1. TTC High Q0 Working Group Summary of developments since last TTC meeting
C. Reece

2. TTC High Q working group
Informal working group from TTC-member institutions pursuing routes to minimized rf surface resistance in SRF applications.
Periodic web-based meetings (1-2 months)
Information exchange
Solicitation of technical feedback
Learning from each other
Not project-specific
Prepared short presentations
Posted on Indico
For informational and learning purposes within TTC
Not for technical reference
All material considered “preliminary”

3. Since the DESY TTC Meeting three web-based virtual meetings were held May 15, July 31, and October 16.
Indico sites:
Although most of the presentations were from US participants due to aggressive LCLS-II investments, there was broad and growing participation from all regions and good stimulating discussion, especially regarding the puzzling Q0 sensitivity to cooldown conditions.

4. A variety of topics are being discussed (and continue in WG 3 this week)
Q0 variability with cooldown conditions
Pressure to better quantify external magnetic fields and amount of flux trapped
Pressure to scrutinize temperature profile and change rate through Tc
How to exploit the phenomenon for best performance?
Analysis of Q0 dependence with rf Bpk as a function of various treatments
HT-N = 800C + N doping – protocol studies EP
BCP
1400°C clean bake without subsequent chemistry
Comparison of bulk and film Nb surfaces

5. What Has Been Learned toward High Q0?
Subtle material details within L matter a lot
High temperature diffusion doping can lower Rresidual and R“BCS”
The best surfaces show decreasing Rs to ~5 mT, 2 K, 1.3 GHz
Magnetic flux must be carefully managed
Cavity Q performance is easily limited by extrinsic effects that yield frozen-in flux that
“Magnetic hygiene” standards must improve
Fixturing hardware, instrumentation connectors, etc. non-magnetic, low permeability
Temperature gradient across cavity during Tc transition is very helpful for expelling residual flux. (“Fast” is but a means of creating gradient.)
Asymmetric temperature gradient across cavity assembly with dissimilar metals during Tc transition may generate currents, large B-field, and frozen flux.
Cavity quench presents opportunity for flux entry – and Q degradation
N-doped material appears to trap available flux more readily than non-doped material.
Higher doping appears correlated with lower cavity quench fields.

6. R&D for Optimum Q0 from Bulk Nb
Quick Review
Unexpected discovery that surface diffusion of some foreign atomic species into Nb at high temperature seems to inhibit RF loss mechanisms that have “normally” been present.
800°C vacuum for 3 hours is used to degas dissolved H from the niobium bulk - routine.
~20 mTorr nitrogen 800°C for a few minutes - new.
Lossy nitrides on the surface are removed by light > 2 µm electropolish.
Resulting rf surface resistance decreases with field to unprecedented low levels (< K, 1.3 – 1.5 GHz)= high Q0
FNAL

7. What Has Been Learned toward High Q0?
Subtle material details within L matter a lot
High temperature doping can lower Rresidual and R“BCS”
The best surfaces show decreasing Rs to ~5 mT, 2 K, 1.3 GHz
With a big push from the LCLS-II project, nitrogen doping for high Q0 has been clearly demonstrated by multiple labs and in a production environment on both single and 9-cell cavities { >150 tests }
FNAL
Cornell
JLab
Many open questions remain regarding detailed mechanism
Immunization against hydrogen?
Low mfp scattering benefit from N?
Why do the best cavities’ R“BCS” match Xiao extension of M-B theory?
Enhanced quench vulnerability from suppressed Hc?
N-doping not yet fully optimized, but clearly ready for use

8. Cornell Nine Cell 2.0K Results, doping recipe 2
100 um bulk VEP, 20min 30 min anneal, final VEP
Q0 > 3E10 at 2K, at 16MV/m (or max field reached) for all nine cells
Quench fields of 14 to 22 MV/m
LCLS-II spec
Similar results from FNAL and JLab
M. Liepe

9. What Has Been Learned toward High Q0?
Temperature gradient across cavity during Tc transition is very helpful for expelling residual flux. (“Fast” is a means) FNAL VTS tests
N-doped bare 9-cell
Slow cooling leads to strong deterioration of Q0
Efficient flux expulsion
Meissner effect flux expulsion is sensitive to the cooling conditions through Tc
Trapped flux adds Rs
Detailed characterization is underway
Poor flux expulsion
Oct, 2013
A. Romanenko et al. J. Appl. Phys. 115, (2014)

10. Turning the two knobs: magnetic field and cooling- measurements at Fermilab
N doped single cell
Helmholtz coils to create close to uniform magnetic field around the cavity
Three fluxgate probes around the cavity equator for precision magnetic field measurements at the important region
Similar Cornell
Alex Romanenko & Anna Grassellino

11. Possible mechanism: thermal force overcoming the pinning force
Thermal gradient at the superconducting/normal conducting boundary is aiding the flux expulsion => the higher dT/dx the better. See J. Appl. Phys. 115, (2014)
A. Gurevich and G. Ciovati, Phys Rev B 87, (2013)
Example of thermal difference across the 1-cell cavity:
fast from 300K leads to full flux expulsion
Slow leads to full flux trapping
Fast from 20K leads to good flux expulsion, but not full
Grassellino
For efficient flux expulsion ~ 13K along the nine cell

12. Image Courtesy of Dan Gonnella
Residual Resistance from Trapped Magnetic Flux in “fast” Cool-down - magnetic field sensitivity studies
Rres
[nOhm]
FNAL
B [mGauss]
Image Courtesy of Dan Gonnella
Cornell University
Turbulent mixing of supplied cold He yields low delta-T across cavity and lower Q0
Palczewski

13. Optimal cooling conditions can produce extra low residual resistance despite large ambient fields !
Reported FNAL
5 nOhm in 200mG!!
Anna Grassellino

14. Importance of Cryomodule Cooldown Conditions

15. What Has Been Learned toward High Q0?
Asymmetric temperature gradient across cavity assembly with dissimilar metals during Tc transition may generate currents, large B-field, and frozen flux. Cornell HTC tests
9-cell cavity in HV
Multiple cooldowns with controlled magnetic field and supplied heat
Dan Gonnella

16. Nb film Rs Dependence on Cooling Conditions
Film Type
Plasma

17. The door has opened to new opportunities.
New performance domain
The door has opened to new opportunities.
New interesting challenges have appeared for exploiting these opportunities with high Q0.