Radio frequency superconductivity for particle accelerators:

Transcription

1 Radio frequency superconductivity for particle accelerators: Recent trends in physics and technology Sergey Belomestnykh (Fermilab) Seminar at John Adams Institute for Accelerator Science Oxford, UK, February 12, 2018

2 Outline Introduction What is RF superconductivity for particle accelerators? SRF basics: surface resistance, Q vs. E Recent SRF science breakthroughs & active areas of research: o Nitrogen doping o Nitrogen infusion o Frequency dependence of R s o SRF in quantum regime SRF over the world Summary 2

3 Introduction Over the past several years, the field of radio frequency superconductivity (SRF) for particle accelerators is going through a period of Renaissance. 5 years ago, most of the community thought that the science and technology reached maturity (even though we lacked understanding of some basic physics) and one can achieve only incremental gains in the niobium cavity performance. The field tended to be mostly technological with only few researchers trying to study fundamental issues of SRF in niobium. Big improvement steps were thought to be possible only with developing alternative materials (e.g. Nb 3 Sn). Recent discoveries of nitrogen doping and infusion, magnetic flux expulsion, opened new horizons and revived interest to studies of SRF basics, both experimental and theoretical. More unexpected and intriguing results have been obtained. In this talk I will try to shed light upon some exciting recent results, show new trends (Fermilab-centric view) and hopefully inspire young generation to turn their attention to this field of research. 3

4 What is RF superconductivity for particle accelerators? 4

5 Discovery of superconductivity: April 8th of 1911 Discovered in 1911 by Heike Kamerlingh Onnes and Gilles Holst after Onnes was able to liquefy helium in 1908 (Nobel Prize in 1913). 5

6 Superconducting elements 6

7 Superconducting state The superconducting state is characterized by the critical temperature T c and field H c 2 T Hc T Hc 0 1 Tc The external field is expelled from a superconductor if H ext < H c for Type I superconductors. For Type II superconductors the external field can partially penetrate for H ext < H c1 and will completely penetrate at H c2. 7

8 Theories explaining superconductivity Early developments: two-fluid model and London equations. Phenomenological Ginzburg-Landau (GL) theory (1950, Nobel Prize in 2003) generalized London equation to nonlinear problems. Microscopic theory of superconductivity was developed by Bardeen, Cooper and Schrieffer (BCS) in 1957 (Nobel Prize in 1972). What do we need to recollect? Magnetic field does not stop abruptly, but penetrates into the material with exponential attenuation. The (London) penetration depth l is quite small, nm. According to BCS theory not single electrons, but (Cooper) pairs are carriers of the supercurrent. However, the penetration depth remains unchanged. The BCS ground state is characterized by the macroscopic wave function and a ground state energy that is separated from the energy levels of unpaired electrons by an energy gap. In order to break a pair an energy of 2D is needed: 8

9 Theories explaining superconductivity (2) GL theory introduced coherence length x a new scale of special variation of the superfluid density and superconducting gap. Also introduced is a dimensionless GL parameter k l / x, which is independent of temperature. 9

10 What happens if AC field is applied? At 0 < T < T c not all electrons are bonded into Cooper pairs. The density of unpaired, normal electrons is given by the Boltzman factor n normal D exp kbt Cooper pairs move without resistance, and thus dissipate no power. In DC case the lossless Cooper pairs short out the field, hence the normal electrons are not accelerated and the SC is lossless even for T > 0 K. The Cooper pairs do nonetheless have an inertial mass, and thus they cannot follow an AC electromagnetic fields instantly and do not shield it perfectly. A residual EM field remains and acts on the unpaired electrons as well, therefore causing power dissipation. 10

