHz cavity !! 1968-1981: Mike McAshan, Alan Schwettman, Todd Smith, John Turneaure and Perry Wilson (Stanford University) Development and Construction of the Superconducting Accelerator SCA. J. Sekutowicz, SRF2007 Beijing Lutz Lilje DESY -MPY-

  7. Why Superconducting Cavities? • Superconducting cavities offer • a surface resistance which is six orders of magnitude lower than normal conductors (NC) • high efficiency • even when cooling is included • low frequency, large aperture • smaller wake-field effects • high accelerating gradients • Theoretical limit for the between 45-55 MV/m depending on the cavity geometry Lutz Lilje DESY -MPY-

  8. SNS (Spallation Neutron Source) Beta = 0.8 800 MHz Beta = 0.6 Lutz Lilje DESY -MPY-

  9. 58 MHz 117 MHz Rare Isotope Seperator 350 MHz 175 MHz 700 MHz 700 MHz Lutz Lilje DESY -MPY-

  10. Accelerator Facilities with SCRF CavitiesNumber of cavities/total active length 1. Introduction and History Dismantled Facilities Operating Facilities • SCA 4 28m • S-DALINAC 10 10m • CESR 4 1.2 m • CEBAF 320 160m • KEK B-Factory 8 2.4m • Taiwan LS 1+1 0.3m • Canadian LS 1+1 0.3m • DIAMOND 3 0.9m • SOLEIL 4 1.7m • FLASH / TTF 48 50m • SNS 81 65m • JLab-FEL 24 14m • LHC 16 6m • ELBE 6 6m • TRISTAN 32 49m • LEP 288 490m • HERA 16 19m J. Sekutowicz, SRF2007 Beijing Lutz Lilje DESY -MPY-

  11. Livingston plot for SRF cavitiesCourtesy H. Padamsee Total >1000 meters > 5 GV Nb/Cu Technology Lutz Lilje DESY -MPY-

  12. High luminosity rings (CESR) • Low Impedance Shape • Beam Power > 270kW Lutz Lilje DESY -MPY-

  13. LHC 400 MHz 16 Nb/Cu Cavities Lutz Lilje DESY -MPY-

  14. Niobium bulk cavities CERN 350 MHz TESLA 1,3 GHz Darmstadt 3GHz Lutz Lilje DESY -MPY-

  15. Accelerator Facilities with SCRF Cavities IINumber of cavities/total active length 1. Introduction and History Tomorrow Facilities Day after Tomorrow Facilities • CEBAF-12GeV 400 216m • SNS-upgrade 117 98m • XFEL 800 832m • ERL-Cornell 310 250m • BESSY 144152m • 4GLS ~40 ~42m • RHIC-cooling 4 4 m • Shanghai LS……. • BEPC II 2 0.6m • RIA option 180 122m • X-Ray MIT option 176 184m • LUX ~40 ~50m) • FERMI Proton Linac 384 370m • ERHIC……… • ELIC ….. • ARC-EN-CIEL 48 50m • ILC ~15764 ~16395m J. Sekutowicz, SRF2007 Beijing Lutz Lilje DESY -MPY-

  16. Overall layout of the European XFEL 3.4km

  17. ILC Layout e+ production e- e+ damping rings e- source+ pre-acceleration e+ pre-acceleration target e- transport line e+ transport line undulator 2-stage bunch compression e+ main linac e- main linac 2-stage bunch compression e- beam dump e+ beam dump IP and 2 moveable detectors Total length: ~31 km Global Design Effort

  18. Outline • Superconducting Radiofrequency Accelerating Sturctures (‘SCRF Cavities‘) • History • Properties • Technology • Example for Accelerator Systems • XFEL and ILC Lutz Lilje DESY -MPY-

  19. Properties of Cavities Example: cylindrically symetric cavity - Pillbox with J0, J1 Besselfunctions Frequency: Stored Energy: Dissipated power: Quality factor: Geometry factor: Lutz Lilje DESY -MPY-

