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

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”

Since the DESY TTC Meeting three web-based virtual meetings were held May 15, July 31, and October 16. Indico sites: http://www.jlab.org/indico/event/TTC_High_Q_WG_3 http://www.jlab.org/indico/event/TTC_High_Q_WG_4 http://www.jlab.org/indico/event/TTC_High_Q_WG_5 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.

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

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 nohms @80 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.

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 gas @ 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 (< 10 nOhm @ 2.0K, 1.3 – 1.5 GHz)= high Q0 FNAL

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 nohms @80 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

Cornell Nine Cell 2.0K Results, doping recipe 2 100 um bulk VEP, 20min N2 @800C, 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

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, 184903 (2014)

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 work @ Cornell Alex Romanenko & Anna Grassellino

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, 184903 (2014) A. Gurevich and G. Ciovati, Phys Rev B 87, 054502 (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

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

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

Importance of Cryomodule Cooldown Conditions

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

Nb film Rs Dependence on Cooling Conditions Film Type Plasma

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.