Pushing SRF Into the Future Materials and Processes R&D

Transcription

1 Pushing SRF Into the Future Materials and Processes R&D Charles Reece It is not yet clear what surface properties are the most important for achieving high Q and high peak RF fields. The answer to this question will be provided by a careful correlation between microwave cavity measurements and surface studies on small samples processed at the same time. It is possible that suitable coatings may finally provide the best solution. A. Septier, SRF Workshop 1980, KfK

2 Pushing SRF Into the Future SRF for accelerators is approaching a watershed date Performance envelope of bulk Nb has reached theoretical limit flux penetration into the ~170 mt, H c1 (~42 MV/m) Note that top ~100 nm material determines SRF properties The challenge for bulk niobium is now Quality Assurance Refine material specs Refine process specs Engineer reliable peak performance Reduce costs Where is the future beyond the limits of bulk niobium? Deposited-film niobium for reduced system costs Deposited-film higher-t c materials, cheaper solutions for some applications Multi-layer films for higher fields and lower costs We have targeted R&D work in both domains

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4 Cavity Fabrication Start with adequate Cavity control? Surface roughness QC how rough? Nb crystal texture Surface chemical composition Minimized contamination Learning Feedback Cavity in Standard Starting State How to get there with active chemistry? Surface smoothing transform topography Remove damage layer Apply Prescribed Transform chemical composition Processing Protocol Green approach? surface melting Remove/prevent particle contamination Cavity in Intended Final State What matters? Surface roughness Nb crystal texture Cavity Test Surface chemical composition Tolerable contamination 35 MV/m, Q >8E9? Pass/Fail Top-level Cavity Process Map Specification of cavity starting conditions Specification of standard protocol & tolerances Specification of cavity final surface conditions TDP1 Sample and model work can greatly aid spec. development. The solution is a package deal Samples & 1-cell R&D Creece 9/12/08

5 Pushing SRF Into the Future - Nb As application requirements approach basic material limits, details take on new importance. Topography, local phase variations of the surface oxide, inadvertent precipitations, etc. matter in ways that we have previously not bothered to measure or control. We must understand better what matters and then control it. Example: topography Sharp topography promotes magnetic field penetration Local quenching results, impairing high field Q Smoother is Better What is smoother?

6 Q0 Q0 JLab cavities: Pushing SRF Into the Future - Nb LL Cavities - VTA Performance 1.E GeV Project Spec LL 29 Watts LL001 LL002 LL003 LL004 1.E+11 HG-6 7/25/2006 Old Performance 9/17/2008 After ~30 melectropolish 1.E+10 1.E+10 LL 29 W T = 2.0K 1.E+09 T= 2.07 K Eacc (MV/m) 3/28/05 cer 1.E Eacc (MV/m) High-field Q drop unresponsive to bake attributed to localized losses due to early magnetic flux penetration at sharp features. Common characteristic of BCP-treated fine-grain Nb cavities We must improve understanding and control of topography

7 Pushing SRF Into the Future - Nb Etching rate varies with local chemical potential influenced by crystal plane, strain, defect density. Buffered Chemical Polish (BCP) is etching. Can produce pits and sharp edges out of planar surface Large grain Nb, BCP etched. Note pattern of pitting. Fresh understanding made possible by the new large-grain niobium. In progress High resolution SEM images. (W&M) Electropolishing at too high temperature (>35C) can add an etching process.

8 First Quantitative Measure of Nb Surface Smoothing with Controlled Electropolishing Ground 30 min EP Roughness on this scale matters in high RF field 60 min EP 90 min EP Work directly links the applied process to surface changes

9 F - % F - % New Understanding of Nb Electropolishing Diffusion Layer (~ um) Sulfuric acid anodizes the Nb - Nb 2 O 5 Nb Bulk Electrolyte HF acid dissolves the Nb oxide with the "at the surface" concentration of F - F - concentration at the surface is limited by how fast it diffuses through the electrolyte ( ~diffusion layer). Distance This is not an electrostatics problem. Nb 2 O 5 Compact Salt Film (~ nm) H. Tian Distance Journal of The Electrochemical Society, D563-D The local gradient in F - concentration produces the desired polishing action. Obtained first temperature-dependent F - diffusion constant measurements enables analytical modeling Local temperature, flow (stirring) & electrolyte composition affect the local F - gradient.

