Efficient Magnetic Flux Expulsion During Cooldown Alexander Romanenko SRF Program Manager, TD Headquarters 7 th International Conference on RF Superconductivity
Main points Discovery at Fermilab in 203 -> Cooling through T c strongly affects Meissner expulsion efficiency [J. Appl. Phys. 5, 4903 (204)] Slow cooling traps ALL flux Efficient cooling exploited Q = 2.7x0 in 27 mg ambient field; Q > 5.5x0 0 in 90 mg ambient field Effect is independent on the field source = static or generated by thermal currents etc SRF 203 recommendation for slow cooldown as best to retain Q in dressed cavities had to be revisited Record high Qs in horizontal tests of fully dressed cavities for LCLS-II using the optimal fast cooling: Q = 3x0 0 at T=2K, Eacc=6 MV/m What have we learnt about physics of the effect since then? Mechanism of expulsion, role of the bulk, geometry 2
Correcting SRF 203 recommendation SLOW COOLDOWN = Best for Q Reason: slow cooldown traps ALL magnetic field making it unsuitable for current magnetic environments in accelerators (e.g. 5 mg ambient field for LCLS-II = the most stringent specification ever) 3
Bare N doped 9-cell in vertical test Efficient flux expulsion Poor flux expulsion Oct, 203 4
Systematic effect for all cavity treatments Single grain EP N doped Fine grain EP By only varying the parameters of cavity cooling through T c =9.25K the residual surface resistance can be systematically and reversibly varied 5
Magnetic probes reveal the new physics Full expulsion of the magnetic field should increase the field at equator ~. times when going superconducting H Efficient flux expulsion Poor flux expulsion Fluxgate magnetometers A. Romanenko, A. Grassellino, O. Melnychuk, D. A. Sergatskov, J. Appl. Phys. 5, 4903 (204) 6
Typical setup on -cell used for studies of expulsion Helmholtz coils to control magnetic field Single axis fluxgate magnetometers (measure magnetic field) Temperature sensors 7
What is the driving factor for better/worse flux expulsion? What makes fast cooldown efficient?
Cooldown rate dt/dt is not the main driving parameter 9
Thermogradient at NC/SC interface is key A. Romanenko, A. Grassellino, A. C. Crawford, D. A. Sergatskov, and O. Melnychuk, Appl. Phys. Lett. 05, 23403 (204) Temperature gradient at the phase front (dt/dx) 0
Why is flux expulsion better for larger thermal gradient? Why is slow cooldown worse?
Mechanism#: Depinning by Thermal Gradient Thermal gradient at the superconducting/normal conducting boundary creates a depinning force [J. Appl. Phys. 5, 4903 (204)] Normal conducting f T ~ - T Require that f T > J c φ 0 to depin flux Force component from inner to outer wall is what helps Cavity interior (vacuum) Cavity exterior (helium) [S. Posen et al, MOPB04] Superconducting 2
Mechanism #2: Unfavourable geometry of SC phase nucleation See [J. Appl. Phys. 5, 4903 (204)] for details Fast Slow Sharp NC/SC interface NC islands encircling the flux For this mechanism uniformity is bad -> leads to islands 3
More Geometrical Considerations: horizontal cavity Gaseous Helium Orthogonal Flux encircled Axial 9.25K Cooling direction Orthogonal magnetic field concentrated on top during the cool-down can not be expelled () flux-hole on top on the cavity equator Axial magnetic field concentrated on top can be expelled with fast cool-down M. Martinello et, al, J. Appl. Phys., 044505 (205) 4
Orthogonal vs Axial ~ MV/m Orthogonal field Fast Cool-down Axial field Fast Cool-down 6 26.7 MV/m 0.000 0.02500 6 26.7 MV/m 0.000 0.02500 0.05000 0.05000 0.07500 0.07500 6 2 26 0.000 0.250 0.500 0.750 6 2 26 0.000 0.250 0.500 0.750 3 0.2000 0.50 3 0.2000 0.50 4 7 0 3 6 Thermometer Number 0.2500 4 7 0 3 6 Thermometer Number 0.2500 [M. Martinello et al, MOPB04] 5
Trapping Efficiency vs Trapped Flux Sensitivity Trapping Efficiency How much of the magnetic field gets trapped for the given ambient magnetic field at Tc Studied in detail -> [S. Posen et al, MOPB04] Bulk treatment is the driving factor Small grain/more dislocations cavities trap more 6
Trapping Efficiency vs Trapped Flux Sensitivity Trapped Flux Sensitivity For the given amount of trapped flux how much additional dissipation (surface resistance) emerges Studied in detail -> [M. Martinello et al, MOPB05] Surface treatment is the driving factor Nitrogen doped cavities are more sensitive BEST to find out optimal cooling conditions to maximize Q (bare, dressed/cryomodule) as they reveal trapped flux the best Whatever is best for N-doped is best for EP, EP/20C etc Use slow cooling to trap all flux and then measure dissipation 7
Utilizing expulsion physics for record Qs
Utilizing new physics for record high Qs bare -cell.5k,.3ghz -cell Ambient magnetic fields are fully expelled Combination of nitrogen doping and efficient flux expulsion => Record high Q >e up to 2 MV/m in SRF cavities A. Romanenko, A. Grassellino, A. C. Crawford, D. A. Sergatskov, and O. Melnychuk, Appl. Phys. Lett. 05, 23403 (204) 9
