Progress in high Q and high gradient R&D Anna Grassellino Tesla Technology Collaboration Meeting Vancouver, TRIUMF, February 2019
Outline • Two main research directions to push high Q at high gradients: – High temperature (> 800C) nitrogen doping – Low temperature treatments ( ~ 50C-200C) with or without nitrogen • With increasing importance of cooldown studies/details in the whole temperature range ~300->2K • A possible common matrix that ties it all together: nano- hydrides? • Theoretical advancements and path forward 2 Grassellino - Progress in High Q/high G
State of the art in high Q and high G (1.3 GHz, 2K) • Q>3e10 @35 MV/m with N doping • Q >1e10 at 49 MV/m (B pk = 210 mT) with 75/120C bake N doping 75/120C N infusion 120C bake EP Important: insufficient bulk removal or high defect density material (insufficiently annealed) will cause extra residual resistance 3 Grassellino - Progress in High Q/high G
Breakdown in Surface Resistance: R BCS and R 0 field dependence for state of the art treatments • Largest advantage in Q from high T doping comes from reversal of R BCS (factor of 3-5 lower at mid field than 120C/EP) PLUS lowest residual resistance (EP grown oxide, on nitrogen enriched layer) • 120C Infusion reduces R BCS compared to regular 120C bake, PLUS has lowest residual (oxide grown in furnace, on oxide enriched layer) • 75/120C gives same R BCS as N infusion…why? But higher residual (as regular 120C) • What is behind these field dependencies? What is this knee between increase and decrease of the BCS resistance? 4 Grassellino - Progress in High Q/high G
High T Doping 5 Grassellino - Progress in High Q/high G
High Temperature Doping is key for Highest Q • Record Q values achieved at all frequencies (see breakout talks by Martinello for 650MHz and Bafia for 1.3GHz) T=2K See also Grassellino et al, Superconductor Science and Technology, Volume 26, Number 10 Martinello et al Phys. Rev. Lett. 121 , 224801 650 and 2.6 data data to be published (FNAL) 6 Grassellino - Progress in High Q/high G
Where do we stand with high T N doping quench fields? • High T N doping is key to record Q values: produces systematically lowest BCS (and residual) surface resistance values: eg Q~6e10 @ 20 MV/m, 1.3 GHz, 2K! Or Q ~ 7-8e10 @ 650 MHz, mid field, 2K • Achievable quench field has evolved – from being limited to ~20 MV/m in earlier days to up to 35 MV/m today, in single and nine cells • What are the important steps that have led to such performance improvement? ? Grassellino et al, Superconductor Bafia et al, TTC ARIES @ CERN Science and Technology, Volume 26, Number 10 7 Grassellino - Progress in High Q/high G
“Recipe” changes yielding gradient advancements • A first big step in ~2014 @ FNAL was achieved moving from longer to shorter duration of doping (example 20/30 à 2/6) • Recently, further improvements have been reached with some new doping “recipes” (see Gonnella, Bafia and Palczewski talks) from simple tweaks (2/0) to more dramatic changes (3/60) 27 MV/m 33 MV/m 23 MV/m 23 MV/m High Q High Q R&D for R&D for LCLS-2 HE, LCLS-2, 2018 2015 8 Grassellino - Progress in High Q/high G
Sequential Doping Study of same cavity 2/6 + 5um EP (baseline): f=1.3GHz +40um EP reset T=2K 2/0 + 5um EP: • Higher Q and quench increases by +6MV/m +40um EP reset 3/60 + 5um EP: • Quench improves by additional +2MV/m , Q 0 =6E10 @ 20MV/m! Grassellino - Progress in High Q/high G 9
Role of mean free path/nitrogen concentration? • One of the leading thoughts on quench in N doped cavities has been that higher concentration/lower mfp could reduce the quench field (corroborated by the fact that lighter doping or deeper EP typically yield higher gradients) • In reality, data does not show a clear correlation with mean free path • More detailed SIMS studies ongoing to systematically relate surface N concentration to achievable field Quench above theoretical H c1 H c1 GL (0K) H c1 GL (0K) Palczewski, Reece MOPB039 Proceedings of SRF2015, Whistler, BC, Canada Open squares data points from M. Checchin talk @TTC RIKEN 2017, new data from D. Bafia in solid colour, presented at TTC ARIES @CERN 10 Grassellino - Progress in High Q/high G
Nano-Hydrides in N doped cavity cutouts (Romanenko/Sung) See Z. Sung (FNAL) breakout talk 300K 300K Nanohydrides form in the range 200-100K, fewer than other treatments and size ~30-50 nm 200K 200K Temp [K] 100K 100K 11 Grassellino - Progress in High Q/high G