10 Electropolishing What are we really doing? Hydrodynamic thermal modeling reveals out-of-control temperatures(> 35C) We are the only ones building simulation models linked to experimental data Feedback to ILC EP activity >> control the temperature move fluid slowly Using these tools to engineer more efficient cavity polishing systems (e.g., ICP with VEP)

11 Cavity Fabrication Start with adequate Cavity control? Surface roughness QC how rough? Nb crystal texture Surface chemical composition Minimized contamination Learning Feedback Cavity in Standard Starting State How to get there with active chemistry? Surface smoothing transform topography Remove damage layer Apply Prescribed Transform chemical composition Processing Protocol Green approach? surface melting Remove/prevent particle contamination Cavity in Intended Final State What matters? Surface roughness Nb crystal texture Cavity Test Surface chemical composition Tolerable contamination 35 MV/m, Q >8E9? Pass/Fail Top-level Cavity Process Map Specification of cavity starting conditions Specification of standard protocol & tolerances Specification of cavity final surface conditions TDP1 Sample and model work can greatly aid spec. development. The solution is a package deal Samples & 1-cell R&D Creece 9/12/08

12 Pushing SRF Into the Future Beyond Nb Limits Low-cost, low-frequency (<700 MHz) channel-cooled 4.5 K systems Deposited-film niobium Much cheaper cryomodules Vital for muon collider? Compact, cheap low-loss accelerators Deposited-film higher-t c modest fields Further reduce cryo heat load and/or raise frequency Compact, very high gradient (>60 MV/m), pulsed accelerators Multi-layer films Exploit the predicted properties of (Gurevich) SIS multi-layers to reach past niobium surface field limits All appear possible; all require new technology development

13 Pushing SRF Into the Future The Challenge We don t know how to grow great SRF films. The potential applications motivate their development. Once appropriate growth parameters are known, engineered coating systems, will be straightforward. Cf. modern electronics and optical industries So, we focus limited resources on parameterizing and understanding film growth characteristics and measure srf properties of affordable samples. Build collaborations with materials experts Establish film characterization toolkit/network Integrate srf/film growth/characterization insights Tighten the process > material > properties feedback loop

14 Biased extraction of Nb ions from ECR plasma Growing Candidate Films Local opportunities HIPIMS with in-situ RHEED in Lukaszew lab at W&M Laser gas nitriding Pulsed laser deposition New JLab Multi-technique Deposition System Deposition Techniques for metals, compounds and insulators: High Power Impulse Magnetron Sputtering (HIPIMS) & Self-Sputtering DC & RF Magnetron Sputtering Reactive Sputtering Ion Etching & Ion Beam Assisted Deposition

15 Characterizing Candidate Niobium Films Nb Inverse Pole Figure Initial film quality assessment is via T c and RRR measurement. Nb thin films deposited by energetic condensation (cathode arc) yield high RRR value (>100) Plus a suite of other techniques Electron backscattered diffraction (EBSD) is used to probe materials crystal structure in 40nm depth, which is ~RF penetration depth. EBSD found a Nb thin film coating on copper substrate has large crystal grain size (~20um), close to that of bulk Nb.

16 R s *Ω+ RF Surface Impedance Characterization (SIC) 7.5 GHz Flat 50mm diameter sample RF-sampled area 0.8cm 2 Sample is thermally isolated from the cavity Cavity and sample temperature can be independently controlled Goal: Measure thin-film R 0~150 mt, 2~20k Successfully commissioned with first Nb 2 mt, expect 20 mt soon. The queue for sample testing will quickly grow Initially designed by J. Delayen, L. Phillips & H. Wang Refined and commissioned by PhD student Binping Xiao (W&M) 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 First R s vs T for Bulk Nb Sample at 7.5GHz BCS T [K]

17 Pushing SRF Into the Future Invited talks Beijing 2007 H. Tian Novel Characterization of the Electropolishing of Niobium with Sulfuric and Hydrofluoric Acid Mixtures Recent XPS Studies of the Effect of Processing on Nb SRF Surfaces AMVF Tutorial SRF Material Other Than Niobium JLab workshop 10 talks MSU workshop 10 talks, 4 invited Workshop organizers: Hosted 1 st and 3 rd International Thin Film Workshops 2004, 2008 Nb Materials Workshop MSU Oct Papers submitted in the past two years Graduate students engaged W&M 3 ODU 2 (+1) UVA 1

