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