11 What is RF superconductivity for accelerators? Radio frequency (RF) superconductivity for particle accelerators is a branch of accelerator physics and engineering dealing with application of superconducting materials to acceleration of charged particles in resonant RF cavities. Input RF power at 1.3 GHz Slowed down by factor of approximately 4x10 9 Niobium ~1 m The science part of this field deals with investigating limitations of and developing methods to improve the SRF cavity performance. In particular, how to reduce power dissipation in SRF cavities and improve accelerating gradients. 11

12 RF superconductivity as a branch of accelerator physics was born 57 years ago In a seminal paper published in June 1961 A. P. Banford and G. H. Stafford described how a future superconducting proton linear accelerator could run continuously, instead of at the 1% duty cycle of the 50 MeV proton accelerator that was operating at the time at the Rutherford High Energy Laboratory in the UK. The basic argument was that, because ohmic losses in the accelerating cavity walls increase as the square of the accelerating voltage, copper cavities become uneconomical when the demand for high continuous-wave (CW) voltage grows with particle energy. It is here that superconductivity comes to the rescue. (from Hasan Padamsee s article Advances in acceleration: the superconducting way, CERN Courier, November 2011) Plasma Physics (Journal of Nuclear Energy Part C), 1961, Vol. 3, pp. 287 to

13 Benefits of RF superconductivity The development of superconducting (SC) cavities for accelerators has enabled new applications not previously possible with normal conducting (NC) structures. SC cavities excel in applications requiring continuous wave (CW) or long-pulse accelerating fields above a few MV/m (up to ~35 MV/m). For NC cavities (usually made of copper) power dissipation in cavity walls is a huge constrain in these cases cavity design is driven by this fact, optimized for lowest possible wall dissipation small beam aperture. The surface resistivity of SC cavities is 5-6 orders of magnitude less than that of copper SC accelerating system is more economical: less wall plug power, fewer cavities required, Additional benefit: the cavity design decouples from the dynamic losses (wall losses associated with RF fields) free to adapt design to a specific application. The presence of accelerating structures has a disruptive effect on the beam and may cause various instabilities, dilute beam emittance and produce other undesirable effects. Fewer SC cavities less disruption. SC cavities can trade off some of wall losses to a larger beam pipe reduce disruption more. 13

14 SRF basics: R s, Q vs. E acc 14

15 Surface resistance A convenient way to characterize power losses at radio frequency resonant cavities is to use a so-called surface resistance. [ For normal conducting cavities R s = 1/(sd), where s is the specific conductivity and d is the skin depth. ] Then the power dissipation per unit area is 1 R 2 And the total power dissipation is obtained by integration over the whole inner surface of the cavity. P diss Calculation of surface resistance must take into account numerous parameters. Mattis and Bardeen developed theory based on BCS, which predicts D Tc 2 k BT c T RBCS A e, T where A is the material constant depends on the electron mean free path s H

16 Surface resistance (2) While for low frequencies ( 500 MHz) it may be efficient to operate at 4.2 K (liquid helium at atmospheric pressure), higher frequency structures favor lower operating temperatures (typically superfluid LHe at 2 K, below the lambda point, K). Approximate expression for Nb: R BCS f [MHz] T e T [Ohm] 16

17 Surface resistance of cavities R s T = R BCS T + R res The BCS surface resistance is described by Mattis-Bardeen theory and comes from thermally excited quasi-particles The residual resistance can come from different extrinsic contributions : o o o R BCS T R res o... = Aω2 T Impurities/defects in the surface Hydrides precipitates Trapped magnetic flux e k B T Residual resistance is significant for cavities operating at 2 K. 17

