VG-1 4/9/08

A Very Brief and Selective Historical Overview of ICF

Capsule Fabrication

Robert Cook Consultant for

General Atomics

Presented at 18th High Average Power Laser Program Workshop

Los Alamos, New MexicoApril 9, 2008

VG-2 4/9/08

• ICF history is full of examples of seemingly insurmountable problems that were solved, sometimes by a totally new approach, sometimes by incremental changes.

What does the history of capsule fabrication tell us?

• The story is constantly changing - both the designs and the fabrication methods.

The current HAPL foam shell development has come a long way - due to advances in

fabrication AND changes in design - but still has a ways to go.

• A firm theoretical understanding of a process generally leads to solutions, and it puts realistic limits on what we can expect from a technology.

• Incremental improvements of existing technologies are important since they cause us to look closely at a process. The understanding that comes from this examination often is what leads to breakthroughs.

• In the ICF capsule area, fabrication and characterization capability have been closely coupled to design, certainly since the early 90’s.

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Quick timeline

1990

2 mm shellsfor NIF

droplet generatorsize control

MICROENCAPSULATION

1972 1980

sorting!!!

drop towerplasticshells

glassshells

Nova0.5 mmcapsules

plasma polymerization

2008

CH, Be, andpolyimidecapsules

foam shells

interfacialtechniques

2000

decomposablemandrels

X

HAPL sizeR/F foam

shells

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• J. Nuckolls, L. Wood, A. Thiessen, and G. Zimmerman, “Laser Compression of Matter to Super-High Densities: Thermonuclear (CTR) Applications,” Nature 239, 139 (1972).

In the beginning…..

- Only a very rudimentary “Design” initially involving glass shells

- Shells available commercially - a huge sorting problem

- Fortunately characterization capability was limited…,.

- M. L. Hoppe, “Large Glass Shells from GDP Shells,” Fusion Technol. 38, 42 (2000).

- Size problem largely solved many years later with a totally new glass shell production method….

- Development of dedicated ICF glass shell towers…. - limited size (not a problem for early ICF

experiments)

from J. H. Campbell, J. Z. Grens, and J. F. Poco, “Preparation and Properties of Hollow Glass Microspheres for Use in Laser Fusion Experiments,” LLNL Report, 1983.

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• The science for thin wall plastic shell fabrication via spray drying existed.- Drop tower methods were perfected for ICF capsule mandrels.

- Size limitation maybe up to 1 mm, most work done at 0.5 mm (Nova Scale).

- Still required lots of sorting….

In the early 80’s the designers found that lower Z - plastic shells - were better

• With the development of surface characterization (SphereMapper - mid 90’s) we found that shells were in fact quite smooth (good thing!) - and the game changed…

AFM

Capsule

AFMtip

Vacuumchuck

Air bearingrotary stage

AFM -3000

-2000

-1000

1000

2000

3000

360270180900

0nm

R() equatorial traces

0.001

0.01

0.1

1

10

100

1000

1 10 100 1000k = mode number

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• Plasma polymerization techniques developed from the late 70’s to current times allowed for the deposition of a thick smooth ablator.

Nova indirect drive capsules required thick (~50 µm) ablators.

H2 + C4H8

70 mTorr

e -

e -

40 MHzRF

Bouncer

Glasstube

Microshelltargets

H.

G. W. Collins, S. A. Letts, E. M. Fearon, R. L McEachern, and T. P. Bernat, “Surface Roughness Scaling of Plasma

Polymer Films,” Phys. Rev. Lett. 73, 708 (1994).

• Our understanding of the deposition - and the conditions that result in very smooth coatings on well defined substrates - is quite advanced.

S. A. Letts, D. W. Myers, and L. A. Witt, “Ultrasmooth Plasma Polymerized Coatings for Laser Fusion Targets,” J.

Vac. Sci. Technol. 19, 739 (1981).

K. C. Chen, Y. T. Lee, H. Huang, J. B. Gibson, A. Nikroo, M. A. Johnson, and E. Mapoles, “Reduction of Isolated Defects on Ge Doped CH Capsules to Below Ignition

Specifications,” Fusion Sci. Technol. 51, 593 (2007).

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• NIF capsules were 4 times larger than Nova capsules - drop tower methods for mandrels simply didn’t work. Several approaches were explored.

The jump to NIF shells offered quite a few challenges.

Microencapsulation is the basis for all capsule targets in both the ICF and IFE

programs

• Microencapsulation was found to be the best answer.

aqueous coreremoval in

vacuum

PMS Shell

b) aqueous bath cure - loss of solventc) shell hardening

a) µ-encapsulation of PMS solution

- An old industrial technique, but developed by the Japanese for ICF.

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How was the first surface specification arrived at for NIF capsules?

10-2

10-1

100

101

102

103

104

105

nm2

2 4 6 810

2 4 6 8100

2 4 6 81000

mode

Nova Capsulesaverage power spectra forcapsules grouped by rms

over modes 10-1000

<15-20 nm rms>11 capsules

<10-15 nm rms>18 capsules

<5-10 nm rms>14 capsules

<20-30 nm rms>8 capsules

NIF Design(rms=9.1 nm)

• In 1999 I analyzed all of the power spectra data that had been acquired on Nova (0.5 mm) capsules used in experiments.