18 Pushing SRF Into the Future Who Niobium optimization Hui Tian postdoc, FT Xin Zhao postdoc Charles Reece Michael Kelley Andy Wu Rongli Geng Curtis Crawford Marjia Raškovid ODU grad student Liang Zhao W&M grad student Chen Xu W&M grad student Thin films Anne-Marie Valente-Feliciano, FT Xin Zhao postdoc Larry Phillips Daniel Bowring UVA grad student Raja Singuravalu ODU grad student Michael Kelley SIC system Binping Xiao W&M grad student, FT (Plus occasional help from several others, two open tech positions)

19 Nb process improvements W&M (EP chemistry, mat l charact.) VT (EP chemistry, mat l charact.) ODU (Plasma etching, NbN) NSU (Mat l charact.) KEK (Sample prep) Boston U (NSLS XPS) NSLS X1B (NSLS XPS) NCSU (SIMS) CNU (EP chemistry) Black Labs LLC (Mat l charact., EP) CEA Saclay (EP) ANL (Mat l charact., sample prep.) Wah Chang (Sample prep) Collaborators SRF thin film developments W&M (Mat l charact., film nucleation) ODU (TEM, {ALD}) NSU (Mat l charact.) Alameda LLC (Sample prep, SBIR) INFN-Legnaro (Sample & cavity prep) INFN-Roma (Sample & cavity prep) PSU (MgB2 {sample & cavity prep}) LBNL (Sample prep) Black Labs LLC (Mat l charact.) MSU (Mat l charact.) ANL ({ALD sample prep}) FSU {Flux penetration theory} MIT {Film growth theory}

20 Pushing SRF Into the Future The challenge for bulk niobium is now Quality Assurance We are defining the quality standards, deepening understanding, and improving cavity performance. Where is the future beyond the limits of bulk niobium? We know where we want to go it will be worth the effort We have sharpened our tools and assembled a viable team We are making progress

21 Supplemental slides

22 EP : a variety of representative of starting surface condition used in cavities Different CBP Nb sample from KEK Iris EBW Equator EBW FG J1 J4 J7 J10 Nb samples - Jlab LG J2 J5 J8 J11 SC J3 J6 J9 J12 Light BCP Grind LG FG 30 µm EP treatment; EIS, AFM and Profilometry measurements have been done, and data analysis ( +PSD) are underway.

23 Rq KEK FG Sample 5 ( 6 hrs large stone CBP) nm Zmax m Rq Z KEK FG Sample 2 ( 6 hrs large stone +6 hrs middle stone1 + 6hr middle stone hrs fine stone) nm max 1.6 m Rq nm after 30 µm EP at 30 C Rq nm Better starting surface decreases the polishing time, and achieves best Dir finishing R&D Rev. 3/09 at 30 cer C

24 A single-crystal Nb sample with EBW. CBP as per KEK cavity treatment. Part of JLab EP topography study. FY09 Process R&D at JLab - 1 Determine the Topographical Transformation Function (TTF) for controlled standard BCP and EP processes. This image was captured by a multi-focus function provided by Hirox microscope. ( about 100 steps) Mechanically ground, CBP, and EBW surfaces, both fine and large grain Nb. (W&M, KEK) Detailed analysis of representative topographical defects with incremental controlled EP. (W&M) The distance between the edge of EBW zone and the deepest site of EBW zone was estimated about 134 microns. How will this structure evolve with incremental EP steps? Develop technique for topographic replica from cavity defect region. (W&M) Quantify the dependence of the EP TTF on surface flow rate and Nb surface temperature. (W&M) Compare BEP (EP with lactic acid) with standard EP electrolyte. Correlate the dynamic impedance response (EIS) during EP with changing surface roughness. (W&M) H. Tian

25 Electropolishing? Distinguish etching from polishing Etching is chemically-driven material removal Local removal rate depends on local chemical conditions. Etching of crystalline materials is anisotropic BCP will not yield smooth surfaces except on defect-free single crystals Electropolishing (ideally) is diffusion-limited material removal Variations in local chemical potential have negligible effect Concentration gradient of the diffusion-limited species produces the desired leveling effect

26 Study of Alternative EP Electrolyte: BEP Vertical BEP system Major Benefits: 1. Smooth surface BEP 21nm BCP 1274nm better RF performance 2. Extremely high removal rate BEP 4.09µm/min EP 0.38µm/min potential huge reduction in cavity fabrication cost A.T. Wu 1.0E E+10 Q 0 1.0E+09 "Fansteel" Cavity BEP_Test Baseline Test BEP without baking MV/m Quench Current Status: 1. BEP on large grain cavity reaches 32MV/m 2. BEP on regular fine grain cavity reaches 23MV/m (quench limited) 3. The inner surfaces of cavities look very smooth indeed for both large and fine grain cavities after BEP 4. Next, compare with standard EP