18 Why Niobium? Type T c H c1 H c H c2 Fabrication - K Oe Oe Oe - Nb II bulk, film Pb I electroplating Nb 3 Sn* II film MgB 2 II film Hg I / Ta I In I *) Other compounds with the same b-tungsten or A15 structure are under investigation as well. Pure niobium has the highest critical temperature T c among single elements, and H c1 and H c are both high. Low R s is needed for operation in superfluid helium at 2 to 4 K (typical accelerator operation domain). High theoretical Meissner state breakup field (Hsh~240 mt) for an ideal surface, which scales with Hc. Good formability is desirable for ease of cavity fabrication. Pure intermetallic compounds, like Nb 3 Sn with a critical temperature of 18.1 K, look attractive for possible 4.2 K operation at first sight as they are clean superconductors. However, so far the gradients achieved in Nb3Sn coated niobium cavities have been limited to below 19 MV/m, probably due to grain boundary effects in the Nb 3 Sn layer. Residual resistance is also high and magnetic flux management is an issue. Alloys are dirty superconductors due to their small mean free path and consequently have large BCS surface resistivity and poor thermal conductivity. High temperature superconductors have been tried in the past and showed very high surface resistances, problems arise from very low coherence length = sensitivity to defects, gap anisotropy etc. 18

19 Q(E) curve It is conventional to evaluate an SRF cavity performance using a Q(E) curve Intrinsic quality factor Q = G/R s Increase Q decrease required power Ideal performance? Typical ILC-prepared cavity at T = 2 K Increase max E acc decrease accelerator length Accelerating gradient E acc = Energy gain/cavity length P diss = E accl 2 R s = E accl 2 G R Q Q R Q L is the cavity length, R s is the average surface resistance R/Q is the cavity impedance (determined only by cavity shape) G is the cavity geometry constant 19

20 Q(E) evolution driven by science GHz, 2 K Q Elliptical Shape E acc (MV/m) 20

21 Q(E) evolution driven by science GHz, 2 K Q Bulk RRR > E acc (MV/m) 21

22 Q(E) evolution driven by science GHz, 2 K Q High Pressure Water Rinse E acc (MV/m) 22

23 Q(E) evolution driven by science GHz, 2 K Q C bake E acc (MV/m) 23

24 Q(E) evolution driven by science GHz, 2 K Q EXFEL E acc (MV/m) Typical ILC-recipe prepared cavity 24

25 Only thin surface layer matters Inner surface nanostructure within ~100 nm completely determines RF losses in the cavity Image from linearcollider.org RF fields Niobium ~3 mm Helium cooling RF fields Nb 2 O 5 <0.1% of thickness RF currents <100 nm Final treatment is crucial to performance 25

26 State of the art Q(E) curve ~5 years ago Increase Q decrease required power LFQS MFQS HFQS Increase max E acc decrease accelerator length Typical ILC-prepared cavity at T = 2 K State of the art until ~5 years ago 26

27 Recent SRF science breakthroughs & active areas of research 27

28 Nitrogen doping 28

29 Nitrogen doping: a breakthrough in Q Discovered while trying to investigate niobium nitride thin films on cavities. Q-factor improvement after N-doping up to 4 times higher Q than standard Nb cavities. Q MFQS Anti-Q-slope Typical Q vs E acc curve obtained with 120 C bake (standard ILC treatment); Avg Q with doping is 2-4 times state of the art; Example, for 1.3 GHz, 2 K, mid-field Q ~ 1.5e10 versus > 3e10; Systematically above Q obtained with any other surface treatment T= 2K E acc (MV/m) Injection of small nitrogen partial pressure at the end of 800 C degassing drastic increase in Q. A. Grassellino et al., Supercond. Sci. Technol. 26, (2013) Rapid Communications 29

30 Doping Treatment: small variation from standard protocol, large difference in performance XFEL FNAL doping for LCLS-II (major steps): o Bulk EP o 800 C anneal for 3 hours in vacuum o C nitrogen diffusion o 800 C for 6 minutes in vacuum o Vacuum cooling o 5 microns EP Ship to DESY HOM Tuning X 120C bake Leak Check Final Assembly Cavity after Equator Welding EP 140 um Short HPR External 20 um BCP Long HPR Ethanol Rinse VT Assembly 800C HT Bake Helium Tank Welding Procedure Long HPR RF Tuning EP 40 um Ethanol Rinse HPR 30