• It was a serious question whether this could be achieved on the larger NIF shells, particularly at lower modes.

NIF Design(rms=9.1 nm)

• The initial design surface finish requirement for NIF shells (2.0 mm) was based on best Nova shells ever shot.

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10-3

10-2

10-1

100

101

102

103

104

105

po

we

r (n

m2 )

10 100 1000mode number

NIFdesign

Mode 2asymmetry

AfterBefore

The early 2-mm microencapsulated shells had problems throughout the power spectrum.

• Mode 2 asymmetry > 5 µm (OOR ~ rn/ n = 2 or 3) Solved by increasing the interfacial tension with PAA (Takagi).

• Mid mode problems were thought to be due to convection cells in the rapidly curing shell wall. Solved by significantly slowing down the curing rate (McQuillan).

Mid modebump

• High frequency roughness was due to surface debris from the curing bath. Solved by an improved washing technique.

High frequencyroughness

The first two of these were NOT incremental changes but were motivated by a theoretical understanding of

relevant parameters.

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Early concern about vacuoles and wall thickness variations in microencapsulated PMS shells led to the development of the “Decomposable Mandrel” technique.

• This was a key development and later was essential in the development of both polyimide and Be shells because the plasma polymer shell is thermally stable.

Plasma polymer

Completed plasticshell

300 °C

Decompose andremove PMS

mandrelPlasma polymer

coater

PMS mandrel

• This is the reason that the NC for NIF capsules is negligible.

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• ~2.2 mm diameter

The current NIF Ge-doped CH shells have very stringent specs.

• 165 µm thick ablator with 4 different Ge-doped layers

Fill tube 10 µm ODLaser drilled fill hole 5 µm

10-2

10-1

100

101

102

103

104

105

power (nm

2)

10 100 1000

mode

rms = 13 nm

rms = 7 nm

Plastic ShellSurface Specifications

mandrels

fullthickness

shells

modes 13-1000

• Outer surface specs are very tight:

VG-12 4/9/08

The NIF capsules meet the surface specs...

10-3

10-2

10-1

100

101

102

103

104

105

power (nm

2)

2 3 4 5 6 7 8 910

2 3 4 5 6 7 8 9100

2 3 4 5 6 7 8 91000

mode number

surface specification for Ge-doped NIF capsules

representative mandrels

power spectra for 9 shells from one batch

Meeting this goal is the result of 35 years of work, a number of key new technologies as well as often slow incremental

improvements to existing technologies, and a good interaction between fabricators and designers.

…and all other specs as well!

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• Suggested first by Sachs and Darling (1987) to symmetrize the fuel - at that time they were thinking about liquid DT!

The history of foam shells is shorter.

• At LLNL there had been a great deal of work on resorcinol/formaldehyde(R/F) low density foams. In the mid 90’s LLNL produced the first R/F foam shells.

• GA now fabricates ~1 mm R/F foam shells for Cryo experiments at Rochester.

• First foam shells fabricated by Takagi, et al. in Japan in early 90’s. He used a methacrylate chemistry, resulting in ~1-2 µm cells and thus opaque shells.- The basic technique for the foam shell formation was microencapsulation

- An interfacial technique was developed to apply a solid outer layer to the shells

• Foam shell fabrication has been extended to a number of different chemistries.

Current effort is to coat HAPL size dry R/F foam shells with GDP

• Work in 2006 indicated that the formation of an outer, gas tight coating by interfacial techniques on HAPL shells was problematic.- Subsequent solvent exchange steps created huge osmotic pressures

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What are the important differences in the fabrication of NIF capsules and HAPL foam shells?

• The GDP coating on NIF shells is on a very smooth substrate - the foam surface is much rougher.

• HAPL shells are twice as large, low mode symmetry will be worse.

• The microencapsulated wall is much thicker for HAPL foam shells, making control of NC more difficult.

• The R/F foam shell microencapsulation system is inverted relative to the NIF mandrel system (and the DVB foam shell system).- The bath additives controlling the interfacial tension are poorer (currently).

inner oildroplet

outer oil bath

inner aqdroplet

outer aq bath

PMS/oil solution

R/F aq solution

R/F foam shellsPMS mandrels

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What are the biggest near-term challenges for meeting the HAPL foam shell specifications?

• Gas tightness and solid layer thickness.

- Optimization of the GDP coating process should address these specifications.

• Out-of-Round, low mode symmetry.

- Controlled during the microencapsulation process by the interfacial tension, , between the aqueous R/F droplet and the oil bath.

- Current results are consistent with OOR ~ r2/.

- The primary handle is - current experiments are focusing on this.

• High mode surface finish.

- However recent experiments at higher coating pressure suggest something new is happening to the foam surface - our ability to find out how it works is critical.

- Experience is that a GDP coating mimics the substrate, and since the substrate is foam, not a smooth solid shell, a rough coating surface will result.

• Non-concentricity problems.- May be helped when OOR is reduced.

- New dielectrophoresis (DEP) technology is intended to address the problem.

VG-16 4/9/08

Summary

The history of ICF is filled with many examples of “insurmountable” problems that have been solved by a combination of new fabrication methods and

steady improvements in existing technologies.

HAPL foam shell development has built effectively on the existing technology, and good

progress has been made due to advances in fabrication AND changes in design.

The remaining challenges will yield to new ideas and increased understanding of our

technology.