A proposed model to explain quench in N doped cavities Field of first vortex entry will depend on size of superficial defects compared to coherence • length • Doping recipe and final N level modifies the coherence length (mfp) but also size of hydrides Think of hydrides as surface ‘defects’ that will lower field of first entry • Possible that N doping brings the coherence to unfavorable point compared to other • treatments, coherence length comparable to size of the hydrides (which is exactly the case) Possible pathway forward: decouple coherence from hydrides size (move to dirtier or cleaner • or longer second step outgassing cycles e.g. 3/60min to reduce hydrides size) EP N doping 120C bake NbH NbH NbH < 10 nm ~40 nm ~40 nm z ~ 1000 nm z ~ 2-10 nm z ~ 40-100 nm Model under development, Grassellino and Sauls (Northwestern U, CAPST) 12 Grassellino - Progress in High Q/high G
Pushing the Q even further via high T doping • The new 3/60 recipe (see Palczewski, Bafia, Gonnella, Martinello) leads to even further reduction in Rbcs (B), leading to extraordinary Q > 6e10 at 2K • To be studied and validated: is this related to smaller size of nanohydrides or fewer due to the longer post doping anneal time (giving a larger avg gap) • Mean free path/concentration of 3/60 seems not too distant from 2/6 recipe so cannot explain by itself the reduction in BCS and especially the stronger reversal • Interesting question: how much lower can we go in Rbcs? Bafia et al, TTC ARIES @ CERN Martinello et al, Appl. Phys. Lett. 109 , 062601 (2016) T = 2K Bafia et al, TTC ARIES @ CERN f=1.3GHz 13 Grassellino - Progress in High Q/high G
Low T treatments 14 Grassellino - Progress in High Q/high G
The new 75/120C findings • We have recently focused our attention to the unexpected finding that a pre-120C bake step of ~4 hours at 75C seem to lead consistently to unprecedented accelerating gradients ~49 MV/m (210 mT, TESLA shape) • However, under the ILC cost reduction effort, as we study more and more cavities, and exchange cavities worldwide, some new interesting findings are emerging in terms of Q and achievable accelerating gradient cooldown dependence 75/120C bake cavities See Grassellino et al arXiv:1806.09824 15 Grassellino - Progress in High Q/high G
Finding 1: the strange ‘branching’ performance for 75/120C • On dozens of tests and several cavities now, we see switch in performance for same cavity with no retreatment in between (always under vacuum) • Effects of magnetic fields, dewars, cables, top plates have been excluded • Some correlation has been found with cooldown speed near room T and starting T ~320-340K • See Daniel Bafia breakout talk for many details on this study Bafia, Grassellino, to be published 16 Grassellino - Progress in High Q/high G
Two 75/120C cavities sent from FNAL to Jlab and Cornell • Cornell gradient matches our 49 MV/m (see Maniscalco breakout talk) • Jlab reproduced exactly the upper/lower branching behavior in two separate cooldowns (see Palczewski breakout talk) • Two more cavities on their way to DESY and KEK 10 11 Cornell FNAL Q 0 10 10 10 9 0 5 10 15 20 25 30 35 40 45 50 E acc (MV/m) Courtesy of Palczewski , Jlab Courtesy of Liepe, Maniscalco, Cornell 17 Grassellino - Progress in High Q/high G
More puzzling differences – infusion cavities at KEK and DESY • FNAL sent infused cavities to KEK and DESY for retest (see Umemori and Wenskat talks in breakouts) • Substantial differences seen in Rbcs and for different cooldowns, but similar residual and quench fields BCS nearly doubled! Why? Grassellino et al, Superconductor Science and Technology, Volume 30, Number 9 Courtesy of Kensei Umemori, KEK 18 Grassellino - Progress in High Q/high G
Finding 2: unequivocal performance change for regular 120C bake • Cooldown From 294K vs ~350K Cooldown from bottom vs top Non equilibrium behavior of surface resistance shifting earlier Grassellino, Bafia, to be published • Substantially lower Q and G from 350K/top cooldown • BCS decreases, residual increases a lot, the “knees” move at corresponding points with a ‘breakdown’ field compatible with the proximity effect model of nanohydrides as introduced by Romanenko Superconductor Science and Technology, Volume 26, Number 3 19 Grassellino - Progress in High Q/high G
Cool Down Profiles ad fluxgates of AES010: zero B field Fully compensated ZERO B field (longitudinal), close to zero transverse 20 Grassellino - Progress in High Q/high G
300K Heating from 300K Preliminary AFM studies of 120C bake sample warming up from 300 to 380K 10 um 21 Grassellino - Progress in High Q/high G Courtesy of Z. Sung, to be published
320K Heating from 300K 10 um 22 Grassellino - Progress in High Q/high G Courtesy of Z. Sung, to be published
Recommend
More recommend
