High Q R&D at JLab

G. Ciovati, P. Dhakal, R. Geng, P. Kneisel, G. Myneni

TTC Topical Meeting on CW SRFCornell Univ., June 12th-14th, 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, 022002 (2010)

Fine-Grain 9-cell cavity results

• 800°C HT without chemistry + 120C 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R. Geng, unpublished, 2010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 1000-1200°C HT than after 800°C

-80

-60

-40

-20

0

20

40

60

500 700 900 1100 1300

Heat treatment temperature (°C)

Q0(

2 K

, 90

mT

) c

ha

ng

e o

ve

r

ba

se

line

(%

)

Beam tubes partially closed with Nb foils

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

0.0 0.2 0.4 0.6

Ti/N

bDepth (mm)

800C

1000C

1200C

15th SRF International Conference, Chicago, IL, 2010, p. 508

Ti Vapor Pressure110-12 Torr

310-9 Torr

810-7 Torr

Streamlined cavity processing

• Proposed at Ingot Nb Symposium (2010) AIP Conf. Proc. 1352, 2011, p. 25

o Ingot Nb cavityo 80 mm material removal by CBPo 20 mm material removal by BCPo Heat treatment at 800 °C/3 h + 120 °C/12 ho Surface passivation with thin nitride layero 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, 042001 (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 Q0 at Ultra High Gradient • Improve Q0 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 Q0 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 Q0 was proposed

– A single-cell achieved Q0=51010 at 90 mT, 1.5 GHz, 2.0 K even without a nitride passivation layer

• A new effort towards increasing the Q0 at very high gradient has started