1. The Continuing Role of SRF for AARD: Issues, Challenges and Benefits SRF performance has been rising every decade SRF installations for HEP (and other applications) have been rising steadily With strong support, SRF can continue to make major impact on future HEP accelerators –ILC, TeV upgrade, Superbeams for Neutrinos, Neutrino Factory, Muon Collider, Multi-TeV colliders Hasan Padamsee Cornell University

2. Steady Increase of Installed SRF Voltage in Accelerators With Time

3. SNS/Jlab KEK-B CESR CEBAF HERA TRISTAN CESR SNS LEP-II Stanford Cornell

4. Steady Increase of SRF Gradients 1980 – 2006 5 MV/m -> 40 MV/m Along the way…

5. Cornell DESY Fermilab Collaboration 1995 Gradients 26 - 27 MV/m reached in Three 5-cell structures 25 MV /m

6. Increase of SRF Gradients 9-cells, 1995 – 2006 -> 40 MV/m

7. 35 - 40 MV/m DESY-KEK 6 best cavities, Vertical Tests SRF for ILC (0.5 TeV) SRF technology in US needs serious catching up ! Need to engage ALL available resources in US

8. Goals of Near Term R&D for ILC Demonstrate reproducibility of performance Perfect EP, HPR, clean room techniques in US Long term performance characterization of full scale structures in cryomodules Assemble and operate one complete RF unit with all ILC building blocks US labs engaged : Fermilab, Jlab, Cornell, Argonne… (SMTF collaboration) See talks by Holmes, Kephart, Kneisel

9. Beyond 35 MV/m for TeV ILC Important work started BUT much more needed! –Better shapes –Better niobium –Better understanding of SRF physics of niobium See Talk by Kneisel See Talk by Gurevich

10. New Shapes Breakthrough 50 MV/m in Single Cells ! Lower Surface Magnetic Field & Lower Losses Re- entrant Cornell KEK Low Loss Jlab KEK Tesla Shape Need Multi-cells Next

11. Better Niobium Started at Jlab Standard fine grain (50  m) Large Grain - 10 cm Single Grain - Reduction of grain boundary density improves performance - Lower cost through ingot stage material

12. SRF for Muon Colliders at Multi-TeV energies Biggest challenge is muon cooling – being addressed elsewhere Muon collider is a cascade of SRF Recirculating Linear Accelerators (many turns each) Starting with linac at low frequency (e.g 200 MHz) at 10 – 15 MV/m, followed by low frequency RLAs TeV scale energies after with several 30 MV/m gradient RLAs Total about 5 km of ILC-like linac

13. Need High Quality Nb Coatings For Low Frequency Applications For 200 MHz, cost of Nb is a major factor => Develop Nb/Cu CERN/Cornell collaboration reached 10 MV/m Improved coatings needed for 15 MV/m Ionize Nb atoms using various methods –Bias magnetron sputtering (Cornell/ACCEL) –Electron cyclotron resonance in a Nb plasma (Cornell/Jlab) –Vacuum Arc (Cornell/INFN-Rome)

14. SRF for the Neutrino Sector Neutrino factory (Muons accelerated to 20 GeV via 5-turn RLA, need about 4 GeV) –Could serve as demonstrator for the muon collider: Neutrino factory may or may not be needed depending on super- beams, as for example 8 GeV Proton Driver Main part of Proton Driver is SRF, ILC-like linac –Could also be test linac for ILC (if needed) See Talk by Foster Neutrino Beams

15. SRF for Colliders Beyond 1 TeV Both gradients > 50 MV/m & Q ≈ 10 11 can contribute Higher gradients –Need New materials, e.g. Nb 3 Sn Simple theory: RF critical magnetic field = Superheating critical field: –Nb : 2200 Oe (57 MV/m) Best achieved 2000 Oe –Nb 3 Sn : 4000 Oe (105 MV/m) Best achieved 1100 Oe…Why? Fundamental research needed on the RF critical field of highly Type II superconductors –Theoretical studies – Experimental work to determine Hcrit –Coupled with fabrication of best materials Present funding level for this effort is nearly zero !

16. Pulsed Measurements of RF Critical Field: Nb & Nb 3 Sn 4000 Oe = 105 MV/m in new shape cavities T. Hays: Cornell Campisi: SLAC Hc1- Nb3Sn

17. Plane Nucleation Model For flux lines Other Models Are these right?? Simple Theory of RF Critical Magnetic Field Best Nb

18. Increased Gradients Must Go Hand-in-hand With Increased Q, to Keep Operating Costs in Line For e.g. : 80 MV/m will need Q = 10 11

19. Best Nb Q: > 10 11 at 25 MV/m, 1.6 K

20. Best Nb 3 Sn Q at 2 K ! (But at low fields) G. Mueller and P. Kneisel

21. Conclusions Basic R&D on SRF has steadily pushed gradients With many benefits to HEP, and non-HEP along the way (See Talk by Swapan C.) We must solidify our gains in the 35 – 40 MV/m range to realize ILC –With other potential benefits to HEP ( Neutrino Factory, Proton Driver, Muon Collider) –And non-HEP (Light sources, Neutron sources…) Support needed to stay on the road to 100 MV/m and Q = 10 11 to realize multi-TeV energy. Continued evolution of the “Livingston Curve”?

22. A Look into the Past and Future Installed SRF Voltage SNS LEP XFEL ILC CEBAF