Efficient Magnetic Flux Expulsion During Cooldown

Size: px

Start display at page:

Download "Efficient Magnetic Flux Expulsion During Cooldown"

Cornelius Berry
5 years ago
Views:

1 Efficient Magnetic Flux Expulsion During Cooldown Alexander Romanenko SRF Program Manager, TD Headquarters 7 th International Conference on RF Superconductivity

2 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

3 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

4 Bare N doped 9-cell in vertical test Efficient flux expulsion Poor flux expulsion Oct, 203 4

5 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

6 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

7 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

8 What is the driving factor for better/worse flux expulsion? What makes fast cooldown efficient?

9 Cooldown rate dt/dt is not the main driving parameter 9

10 Thermogradient at NC/SC interface is key A. Romanenko, A. Grassellino, A. C. Crawford, D. A. Sergatskov, and O. Melnychuk, Appl. Phys. Lett. 05, (204) Temperature gradient at the phase front (dt/dx) 0

11 Why is flux expulsion better for larger thermal gradient? Why is slow cooldown worse?

Mechanism#: Depinning by Thermal Gradient Thermal gradient at

c φ 0 to depin flux Force component from inner to outer wall is

12 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

13 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

25K Cooling direction Orthogonal magnetic field concentrated on

top on the cavity equator Axial magnetic field concentrated on

14 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., (205) 4

15 Orthogonal vs Axial ~ MV/m Orthogonal field Fast Cool-down Axial field Fast Cool-down MV/m MV/m Thermometer Number Thermometer Number [M. Martinello et al, MOPB04] 5

16 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.

17 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

18 Utilizing expulsion physics for record Qs

19 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, (204) 9

20 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, (204) 20

21 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

22 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

23 THANK YOU 23

24 Observing fast and slow cooldown dynamics 24

25 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 Thermometer Number 25

26 Fast Cool-down T-map Starting T: 250K 26

27 Fast Cool-down From 250K Thermometer Number

28 Fast Cool-down From 250K Thermometer Number

29 Fast Cool-down From 250K Thermometer Number

30 Fast Cool-down From 250K Thermometer Number

31 Fast Cool-down From 250K Thermometer Number

32 Fast Cool-down From 250K Thermometer Number

33 Fast Cool-down From 250K Sharp NC-SC transition interface Thermometer Number

34 Fast Cool-down From 250K Sharp NC-SC transition interface Thermometer Number

35 Fast Cool-down From 250K Sharp NC-SC transition interface Thermometer Number

36 Fast Cool-down From 250K Sharp NC-SC transition interface Thermometer Number

37 Fast Cool-down From 250K Sharp NC-SC transition interface Thermometer Number

38 Fast Cool-down From 250K Sharp NC-SC transition interface Thermometer Number

39 Fast Cool-down From 250K Sharp NC-SC transition interface Thermometer Number

40 Fast Cool-down From 250K NC zones encircle by SC material Thermometer Number

41 Fast Cool-down From 250K NC zone encircle by SC material Thermometer Number

42 Fast Cool-down From 250K Thermometer Number

43 Slow Cool-down T-map Starting T: 2K 43

44 Slow Cool-down From 2K Thermometer number

45 Slow Cool-down From 2K Thermometer number

46 Slow Cool-down From 2K Thermometer number

47 Slow Cool-down From 2K Thermometer number

48 Slow Cool-down From 2K Thermometer number

49 Slow Cool-down From 2K Thermometer number

50 Slow Cool-down From 2K Thermometer number

51 Slow Cool-down From 2K Thermometer number

52 Slow Cool-down From 2K Thermometer number

53 Slow Cool-down From 2K Thermometer number

54 Slow Cool-down From 2K Thermometer number

55 Slow Cool-down From 2K Thermometer number

56 Slow Cool-down From 2K Thermometer number

57 Slow Cool-down From 2K Thermometer number

58 Slow Cool-down From 2K Thermometer number

59 Slow Cool-down From 2K Thermometer number

60 Slow Cool-down From 2K Thermometer number

61 Slow Cool-down From 2K Thermometer number

62 Slow Cool-down From 2K Thermometer number

63 Slow Cool-down From 2K Thermometer number

64 Slow Cool-down From 2K Thermometer number

65 Slow Cool-down From 2K Thermometer number

66 Slow Cool-down From 2K Thermometer number

67 Slow Cool-down From 2K Thermometer number

68 Slow Cool-down From 2K Thermometer number

69 Slow Cool-down From 2K Thermometer number

70 Slow Cool-down From 2K Thermometer number

71 Slow Cool-down From 2K Thermometer number

72 Slow Cool-down From 2K Thermometer number

73 Slow Cool-down From 2K Thermometer number

74 Slow Cool-down From 2K Thermometer number

Bipolar EP: Electropolishing Without Fluorine in a Water Based Electrolyte

Bipolar EP: Electropolishing Without Fluorine in a Water Based Electrolyte Allan Rowe Fermilab-Technical Division Applied i What is bipolar EP? Anodic Pulse Tuned to: - Control current distribution Eliminates