31 Surface post nitrogen bake, pre-ep: poorly SC nitride phases Flat Nb sample baked at 800C for 2 min with N2 + 6 min annealing Flat Nb sample baked at 800C for 20 min with N min annealing Bad (poorly SC) nitride phases that need to be removed via EP correlate with poor performance (pre-ep) Q~1e7 Few Nb-nitride features (Nb 2 N reflections) in Nb near-surface. Nitride teeth go ~0.2 μm deep. Pt layer Nb [113]+Nb2N [210]+? Y. Trenikhina, MOPB055, SRF15 31

32 Origin: reversed field dependence of R BCS R s T = R BCS T + R res Reverse field dependence of the BCS surface resistance component lowest R BCS. Lower than typical residual resistances (seems to zero all contributions but trapped flux). A. Grassellino et al, 2013 Supercond. Sci. Technol (Rapid Communication) A. Romanenko and A. Grassellino, Appl. Phys. Lett. 102, (2013) 32

33 Physics perceived BCS limit has been overcome Anti-Q-slope emerges from the BCS surface resistance decreasing with field. This was thought to be the lowest possible BCS resistance. A. Grassellino et al, 2013 Supercond. Sci. Technol (Rapid Communication) A. Romanenko and A. Grassellino, Appl. Phys. Lett. 102, (2013) N doping brings also lower than typical residual resistance < 2 nanoohms (non trapped flux related). 33

34 Nitrogen doping from research to production Shortly after its discovery, the nitrogen doping was adopted by LCLS-II project. After a short R&D period (Fermilab, JLab and Cornell), the recipe was successfully transferred to industry. Plot shows performance of SRF cavities from two prototype LCLS-II cryomodules (Fermilab and JLab): avg. Q = 3.6e10, avg. E acc = 22.2 MV/m highest average Q ever demonstrated in vertical tests of 1.3 GHz nine-cell cavities at 2 K, 16 MV/m. Cavities from vendors demonstrate similar performance. Higher Q would allow SLAC to use only one cryoplant (of purchased two) to run the machine and use the second cryoplant to support the energy upgrade of LCLS-II from 4.2 GeV to 8 GeV. Two drawbacks of N-doping: 1. Achievable accelerating gradient is lower than that of 120 C baked cavities (35-40 MV/m) 2. Nitrogen-doped cavities are more sensitive to trapped flux losses. 34

35 Next step in the cavity performance GHz, 2 K Nitrogen doping Q LCLS-II, PIP-II, PIP-III E acc (MV/m) 35

36 Do we understand how to choose best cavity treatment? R S 2 K = R BCS 2 K + R fl + R 0 M. Martinello et al, Appl. Phys. Lett. 109, (2016) Residual resistance: 4 nw: 120 C baked cavities 2 nw: EP and optimally N-doped cavities R Fl = B ext η S B ext : external magnetic field η: flux trapping efficiency N-doping modify the mean free path close to theoretical minimum of R BCS N-doping seems to increase the reduced energy gap D/k B T c Adding together all the R S contributions, it is possible to predict which treatments lowers Rs, taking into account also trapped flux Best compromise is given by light N-doping treatments 36