Expelling 90 mg Q > 2.7e0 at 2K, 6MV/m in 90 (!) mg ambient field A. Romanenko, A. Grassellino, A. C. Crawford, D. A. Sergatskov, and O. Melnychuk, Appl. Phys. Lett. 05, 23403 (204) 20
Bringing it all together: N-doped dressed cavity in cryomodule environment 9-cell N-doped fully dressed cavity for LCLS-II with high power coupler, tuner, HOM couplers in horizontal test Ambient field < 4 mg [see MOBA06, MOP02, THBA06, MOPB07] 2
Summary Discovery at Fermilab Magnetic flux can be expelled from cavity walls by fast/high thermal gradient cooldown Allows achieving record low residual resistances Examples: Q = 2.7x0 in 27 mg ambient field; Q > 5.5x0 0 in 90 mg ambient field Understanding has much progress Mechanism of expulsion (or non-trapping) [MOPB04] Importance of bulk properties [MOPB04] Surface treatment effect [MOPB05, MOPB020] Geometry effect (horizontal vs vertical) [MOPB04] Is cryomodule configuration/performance any different? [MOBA06, MOP02, THBA06] Slow cooldown = best (SRF 203)? Not true for current magnetic environments
THANK YOU 23
Observing fast and slow cooldown dynamics 24
T-map cooling capturing IRIS L EQUATOR IRIS R EQUATOR 3 5 TOP NC REGION IRIS L IRIS R BOARD #3 BOARDS #,4 Top Mid 3 BOTTOM SC REGION BOARD #3 Bottom 2 4 6 0 2 4 6 Thermometer Number 25
Fast Cool-down T-map Starting T: 250K 26
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 4 7 0 3 6 27 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 4 7 0 3 6 2 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 4 7 0 3 6 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 4 7 0 3 6 30 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 4 7 0 3 6 3 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 4 7 0 3 6 32 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 Sharp NC-SC transition interface 2 4 6 0 2 4 6 33 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 Sharp NC-SC transition interface 4 7 0 3 6 34 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 Sharp NC-SC transition interface 4 7 0 3 6 35 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 Sharp NC-SC transition interface 4 7 0 3 6 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 Sharp NC-SC transition interface 4 7 0 3 6 37 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 Sharp NC-SC transition interface 4 7 0 3 6 3 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 Sharp NC-SC transition interface 4 7 0 3 6 39 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 NC zones encircle by SC material 4 7 0 3 6 40 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 NC zone encircle by SC material 4 7 0 3 6 4 Thermometer Number
Fast Cool-down From 250K 9.25 0.40 0.9.55 5 2.3 2.70 3.27 3.5 4.43 5.00 4 7 0 3 6 42 Thermometer Number
Slow Cool-down T-map Starting T: 2K 43
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 44 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 45 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 46 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 47 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 4 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 49 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 50 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 5 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 52 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 53 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 54 Thermometer number
Slow Cool-down From 2K 6 9.305 9.0 9.45 9.470 9.525 6 2 26 9.50 9.635 9.690 9.745 9.00 3 4 7 0 3 6 55 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 56 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 57 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 5 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 59 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 60 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 6 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 62 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 63 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 64 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 65 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 66 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 67 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 6 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 69 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 70 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 7 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 72 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 73 Thermometer number
Slow Cool-down From 2K 9.305 9.0 9.45 9.470 5 9.525 9.50 9.635 9.690 9.745 9.00 4 7 0 3 6 74 Thermometer number