27 Surface Evolution with Controlled Treatments The general trends of average roughness of Nb surfaces with BCP or EP material removal have been clear for some time. One would like to understand: Saito PAC 2003 The role of lateral scale of the evolving topographical structure and its correlation with crystallographic structure, if any. The effect that particular topographical features have on SRF performance. How to engineer processes to obtain desired performance.

28 SRF Process and Materials R&D Green Support exploration of dry and/or green cavity surface processing techniques partnering with others (SBIRs & universities) Cost/MV Plasma ion etching (ODU) HF-free electropolishing (W&M, VT, Black Labs, CNU) Laser surface processing Essentially chemical free and non-contacting Fundamentals are well established adaptation required Melting/glazing to smooth topography Laser gas nitriding PLD We have equipment, expertise, and partners Etching rate of 1.7 ± 0.2 m/min was demonstrated: 3% Cl 2 in Ar Input power density 2.5 W/cm 3 Pressure in reaction chamber 1200 mtorr Surface smoothness comparable to BCP not yet optimized In collaboration with M. Raškovid, J. Upadhyay, L. Vuškovid, and S. Popovid Department of Physics - Center for Accelerator Science, ODU JLab, Accelerator Division supports M. Raškovid and J. Upadhyay through fellowships.

29 SRF Process and Materials R&D Cleaning Objective 2: Specification of a cavity surface cleaning protocol which efficiently leaves the rf surface free of field emitting particulates to surface field of 100 MV/m. Maximum G, Q, Cost/MV Improve characterization of cavity surface cleaning techniques Improve on present rough, empirical HPR protocols Improved understanding could enable intentional system engineering. Quantitative assessment of cleaning effectiveness is proposed staff and student projects. Nb 2 O 5, metallic particulates HPR - time, distance, nozzle, angle, pressure CO 2 snow - time, distance Ultrasonic detergent/upw To begin this summer

30 JLab UHV Multi-Technique Deposition System for Engineered SRF Film Surfaces A unique, versatile thin film deposition system Designed to enable rapid exploration of the production parameter space of: Nb films Alternative material films like NbN, NbTiN Multilayer structures S-I-S Investigation of the A. Gurevich super-critical field model 2 self-sputter magnetrons 3 RF/DC 2 magnetrons 1 RF/DC Ion source (2 beam) Deposition with Ar, Kr or Xe or in vacuum. Multiple sample stages with heating capabilities (500ºC) and shutters NEG chamber RGA with differential pumping Deposition Techniques for metals, compounds and insulators: High Power Impulse Magnetron Sputtering (HIPIMS) & Self- Sputtering DC & RF Magnetron Sputtering Reactive Sputtering Ion Etching & Ion Beam Assisted Deposition Presently under Commissioning

31 SRF Process and Materials R&D - BCP Objective 1: Specification of starting Nb surface condition and a chemical processing protocol which reproducibly and economically avoids all SRF cavity field performance limitations due to surface roughness to 170 mt surface fields. Maximum G, Q, Cost/MV A. Improve understanding of topographic effects of standard BCP etching Pit (sharp edge) creation mechanism via crystallographic defects Rediscovered via use of large-grain Nb Performance-limiting effects at high surface magnetic fields Quantify the implications for fine-grain Nb and large-grain Nb Candidate cause of Q-drop in many BCP d cavities (unresponsive to baking) In progress

32 Study of Etching Pits on a BCP Large-Grain Single Cell SRF Accelerator Cavity Equato r Quench location 12 samples (9 hotspots and 3 cold spots) were cut from the cavity by milling for surface analysis Photos of samples High resolution SEM micrograph revealed anisotropic etching pits at tricrystal grain boundary Etching pits could be in grain. Cold spots have very low density or just a few etching Dir pits, R&D compared Rev. 3/09 to cer hotspot and the quench spot

33 Etching Pits have sharp geometrical feature and they might stem from dislocation anisotropic etching

34 SRF Process and Materials R&D BCP>>EP Dramatic performance improvement due to elimination of many BCP-induced microscopic sharp edges?