37 Nitrogen infusion 37

38 Pressure (Torr) Temperature ( C) Pressure (Torr) Temperature ( C) Pressure (Torr) Temperature ( C) Pressure (Torr) Temperature ( C) Nitrogen infusion: higher Q at higher gradients TE1AES SKC 900 o o o o Chamber pressure Cavity temperature Heat treatment: 800 C, 3 h in UHV 120 C, 48 h with N 2 at Torr Elapsed time (h) TB9AES SKC Bulk electro-polishing Chamber pressure Cavity temperature High T furnace: 800 C, Heat treatment: 3 hours, high vacuum 800 C, 3 h in UHV 120 C, 160 C, 48 h hours with N 2 at with -3 Torr N 2 (25 mtorr) No chemistry post-furnace HPR, VT assembly Slides on N-infusion are courtesy of A. Grasselino Composition and mean free path in first nanometers 10of cavity surface have been shown to be crucial for -1 TE1PAV SKC 900 both Q and gradient performance. 10N-doping at T > 800 C proven to manipulate mean -3 Cavity temperature 700 free path, but constantly throughout several 10-4 Heat treatment: 600 microns, giving 800 C, 3 high h in UHV Q. 120 C bake known to manipulate mean free path at 10 very near surface on clean bulk, and produce the 10 highest gradients. Therefore, it was decided to study how to better 10 engineer a dirty layer on top a clean bulk Nb, 10 using low 10 T 20nitrogen treatments aim to create a 10 few -1 TE1AES015 & TE1PAV SKC 900 to several Elapsed nanometers time (h) of nitrogen-enriched 10 layer on top of clean EP Chamber bulk, pressure to attempt to bring 10 together -3 Cavity temperature 700 the benefit of the Q and gradient Heat treatment: Nitrogen enriched nanometric layer to be created in 10 the C, 3 h in UHV 160 C, 48 h with N 500 furnace post 800 C treatment Torr when no oxide 10 is C, 96 h in UHV 400 present at the moment of injection of nitrogen at 10 low -7 T Studies aim also at fundamental understanding of 10 HFQS -9 and 120 C cure of high field Q-slope Elapsed time (h) Elapsed time (h) C, 48 h in UHV Chamber pressure

39 New potential breakthrough: very high Q at very high gradients with low temperature (120C) nitrogen treatment Results: ILC recipe vs. nitrogen infusion Same cavity, sequentially processed, no EP in between - Record Q at fields > 30 Achieved: 45.6 MV/m 194 mt, with MV/m Q ~ 2e10! - Preliminary data indicates potential 15% Q ~ 2.3e10 at E acc ~ 35 MV/m Repeatable increase of Q by a factor of boost two, increase in of gradient ~15% achievable quench fields Potential application ILC, significant - Can cost be reduction game of the machine. changer for ILC! 34 Slides on N-infusion are courtesy of A. Grasselino Alexander Romanenko FCC Week Rome 4/12/16 39

40 Kubo and Checchin models on bi-layer potentially increasing achievable accelerating gradients This idea is supported by Checchin (FNAL) and Kubo (KEK) models on bi-layer structure (e.g. dirty N-doped layer on clean Nb) claim that can enhance the achievable accelerating gradient. Ideal Depth of this layer? Can this trick help push beyond the 200 mt or achieve 200 mt with higher yield? We are investigating this empirically via low-t N-infusion (different T and durations) TTC@Saclay 40 In addition to the BL barrier, we have the second barrier due to the S-S boundary. The second barrier is also imperfect: easily weakened by defects. However, we have a second chance to stop the vortex penetration. The S-S bilayer structure defect defect 40 defect T. Kubo, TTC Meeting 2016

41 Exploring doping/infusion parameter space High Q 0 (e.g. LCLS-II) High Q 0 & High E acc (e.g. ILC) This is still very active area of research: o o o Nitrogen infusion at various temperatures / exposure times Doping with other materials Better understanding surface properties to get insight on how to nanoengineer niobium for different applications Slides on N-infusion are courtesy of A. Grasselino 41

42 Large parameter space T and duration being explored Q ~ 6e10 at 15 MV/m! Q > 3e10 at 31.5 MV/m! Slides on N-infusion are courtesy of A. Grasselino 42

