IFE Target Fabrication Update Presented by Jared Hund 1 J. Bousquet 1, Bob Cook 1, D. Goodin 1, R. Luo 1, B. McQuillan 1, R. Paguio 1, R. Petzoldt 1, N. Petta 2, N. Ravelo 1, D. Schroen 1, J. Streit 2, B. Vermillion 1, W. Holloway 3, N. Robertson 3, M. Weber 3 1 General Atomics, Inertial Fusion Technology, San Diego, CA 2 Schafer Corporation, Livermore, CA 3 UC San Diego, San Diego, CA HAPL Workshop Princeton, New Jersey December 12-13, 2006 IFT\P

The current HAPL target design is a 4.6mm foam capsule DT Vapor Foam + DT Thin ( Å) High Z coating ~ 2.3 mm rad 5  m CH Overcoat DT Foam layer: ~0.18 mm divinyl benzene (DVB) We have demonstrated basic feasibility of the foam shell (Aug 06) The current challenge is developing the HAPL specified CH coating –Gas tight –Smooth (50 nm RMS)

Achieving this is a hard problem because Low buckle and burst strength of shells Impacts Fabrication Permeation Filling Layering Covering large pores of DVB –Foam has pores of ~1μm width that coating must cover Smoothness –Related to covering porous structure

Current strategies for improving the CH overcoating 1.Keep the interfacial coatings from breaking –Reduce Δpressure in interfacial polymerization fabrication (PVP) Osmotic pressure: Solvent exchanges – eliminate IPA step Better control pressure drops in CO 2 dryer 2.Improve 2 layer coating by making a better interfacial layer – modify chemistries to better cover large pores and make a smoother interface for dual layer coating 3.“Repair” damage to the interfacial coating layer –Parylene coating 4.Smoothing – make everything smoother in the end

A challenge of fabricating a continuous overcoat is the low buckle strength of any 5μm polymer coating Material Constant w = coating thickness r = radius Buckle Strength*: This term is similar for most types of polymers that can be used Calculated Buckle Strength of Parylene Topic #1 Reduced ΔP Buckle Pressure (atm) Polymer E (kpsi) Polystyrene Polyimide Parylene348 Elastic (tensile) modulus ( E ) of various polymers Wall thickness (μm) Alternate form from Roark* sugests buckle may be even less *Roark and Young, Formulas for Stress and Strain (1982)

The buckle strength of DVB shells with thick coatings has been measured 4.1mm dia 4.6mm dia Topic #1 Reduced ΔP The buckle pressure of a HAPL target will be ~0.1 atm* ~2-5atm The Burst Strength is higher: *assuming no foam contribution Buckle Data of GDP/PVP Coated DVB Shells Curve fits based on buckle equation Material Constant w = coating thickness r = radius Buckle Strength: S = tensile strength

There are several process steps that contribute to pressure differentials across the capsule wall The early process steps can create microcracks that are “healed” with GDP Dual Layer Process PVP coating Solvent exchange IPA Osmotic Buckle Pressure CO 2 drying Buckle and (venting) Burst Pressures GDP or Parylene Coating DEP – diethyl phthalate IPA – isopropyl alcohol If we can control Δpressure better we may improve gas retention DEP IPACO 2 IPA CO 2 Buckle and Burst Pressures Topic #1 Reduced ΔP

The solvent exchanges (DEP to IPA) can generate huge pressure differences across overcoat.  P osmotic is the pressure difference which stops flow across the overcoat –Assuming DEP diffuses much faster than IPA:  P osmotic = 85 atm (  X/X DEP ) X DEP = mole fraction of the diffusing solvent (DEP);  X = X(inside) - X(outside) X DEP (1-X) IPA X-  X DEP ( 1-X+  X ) IPA DEP flow IPA flow One needs very small steps of  X/X DEP Exact diffusion rates are unknown To be absolutely safe, long exchange times- >400 days could be needed Topic #1 Reduced ΔP It is best to avoid DEP-IPA-CO 2 ; go from DEP to CO 2 directly

Coated capsules are more sensitive to pressure changes in the CO 2 drying process than bare foam shells Possible problem steps: In step 2, bubbles nucleated in the liquid-and possibly in foam/overcoat Steps 2-4 Osmotic pressure (CO 2 diffusion vs. IPA diffusion) Step 6 is a vent that can subject the shells to a large pressure differential IPA CO 2 (l) 1) Pressurize with liquid CO 2 3) Refill liquid CO 2 4) Repeat steps 2&3 (~25x) 2) Drain liquid CO 2 CO 2 (g) 5) Heat CO 2 to supercritical fluid (90 atm, 38°C) 6) Vent CO 2 (SCF) Osmotic Pressures Pressure Differentials IPA shells Vent rate ~9 hrs corresponds to ~3 atm burst pressure Pressure vessel vial IPA/CO 2 mix Topic #1 Reduced ΔP

