High Q R&D at JLab G. Ciovati, P. Dhakal, R. Geng, P. Kneisel, G. Myneni TTC Topical Meeting on CW SRF Cornell Univ., June 12 th -14 th, 2013

Beginnings Collaborative studies of H in Nb samples (2002 – 2008) indicated the possibility of H causing additional RF losses in Nb cavities at medium or high fields, without Q-disease ( G. Myneni ). G. Myneni, CERN Courier Viewpoint, Oct. 20, 2008 G. Myneni proposed a process for “H-free” Nb surfaces ( US Patent 7,151,347, Dec. 19, 2006 ) and to do 2 K RF measurements of Nb cavities after 800°C heat treatment (HT) without subsequent chemistry at JLab (2009)

Results from 800°C/3 h + 120°C HT without chemistry Large Grain Single Crystal Fine Grain Phys. Rev. ST Accel. Beams 13, (2010)

Fine-Grain 9-cell cavity results 800°C HT without chemistry  C bake was tried on three fine-grain, 9-cell ILC cavities ( R. Geng ) Two cavities were limited by field emission R. Geng, unpublished, 2010

HT extended up to 1200°C HT study of large-grain SC extended up to 1200°C/2h (2010) SIMS analysis of samples treated with cavity show much higher Ti conc. after °C HT than after 800°C Beam tubes partially closed with Nb foils 15 th SRF International Conference, Chicago, IL, 2010, p. 508 Ti Vapor Pressure 1  Torr 3  Torr 8  Torr

Streamlined cavity processing Proposed at Ingot Nb Symposium (2010) AIP Conf. Proc. 1352, 2011, p. 25 o Ingot Nb cavity o  80  m material removal by CBP o  20  m material removal by BCP o Heat treatment at 800 °C/3 h °C/12 h o Surface passivation with thin nitride layer o High-Pressure Rinse

New Furnace A new UHV induction furnace with all Nb hot-zone was designed and procured to reach up to 2000°C ( G. Myneni ) (2011)

HT extended up to 1400°C with new furnace Samples’ analysis after 1400°C show: –Reduced H content and ~1 at.% Ti content –Higher energy gap and reduced broadening parameter –Unchanged mechanical properties Ingot Nb cavity ( RRR~200, Ta~1375 wt.ppm ), treatment sequence after fabrication: CBP, BCP, HT, HPR Phys. Rev. ST Accel. Beams 16, (2013) To be published

Planned studies Repeat 1400°C HT on cavities with Nb flanges Extend HT temperature up to ~1800°C on cavities with Nb flanges Repeat 1400°C HT on: –Same and new large-grain cavities –Fine-grain cavities

High Q 0 at Ultra High Gradient Improve Q 0 at very high gradient for ILC-type accelerator ( R. Geng ) Key points: Large-Grain Nb material for low residual surface resistance EP as final surface treatment for reduced HFQS Mirror finish for low field emission Cryogenic thermal annealing for minimum frozen flux To be published

Summary JLab pioneered research on the improvement of Q 0 at medium field by high temperature annealing of large-grain cavities –motivated by the need to establish low H-content near the Nb surface –a new furnace was commissioned A cost-effective process to produce SRF Nb cavities with high Q 0 was proposed –A single-cell achieved Q 0 =5  at 90 mT, 1.5 GHz, 2.0 K even without a nitride passivation layer A new effort towards increasing the Q 0 at very high gradient has started