43 Is the evolution nearly complete? ILC cost reduction Nitrogen infusion Q E acc (MV/m) 1.3 GHz, 2 K 43

44 Frequency dependence of R s 44

45 Frequency dependence of R s and non-equilibrium SC Mattis-Bardeen theory predicts quadratic frequency dependence of R s. However, the theory is valid only at zero fields. Does it need modifications when we consider field dependence? Another active research area. The following cavities were studied so far: N-doping 120 C baking Q EP/BCP 1.3 GHz, 2 K EP BCP 120 C baking E acc (MV/m) 2/6 N-doping Slides on frequency dependence are courtesy of M. Martinello 650 MHz 1.3 GHz 2.6 GHz 3.9 GHz 45

46 Normalized R T 2 K for 120 C Baking *Some measurements were admin limited between MV/m to avoid quench so, in order to compare the different curves, only data till ~20 MV/m are shown Slides on frequency dependence are courtesy of M. Martinello 46

47 Normalized R T 2 K for 120 C Baking At low field R T follows the 2 trend suggested by the Mattis-Bardeen theory Slides on frequency dependence are courtesy of M. Martinello 47

48 Q-factor of 2.6 GHz at high field tends to the one at 1.3 GHz Q-factor of 2.6 GHz cavity converge to the one at 1.3 GHz at high gradients 120 C baked cavities T=2 K Slides on frequency dependence are courtesy of M. Martinello 48

49 Normalized R T 2 K for N-doping Higher frequency leads to stronger anti-q-slope! Higher frequency is favorable for Q, and can be also for higher gradients. Understanding the reversal of R BCS with the RF field: o The non-equilibrium quasiparticle distribution driven by microwave fields o Need solid theoretical basis and measurements of some Nb properties M. Martinello et al, 49

50 Comparison in terms of Q-factor at 2.0 K T = 2 K Slides on frequency dependence are courtesy of M. Martinello 50

51 Comparison in terms of Q-factor at 2.0 K 1.3 GHz wins over 650 MHz at ~10 MV/m 3.9 GHz wins over 2.6 GHz at ~13 MV/m T = 2 K Slides on frequency dependence are courtesy of M. Martinello 51

52 Unprecedented medium field Q 0 at 3.9 GHz Q-factor of N-doped 3.9 GHz comparable to 120 C baked 1.3 GHz cavity at ~ 20 MV/m Further improvement with 900 C bake Q 0 ~ T = 2 K Slides on frequency dependence are courtesy of M. Martinello 52

53 Enabling future efficient HEP accelerators Q Nitrogen Infusion Q > 2e10 At high field E acc > 100 MV/m Q > 3e10 non-equilibrium SC? new materials? E acc (MV/m) Nitrogen infusion showed that it is possible to achieve both high Q and high gradient at the same time. We hope that further progress in SRF experiment and theory will allow us to achieve much better performance and enable new particle accelerators. 53

54 SRF at low T & low field (toward mk / single-photon scale): from accelerators to quantum computers 54

55 Renewed (now practical) importance of the LFQS and low T How will the best cavities we have behave at ultralow fields for various applications? Quantum computing / quantum memory Dark sector photons searches Gravitational effects search. These applications are interested in high Q at very low fields LFQS is present after all treatments What is the cause of the low field Q slope and what happens with Q as we decrease the field further LFQS A. Romanenko et al, Appl. Phys. Lett. 105, (2014) 55

56 tan d Quality Factor LFQS measurements toward quantum regime & TLS model Q measured using a single-shot method (decay from PLL state) 9x x10 10 T = 1.5 K Fit to TLS model Ec = 0.1 MV/m b = 0.19 Good news: LFQS stops below 0.1 MV/m with Q ~ 3x x10 10 Saturation of the Q decrease Previous models: Halbritter, Palmieri, Weingarten From 2D resonator world: nonlinear dissipation in two-level systems of an amorphous dielectric layer 3x x10 10 CW SS RBW=10 khz SS RBW=30 Hz E acc (MV/m) A. Romanenko and D. I. Schuster, Phys Rev Lett. 119, (2017) tand µ E -1 2 w R TT 1 2 E = 1 56

