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LLNL-PRES-748677
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC
Superconducting Thin Films For Levitation of NIF Targets3rd Workshop on Microwave Cavities and Detectors for Axion Research
Alex BakerPost-Doctoral Appointee
August 22, 2018
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NIF Places Fuel Capsules Inside Hohlraums…
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▪ Fuel capsule must be supported within center of hohlraum
▪ Tents can cause perturbations that affect the implosion
… but Capsule Supports Cause Perturbations
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Levitation Offers an Alternative
Earnshaw’s Theorem: a collection of point charges cannot be maintained in a stable stationary equilibrium configuration solely by the electrostatic interaction of the charges
Holding capsule in place without supports would allow fully symmetric implosion
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Levitation Offers an Alternative
Earnshaw’s Theorem: a collection of point charges cannot be maintained in a stable stationary equilibrium configuration solely by the electrostatic interaction of the charges
Holding capsule in place without supports would allow fully symmetric implosion
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▪ Expelling the magnetic field enables levitation
Superconducting Levitation
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Upward
Lorentz
Force
▪NIF is already surprisingly compatible with superconducting levitation:
✓Targets at ~20K within chamber
✓Shape of capsule is favourable for levitation
➢Coat with thin layer of low Z material
Bringing Levitation to NIF
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▪Simple binary compound
▪Range of fabrication routes
▪Low Z
▪High Tc (39 K),
▪High Jc (~1e9 A/cm2 at 1 T),
▪High Hc2 (>50 T)
Magnesium Diboride
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Fabrication of MgB2 Thin Films
Film Characterization
Glassy Carbon Substrates
Boron Sputter Deposition
Mg Vapor Anneal at 850ᵒC
Vary film thickness
Vary dwell time
Vary temperature ramp
Tw
o-s
tep s
ynth
esis
SQUID-magnetometry
Rutherford Backscattering
Raman Spectroscopy
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Film Thickness and Temperature Profile Matter
▪ Faster temperature ramps and shorter dwell times give higher Tc
and sharper transition
Bulk
MgB
2
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Film Thickness and Temperature Profile Matter
▪ Faster temperature ramps and shorter dwell times give higher Tc
and sharper transition
Bulk
MgB
2
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Rutherford Backscattering Probes Composition
▪ Films are chemically homogeneous
▪ Oxygen distributed throughout the film
▪ Films tend to be slightly Mg-rich
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▪ Thinner films have significantly higher oxygen concentrations
Oxygen Depresses Tc
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▪ Thinner films have significantly higher oxygen concentrations
▪But this is not the whole story
Oxygen Depresses Tc
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▪Defects and inclusions suppress Tc
▪Originates in non-epitaxial growth, mitigated by faster thermal processing
Raman Spectroscopy Reveals Disorder
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Surface Roughness Must be Controlled
100µm
600 nm, low magnification
1µm
▪ Thicker films are blister and peel
▪Needle-like surface texture
600 nm, high magnification
160 nm, low magnification 160 nm, high magnification
100µm 1µm
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Surface Roughness Must be Controlled
100µm
600 nm, low magnification
1µm
▪ Thicker films are blister and peel
▪Needle-like surface texture
▪ Thin films are smooth, < 1nm rms.
600 nm, high magnification
55 nm, low magnification 55 nm, high magnification
100µm 1µm
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▪Deposition on spheres will require bounce pan and rotating tube furnace
▪ Early attempts to measure levitation were a mixed success, but have plans for custom modification to PPMS in coming year
▪ Studying ion irradiation to enhance critical current
Next Steps
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▪ L. Bimo Bayu-Aji, Dante O’Hara (summer student), John Bae (GA), Elis Stavrou, David Steich, Scott McCall, Sergei Kucheyev
Acknowledgments
LDRD 17-ERD-040
Goal: Demonstrate a path to ideal ICF capsule support based on quantum levitation
Approach: Thin films of superconducting MgB2
“Vapor annealing synthesis of non-epitaxial MgB2 films on glassy carbon”, A A Baker et
al (2018) Supercond. Sci. Technol. 31 055006
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▪Measured in plane of film
▪Good zero field values, but drops rather precipitously.
▪Hc2 ~ 20 kOe
▪ Simulations predict ~103A/cm2 required
Critical Current Limits Levitation