Update on SRF Activities at Argonne

1. Update on SRF Activities at Argonne Mike Kelly, Peter Ostroumov, Mark Kedzie, Scott Gerbick (PHY) Tom Reid, Ryan Murphy (HEP) Thomas Proslier, Jeff Klug, Mike Pellin (MSD) June 7, 2010

2. ATLAS • ILC • National Security • Atomic Layer Deposition • SRF at the Advanced Photon Source (SC undulator and crab cavity)

3. I. ATLAS Energy Upgrade: Commissioned June 2009Exceeds previous state-of-the-art (at TRIUMF) by ~50% EP in Joint Facility

4. I. ATLAS Efficiency and Intensity Upgrade Phase I: RFQ and new cryomodule • New 60.625 MHz CW RFQ • New cryomodule with 7 QWRs OPT=0.077 • Total $9.86M ARRA funds • Complete in March 2013

5. I. ATLAS Energy and Intensity Upgrade: CryomoduleCommissioning in July 2012 (for scale)

6. 30 cm I. Pushing performance for low-beta SRF cavities b=0.077 f=72.5 MHz Bp/Eacc= 4.8 mT/MV/m Ep/Eacc=3.25 • Obvious benefits for ATLAS • Replace aging split-ring cryomodules • Higher energies (30-40% beam energy increase with Phase I) • Higher intensities • Real possibilities for high-gradient low-beta for applications • National security (non-destructive interrogation methods) • Nuclear medicine (accelerators as solution to Mo99 crisis) • Renewed interest for waste transmutation • To push for better performance in the intensity upgrade… • VCX fast tuner g Piezoelectric transducer + 4 kW coupler • Better performance through the use of techniques learned in FNAL collaboration; particularly horizontal electropolishing on completed jacketed niobium cavity New center conductor die Courtesy AES, June 4, 2010

7. II. Joint ANL/FNAL Cavity Processing Facility Ultrasonic Cleaning Electropolishing • Full operations (chemisty/clean room) since Mar 2009 • Two new EP operators trained • Excellent single cell results, recent good 9-cell results • Electropolishing system refinements • Possible improvements still to be had in operating parameters • Collaboration with JLab on KEK on EP optimization High-pressure rinse

8. II. Cavities Electropolished/Assembled at the ANL/FNAL SCSPF in 2010

9. II. Feedback from FNAL SRF Cavity Diagnostics (KEK Camera) to ANL Cavity Processing • Intra-grain structure is due to disruption of viscous layer from acid injection • Cathode holes covered and orientation changed to upward to reduce/remove this effect

10. II. New Low Voltage 9-cell cavity electropolishing parameters Cavity Temp. Current Voltage Acid Temp. Acid Temp. Water Temp Acid Flow

11. II. In the 2nd Chemistry Room: QWR electropolishing based on existing mechanical and electrical hardware rotating carbon brush assembly sliding Bosch rail

12. II. Electropolishing for 650 MHz 5-cell cavity • Scaled cavity geometry shown with the existing EP hardware • Cavity with twice radial dimension of the 1.3 GHz 9-cell fits into the existing system with modest modification (no cavity frame shown, may need to shim under blue stands) • 55 gallon acid handling limit OK • 2 ½ times surface area, EP supply OK, 50% larger chiller • Cavity handling similar to 9-cell (crane in hi-bay, hoist in chemistry room) • No major difficulties in adapting EP to this geometry

13. III. SRF for National Security • Accelerators for interrogation of special nuclear materials • Based short high-intensity pulse of protons • Secondary neutron production induces detectable g-rays • Very high accelerator real estate gradients needed (both low and high-b) • ANL-PHY funded to develop high real estate gradients for low-b • Fabrication/processing/diagnostic technique to achieve ILC type surface fields (~120 mT) • Innovative design techniques to reduce surface fields/increase packing factor Concept for a “stackable” half-wave cavity with very low surface fields

14. IV. Surface impedance & Magnetic impurities: the residual resistance and more Experimental evidence: -Data courtesy JLab (Ciovati) -Theory, Argonne Hot spots have higher concentration of Magnetic impurities than cold spots Bake 120C Bake 180C 3 parameters: -ε, α: effect of Magnetic impurities on the Nb superconductivity, give Rres , Δ and TC. Here ε=0.2 fixed. -Normal conductivity σ0: shift RS[T] vertically, give the mean free path L

15. IV. Surface impedance & Magnetic impurities: the residual resistance and more • Summary of results: • More magnetic impurities after baking (consistent with Casalbuoni SQUID), Conc ~ 200 ppm • Longer mean free path thus cleaner after baking. • Smaller gap but larger Δ/kTc after baking. • Unknowns and next experiments: • Where are the magnetic impurities coming from: Oxides for sure but something else also? • EPR (electron paramagnetic resonance) to probe mag. moments on EP samples. • -Refine the model: introduce inhomogeneity or surface layer.

16. IV. Superconducting layer by ALD

17. Thin films: 10 nm

18. Summary SRF at ANL • Phase I ATLAS Intensity Upgrade funded; work proceeding; completion in 2013 • Cavity processing at the joint ANL/FNAL facility • Good cavity throughput • Tweaking chemistry and clean room techniques based on test results and discussions with JLab/KEK • Interest and support for SRF for non-basic science applications • Material Science • Atomic layer deposition to produce new superconducting layers for cavities • Magnetic impurities to explain SRF properties of niobium