57 Effect of the oxide layer Thicker oxide (anodization) has a drastic effect at low fields Similar to how we characterize losses due to surface currents via the geometry factor, one can introduce a similar term, the surface participation ratio, characterizing dissipation due to surface dielectric losses 100 nm Nb 2 O 5 Nb 5 nm Nb 2 O 5 Nb A. Romanenko and D. I. Schuster, Phys Rev Lett. 119, (2017) 57

58 Next step: toward quantum regime Demonstration of T = 10 mk and <N> ~ 1 photon high Q in Large dilution refrigerator allows exploring fundamental physics of residual resistance of SRF cavities at very low temperature. First SRF cavity is being mounted inside the dilution refrigerator at Fermilab 58

59 SRF accelerators around the world ISAC CLS FRIB PIP-II/III EIC CESR CEBAF ATLAS LCLS-II MaRIE SNS ESS ALICE SPIRAL2 BESSY ELBE Soleil XFE LHC FLASH FCC L ALPI HIE-ISOLDE SARAF BEPC-II SC-LINAC ADS ISNS C-ADS ILC RAON CepC-SppC ANURIB TLS LIPAc cerl J-PARC Circular Linear HEP NP Light src USP ANU Oper n Produc n <10 cavities cavities cavities >1000 cavities Sam Posen 59

60 SRF projects in progress / planned ISAC CLS LCLS-II FRIB PIP-II/III EIC CESR CEBAF ATLAS MaRIE SNS ESS ALICEBESSY SPIRAL2 ELBE Soleil XFE LHC FLASH FCC L ALPI HIE-ISOLDE SARAF BEPC-II SC-LINAC ADS C-ADS ILC RAON ANURIB ISNS CepC-SppC TLS LIPAc cerl J-PARC Circular Linear HEP NP Light src USP ANU Oper n Produc n Planning <10 cavities cavities cavities >1000 cavities Sam Posen 60

61 Major SRF Projects in Progress Project No. of Cryomodules No. of Cavities LCLS-II 1.3 GHz LCLS-II 3.9 GHz SCLF Voltage (MV) FRIB /nucleon RISP /nucleon SNS-upgrade ESS LHC-HL 8 16 Total GV 61

62 Planned SRF Projects, Some? Project No. of Cryomodules No. of Cavities LCLS-II-upgrade KEK-ERL euv ERL Voltage (MV) FRIB-upgrade /nucleon erhic ERL PIP-II PIP-IIIa PIP-IIIb India SNS C-ADS Total GV 62

63 International Linear Collider Japanese Association of High Energy Physics (JAHEP) proposed the prompt construction in Japan of ILC as a Higgs Factory at 250 GeV. ICFA expressed its support for this ILC option. The Linear Collider Board estimates cost of 250 GeV starting point will be 40% less than the cost of 500 GeV TDR cost. A decision from the Japanese government is expected soon, in 2018? Continued SRF performance improvement R&D, cost reduction R&D and optimization R&D will be very beneficial. International Linear Collider (ILC) This will be the largest SRF project, >10 times E-XFEL 63 e + e - linear collider with Superconducting RF linac Baseline: s = 500 GeV (31 km) upgrade later to ~ s= 1 TeV (50 km),

64 Summary SRF science and technology is going through a period of Renaissance. There are several new research directions opened up in the last ~5 years, four of each I reviewed in this presentation: nitrogen doping, nitrogen infusion, frequency dependence of surface resistance, and SRF in quantum regime. Other research areas, which I did not have time to discuss, include: nature of losses due to trapped magnetic flux; study of flux expulsion; nonequilibrium superconductivity and ultimate gradient limit; Nb 3 Sn SRF cavities and other materials. Nb-based SRF accelerator technology is mature and became the technology of choice for many new SRF accelerators. BUT: there are still many problems that need attention and careful investigation. These are exciting times and the field needs more young and energetic researchers! There will always be ample opportunities for imagination, originality, and common sense. 64

65 Thank you! 65