IFE Target Fabrication Update Presented by Jared Hund

IFE Target Fabrication Update Presented by Jared Hund 1 J. Bousquet 1, Bob Cook 1, D. Goodin 1, R. Luo 1, B. Mc. Quillan 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, CA HAPL Workshop Princeton, New Jersey December 12 -13, 2006 IFTP 2006 -154

The current HAPL target design is a 4. 6 mm foam capsule Thin (300 -1200 Å) High Z coating 5 m CH Overcoat Foam + DT mr ~2 . 3 m DT Vapor ad 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 • Buckle Strength*: Material Constant w = coating thickness r = radius This term is similar for most types of polymers that can be used Wall thickness (μm) Elastic (tensile) modulus (E) of various polymers Alternate form from Roark* sugests buckle may be even less *Roark and Young, Formulas for Stress and Strain (1982) Buckle Pressure (atm) Calculated Buckle Strength of Parylene Polymer E (kpsi) Polystyrene 260 -490 Polyimide 189 - 580 Parylene 348 Topic #1 Reduced ΔP

The buckle strength of DVB shells with thick coatings has been measured Buckle Data of GDP/PVP Coated DVB Shells • Buckle Strength: Material Constant Curve fits based on buckle equation 4. 1 mm dia w = coating thickness r = radius 4. 6 mm dia • The Burst Strength is higher: The buckle pressure of a HAPL target will be ~0. 1 atm* *assuming no foam contribution Topic #1 Reduced ΔP ~2 -5 atm 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 CO 2 drying DEP IPA DEP – diethyl phthalate IPA – isopropyl alcohol IPA Osmotic Buckle Pressure CO 2 Buckle and (venting) Burst Pressures GDP or Parylene Coating Buckle and Burst Pressures If we can control Δpressure better we may improve gas retention Topic #1 Reduced ΔP

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

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 Osmotic Pressures 1) Pressurize with liquid CO 2 CO Pressure vessel vial (l) 2 CO 2 (g) IPA 3) Refill liquid CO 2 4) Repeat steps 2&3 (~25 x) IPA/CO 2 mix shells 5) Heat CO 2 to supercritical fluid (90 atm, 38°C) 2) Drain liquid CO 2 6) Vent CO 2 (SCF) Topic #1 Reduced ΔP Pressure Differentials Vent rate ~9 hrs corresponds to ~3 atm burst pressure

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 Vent Backpressure regulator CO 2 (l) Vent Sample chamber Liquid drain Dead volume A 29 hour vent is required so that no more than a 1 atm buckle pressure is applied Topic #1 Reduced ΔP

By creating a smoother under coating, we may be able to improve gas retention Coatings currently investigated: • 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 PVA benzoyl chloride Melamineformaldehyde None Resorcinol Isophthalyol Hydroxyethyl cellulose isophthalyol dichloride 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 Hydrogen bonding value diethyl phthalate 0. 0 dimethyl maleate 8. 5 11. 8 Shells wet 50 μm Shells dry not yet dry 50 μm More interfacial polymerization experiments are underway 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 PVP/GDP Dual Layer Gas Retention thicker than specification Gas Retention to hold gas Yield 5 µm PVP GDP DVB Current Spec Cross section of coated DVB shell Topic #3 Top layer coating

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 Parylene overcoated PVP/DVB shell 0. 5 μm SEM of a parylene overcoated DVB shell 1 mm Topic #3 Top layer coating stalk

Smoothness specification is also a challenge • • The smoothness specification is 50 nm 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 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 Wyko Expose to solvent vapor Wyko re-measure Topic #4 Smoothing

The solvents tried either had no surface effect, or wicked into the foam and compromised the shell * Roughness data is reported in rms (nm). Solvent Toluene Shell. Top layer RMS* Before Dichlorohexane CH 2 Cl 2 RMS* After Before After DVB-PVP 612 1920 434 444 DVB-PVP-GDP 326 1610 323 320 RF-GDP 79 1320 728 739 GDP alone 18 15 RMS* Before After 79 1320 18 16 It is unlikely a suitable vapor smoothing solvent can be found for GDP or PVP. Topic #4 Smoothing

The best surface finish on foam capsules so far is on resorcinol formaldehyde shells • Roughness Spec can be Power Spectrum of GDP Coated met on solid polymer RF 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 300 -1000 nm RMS over patches ~200 x 300 μm Topic #4 Smoothing ~900 μm dia shell

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)