  20. Electric Field (Pillbox): Field Distributions in Cavities TM010 : accelerating mode  L (side view) Magnetic Field : Other modes : e.g. deflecting modes  D (front view) Lutz Lilje DESY -MPY-

  21. Field Distributions in Cavities Elliptical cavity: Numerical solution for surface fields:  Relations for the surface fields to acclerating gradient: Epeak/Eacc = 1,98 minimize this to reduce field emission Bpeak/Eacc = 4,17 [mT]/[MV/m] minimize because of maximum critical field of the superconductor  Lutz Lilje DESY -MPY-

  22. Why Superconducting Cavities? • SC cavities offer • a surface resistance which is six orders of magnitude lower than normal conductors (NC) • high efficiency • even when cooling is included • low frequency, large aperture • smaller wake-field effects • high accelerating gradients • Theoretical limit for the between 45-55 MV/m depending on the cavity geometry Lutz Lilje DESY -MPY-

  23. Superconductor (Blue) / Superconductor under pressure (Light Blue) From: SUPERCONDUCTING MATERIALS – A TOPICAL OVERVIEW, R. Hott et al. FZK Enter Superconductivity: Pure Elements Lutz Lilje DESY -MPY-

  24. Superconductor (Blue) / Superconductor under pressure (Light Blue) From: SUPERCONDUCTING MATERIALS – A TOPICAL OVERVIEW, R. Hott et al. FZK Enter Superconductivity: Pure Elements Lutz Lilje DESY -MPY-

  25. Critical magnetic field for the RF case • RF field at 1,3 GHz is ‘ON’ for less than 10-9 s • If there are no nucleation centers (surface defects...) the penetration of the magnetic field can be delayed. Superheating! Superheating fields: Niobium properties:  Theoretical accelerating field limits What is really the fundamental limit for RF cavities? } Lutz Lilje DESY -MPY-

  26. RF superconductivity • The superconducting Cooper pairs have inertia. • Therefore the unpaired normalconducting ‘feel’ also a part of the electromagnetic RF fields.  Superconductors have for temperatures T>0 K a surface resistance! Lutz Lilje DESY -MPY-

  27. Electric conductivity and Surface resistance Surface resistance in two-fluid model (analogous to skin depth): • Resistance depends • strongly on the temperature, we need 2 K • quadratically on frequency: Limit for 3 GHz would be 30 MV/m. • on the mean free path, what purity do we need? Surface resistance for superconductors in BCS theory: Effective penetration depth: Lutz Lilje DESY -MPY-

  28. Geometry factor: G = 270 Ohm Surface resistance: Surface resistance Rs RBCS(T) 700 nOhm @ 4,2K RRES ~10 nOhm @ 2K Lutz Lilje DESY -MPY-

  29. Flux penetration into a superconductor Electron holography is used to make magnetic fluxons visible (Tonomura et al.) Fluxons stick to defects ! This is good for magnets, but bad for cavities. Lutz Lilje DESY -MPY-

  30. Surface resistance and accelerating gradient • One usually measures the Q(Eacc) curve: • Q0~(1/ Rsurf) • Quality factor will tell you how much you have to pay for the cooling power • Depends on the accelerating gradient e.g. field emission • Helps to understand the loss mechanisms especially is supported by temperature mapping Lutz Lilje DESY -MPY-

  31. Surface resistance and accelerating gradient Test temperature: 1,6 K Test temperature: 2K XFEL ILC One-cell cavities Lutz Lilje DESY -MPY-

  32. Outline • Superconducting Radiofrequency Accelerating Sturctures (‘SCRF Cavities‘) • History • Properties • Technology • Limitations and Remedies • Example for Accelerator Systems • XFEL and ILC Lutz Lilje DESY -MPY-

