Be Coating on Spherical Surface for NIF Target Development

H. Xu, J. Wall, and A. Nikroo General Atomics 3550 General Atomics Court San Diego, CA 92121

Beryllium Targets are essential for Inertial Confinement Fusion because of high x-ray absorption with stable implosions It requires ~160μm Be/Be(Cu) coatings.

10 5

300 eV design

10 4

1000 915 900 850 845 840 760 radii in µm (not to scale) DT 0% 0.35% 0.70% 0.35% 0% Cu dopant atom %

10 3 10 2 10 1 10 0 10 -1 1 MJ graded Be:Cu design

Graded Be NIF spec Graded CH NIF spec

2 4 6 8 10 2 4 6 8 100 mode number 2 4 6 8 1000 better stability for Be capsule

Magnetron Sputtering is One Way of Making Be Coatings on Sphere Mandrels Vacuum Chamber Be sputter targets Cu sputter target CH mandrels voltage bias Piezoelectric oscillator Bounce pan - energetic deposition beam - low temperature - relative high rate

Be Coatings Characterized by X-ray Radiograph and SEM 0.4-0.5μm/hr

27µ m CH Be

X-ray radiograph shows ~100μm Be coating and inside CH mandrel.

SEM showed columnar grain growth of Be coatings.

Higher Coating Rates Lead to Twisted Grain Structure 0.75 μm/hr 0.5 μm/hr More directional beam at higher power plays a role here.

Be Coating Density Measurements

ρ = M/V Be shells Density (g/cc)+3% Be050325, 49μm 1.74

Be041206-2, 95μm 1.77

Be050414, 43μm Be050420, 53μm 1.74

1.74

94-95% bulk density

TEM showed low void density and small void size voids Low void density and small void size are consistent with density measurements, which indicate Be films are a few percent from bulk density.

Roughness Evolution versus Thickness

1000 100 y = 4.88x

1.1908

y = 9.5272x

0.7931

2004 2005 Power (2004) Power (2005) 10 1 0 20 40 60 Thickness (um) 80 100 120 Roughness RMS ~ d β and β is 0.8-1.2, which is consistent with shadowing dominated roughening.

Evidence of Self-shadowing

FIB cross section 20º 40º 60º Striations seen at high magnification are consistent with shadowing effects. J. Appl. Phys. 91, 1963 (2002)

Film Growth Structure Zone Model Microstructure of sputter-deposited coating

T/T m ~0.25

Structure Zone Model (SZM) proposed by Thornton (1977) At 3.7-10mTorr and deposition temperature of 130ºC-150ºC (T/T Zone I and Zone II.

m ~0.25) microstructure of our Be film is consistent with a transition regime between

Challenge for full thickness capsule-beyond thin film regime

• • • •

High density up to 150μm thickness: repeatedly demonstrated at ~50μm; what will be the density at ~150μm?

Some problems for getting thicker films (100-150μm): agitation, stoppage etc.

power law predicts rougher surface with increasing thickness; one approach is to divide full thickness growth into a few thinner layer growth and apply polishing/ion etching at each interfaces.

Smooth surface finish: polishing has been demonstrated to improve surface finish. High accuracy film density and doping level measurements.

~100μm coating with columnar grains ~100μm coating with nodular growth

Summary

• • • • •

Be coatings on sphere mandrels have been demonstrated.

Columnar grain growth was obtained up to 100μm.

Higher density columnar structure has been demonstrated repeatedly up to 50μm.

Film roughening mechanism on spherical surface is analyzed and believed to be dominated by shadowing effect.

Future efforts are focused on obtaining full thickness NIF capsule with high density and smooth finish.