The CO 2 dryer has been recently improved to minimize pressure differences across the shell walls An automated venting system reduces the delta P at final vent to prevent bursting A dead volume avoids bubble nucleation cause of buckling A 29 hour vent is required so that no more than a 1 atm buckle pressure is applied Sample chamber CO 2 (l) Dead volume Vent Liquid drain Backpressure regulator Vent Topic #1 Reduced ΔP

By creating a smoother under coating, we may be able to improve gas retention Shells are being fabricated using several interfacial chemistries Organic reactant can play a role in reaction speed Literature* suggests that the properties of the solvent can effect surface finish *Fusion Technology 31, 391 (1997) Polymer Coating Organic reactant PVP isophthalyol dichloride Polyvinyl alcohol (PVA) isophthalyol dichloride PVA sebacoyl chloride PVAbenzoyl chloride Melamine- formaldehydeNone ResorcinolIsophthalyol Hydroxyethyl cellulose isophthalyol dichloride Coatings currently investigated: Topic #2 Improve Interfacial Layer

To study the effect of solvent on the PVP coating, 3 solvents with different solubility parameters were chosen The original solvent was p-chlorotoluene. Shells wet Shells drynot yet dry p-chlorotoluenediethyl phthalatedimethyl maleate Hydrogen bonding value More interfacial polymerization experiments are underway 50 μm Topic #2 Improve Interfacial Layer

Our baseline method has been to create an interfacial polymerization layer and cover with GDP Poly vinyl phenol (PVP) covers the porous foam and glow discharge polymer is deposited on top To date, this technique requires coatings much thicker than specification to hold gas PVP/GDP Dual Layer Gas Retention Gas Retention Yield Topic #3 Top layer coating Current Spec Cross section of coated DVB shell PVP DVB GDP 5 µm

Parylene is an alternative coating or secondary coating for repairing damage in under layer Advantages: –Covers dry shell, so no problems with solvent exchanges or drying –Only one pressurization (venting) step at end of process –More conformal than GDP –Can be used as a coating over interfacial polymerization layer (similar to PVP/GDP) Disadvantage –Will it be able to meet smoothness spec? –Sticking during coating? –Others? Topic #3 Top layer coating

Stalk mounted DVB shells have been test coated with parylene Initial coated capsules collapsed due to fast vent (good sign that the shells hold gas) Now have better control over vent rate so that more overcoated shells survive Gas testing in progress 0.5 μm SEM of a parylene overcoated DVB shell 1 mm stalk Parylene overcoated PVP/DVB shell Topic #3 Top layer coating

Smoothness specification is also a challenge The smoothness specification is 50nm RMS (over lengths of 50 to 100 μm) Possible ways of meeting spec 1.Make an inherently smooth coating 2.Vapor smooth the coating 3.Mechanical polish Topic #4 Smoothing

A series of basic vapor smoothing experiments were performed Wyko Expose to solvent vapor re-measure Vapor smoothing is a process in which a solvent is used to swell the polymer to help asperities sink back into the surface due to surface tension. Basic experiment of solvent effects on dry, coated shells Topic #4 Smoothing Wyko

The solvents tried either had no surface effect, or wicked into the foam and compromised the shell Solvent Shell - Top layer TolueneDichloro- hexane CH 2 Cl 2 RMS* Before RMS* After RMS* Before RMS* After RMS* Before RMS* After DVB- PVP DVB-PVP- GDP RF -GDP GDP alone It is unlikely a suitable vapor smoothing solvent can be found for GDP or PVP. * Roughness data is reported in rms (nm). Topic #4 Smoothing

The best surface finish on foam capsules so far is on resorcinol formaldehyde shells Roughness Spec can be met on solid polymer shells without post coating smoothing Creating a smooth coating on rough foam substrate is more difficult DVB coated with PVP, GDP/PVP or parylene is typically nm RMS over patches ~200 x 300 μm Power Spectrum of GDP Coated RF shell ~900 μm dia shell Topic #4 Smoothing

Timeline – what’s next? Reduce delta p –Will have sets of shells though the new drying process by February 07 Alternate interfacial polymerizations –PVP solvent experiments: Jan 07 Parylene testing –Coating tests (stalk mounted): Jan-Mar. 07 If promising results work on freestanding coated shells HAPL scale RF shell –Fabricate and GDP coat first set of HAPL scale shells: Feb. 07

Conclusion We are refining our process to reduce the delta pressure –The baseline design has a 0.1 atm buckle strength (extrapolated from data) We are evaluating alternate interfacial chemistries Trying new ways to repair coatings in 2 layer process - (GDP, Parylene) We have evaluated chemical smoothing –Result: Not feasible for PVP or GDP on DVB shells Trial fabrication of small pore foam with a single layer overcoating - (GDP on RF)