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Some Potential Solutions to the Overcoat Survival Issue

Some Potential Solutions to the Overcoat Survival Issue. Barry McQuillan & Reny Paguio HAPL Project Review Livermore CA June 20-21,2005. IFT/P2005-073. We do not yet have a consistent overcoat process. Develop the PVP overcoat (1-5 m m) process high yield & reproducible

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Some Potential Solutions to the Overcoat Survival Issue

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  1. Some Potential Solutions to the Overcoat Survival Issue Barry McQuillan & Reny Paguio HAPL Project Review Livermore CA June 20-21,2005 IFT/P2005-073

  2. We do not yet have a consistent overcoat process • Develop the PVP overcoat (1-5 mm) process high yield & reproducible • Seek uniform adherent PVP overcoat on DVB foam shell • We have made ~ 40 batches: • About 80% are imploded shells (most in IPA) • About 20% are spherical, permeation tests show not gas tight • We theorize the implosions are from osmotic pressure>buckling pressure • We have a more refined understanding of the process to prevent implosions • We have alternative paths to achieve overcoat

  3. DEP DVB The reference process for overcoating has 2 fluid exchanges Overcoating fills in the 1-3 mm foam pores, to make a smooth diffusion barrier PVP IPA DEPfilled foam shell DEP to IPA solvent exchange IPAto liquid CO2 exchange, then warm to remove CO2(g)

  4. A fundamental problem arises with the DEP-IPA exchange We are seeing imploded shells, or broken overcoat DEP to IPA solvent exchange We estimate the osmotic force at the DEP-IPA exchange, leads to large pressure drop across the overcoat - We propose some potential solutions

  5. Consistently see imploded shellsor spheres which don’t hold gas An imploded shell during exchange may be evidence of continuous overcoat No Ar signal observed in permeation test of dry spherical target The good overcoated shell fails to survive The leaking overcoat seems to survive!

  6. DEP coming out creates a pressure drop across wall At start, DP =0 across wall DP >0 across wall • Nucleate a bubble • Damages foam • Crack the overcoat • No longer continuous • Shrink or implode the shell DP causes 3 bad features:

  7. What is the magnitude of thebuckling pressures for IFE shell? DP = 2E (w/r)2/(3(1-0.342))1/2 ~ 0.365 [Cr2] (w/r)2 Assume foam wall bears the DP - Schwendt et al, Fusion Science & Technology, 43, 217,2003. • DP = 0.062 atm (0.91 psi) • Petzoldt estimate for E = 0.76 Mpsi, DVB foam, 100 mg/cc, HAPL 2003 & 2004. • GA ICF program routinely fills shells with pressure steps of about 1 psi.

  8. What Is the osmotic pressure? • = (RT/V*)(moles IPA/moles DEP) • P = 0.0643 atm • Shells immersed in 0.020 vol % IPA in DEP • P = 0.0965 atm • Shells immersed in 0.030 vol % IPA in DEP DEP will continue to come out, until the pressure across the wall reaches these values

  9. If IPA diffuses in (slowly), DP is relieved Immerse DEP filled shells into 0.030 vol% IPA/DEP k = Perm(IPA)/Perm(DEP) If IPA is impermeable or very slow target buckles If IPA diffuses in at rate similar to DEP out, DP is relieved and target does not buckle if you wait! 0.060% IPA 0.030% IPA

  10. Proposed experiments for DEP/IPA • Measure fundamental factors to design exchanges which will not buckle target • What is permeation rate of DEP, IPA, and CO2 thru PVP overcoat? • Make films for IPA and CO2; measure DEP leaving shells • What is the buckling pressure of the DVB foam? Smaller the pressure, longer the exchange • Put overcoated shells in variety of IPA/DEP to see when implosions stop • Other methods?

  11. Can we avoid DEP/IPA exchange? • Can we remove DEP from overcoated shells, with CO2 directly? • Has been done for 1 mm OD RF shells, but long times • DEP not very soluble in CO2 • There is a similar DP issue in performing the CO2 extraction. Permeation rates of DEP coming out and liquid CO2 going in across overcoat will be important.

  12. Can viable targets be made with GDP coated microcracks? One solution: overcoat PVP with GDP and let GDP be continuous film • Can we develop a consistent process? • How thick a GDP layer will seal the microcracks, and is this GDP thickness tolerable to target design? • Will target be smooth enough? DVB foam PVP GDP

  13. We have moved to new strategies • Fundamental factors have been identified, and need to be quantified • The DEP/IPA exchange is still viable • yet not as easy as first thought. • We can design a process, once fundamental factors are quantified • Paths around the DEP/IPA problem are also viable

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