  33. Temperature Mapping System • Example of a Temperature mapping: • the picture shows a Mercator projection of a single-cell cavity • strong localised heating spot on the equator • another band of heating around the equator in the high magnetic field (high current) region Lutz Lilje DESY -MPY-

  34. Example Of A Material Defect Defect found with X-ray technique: Tantalum Heating on the outside surface measured with carbon resistors Lutz Lilje DESY -MPY-

  35. Quality Control Of Nb For Cavities • Eddy current scanning of all sheets • measures change of electric resistance • 0.5mm depth, 40 μm defect dia. sensitivity • rejection rate of sheets about 5 % • SQUID scanning under development • Some special investigations are possible on demand • x-ray radiography (defect visualization) • x-ray fluorescence (defect element determination) • neutron activation (Ta distribution) Lutz Lilje DESY -MPY-

  36. Eddy Current Scanning • Large tantalum inclusions (~200 mm) and places with irregular patterns from surface preparation (grinding) Grinding mark Lutz Lilje DESY -MPY-

  37. Result of eddy current scanning a Nb disc, dia. 265 mm Real and imaginary part of conductivity at defect, typical Fe signal Global view, rolling marks and defect areas can be seen Lutz Lilje DESY -MPY-

  38. Principal arrangement of SQUID scanning Measured response from the back side of the sheet Nb test sheet with .1mm Ta inclusions Lutz Lilje DESY -MPY-

  39. High-Resolution Optical Inspection • New development by japanese colleagues to reduce reflections • Improved lighting technique using flat electro-luminescense strips • Damping of vibrations • Resolution down to a few micrometer! Lutz Lilje DESY -MPY-

  40. Courtesy Kyoto University / KEK Lutz Lilje DESY -MPY-

  41. AC80: cat’s eye @cell#5_equator 100μm Courtesy Kyoto University / KEK 1mm Lutz Lilje DESY -MPY-

  42. Electron Effects:Field Emission • Emission of e- from cavity surface in presence of high surface E-fields • Emitted e- impacts elsewhere on cavity surface, heating the surface and increase Rs • Limits the achievable Eacc in cavity • Primary limitation over past 5-10 years • Strongly related to the cavity handling • Very clean surface preparation and handling are needed • This is by no means trivial! • Especially for multi-cell cavities • On-going effort needed in quality control Lutz Lilje DESY -MPY-

  43. Field Emission Pictures taken from: H. Padamsee, Supercond. Sci. Technol., 14 (2001), R28 –R51 Particle causing field emission Temperature map of a field emitter Simulation of electron trajectories in a cavity Lutz Lilje DESY -MPY-

  44. Cleanroom Technology for SC Cavities • the small surface resistance of the superconducting necessitates avoidance of NC contaminations larger than a few mm • detailed material specification and quality control need to be done • tight specification for fabrication e.g. welds have to be implemented • clean room technology is a must (e.g. QC with particle counts, monitoring of water quality, documentation of processes) The inter-cavity connection is done in class 10 cleanrooms Lutz Lilje DESY -MPY-

  45. State-of-the-Art:Multi-cell Accelerating structures for XFEL and ILC XFEL ILC Lutz Lilje DESY -MPY-

  46. Outline • Superconducting Radiofrequency Accelerating Sturctures (‘SCRF Cavities‘) • History • Properties • Technology • Limitations and Remedies • Example for Accelerator Systems • XFEL and ILC Lutz Lilje DESY -MPY-

  47. Properties of XFEL radiation • X-ray FEL radiation (0.2 - 14.4 keV) • ultrashort pulse duration <100 fs (rms) • extreme pulse intensities 1012-1014 ph • coherent radiation x109 • average brilliance x104 • Spontaneous radiation (20-100 keV) • ultrashort pulse duration <100 fs (rms) • high brilliance spontaneous radiation

  48. Overall layout of the European XFEL 3.4km

  49. XFEL site in Hamburg/Schenefeld

  50. … after construction (computer simulation) Building Phase 1

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