Some Potential Solutions to the Overcoat Survival Issue

Barry McQuillan & Reny Paguio

HAPL Project ReviewLivermore CA

June 20-21,2005

IFT/P2005-073

We do not yet have a consistent overcoat process

• Develop the PVP overcoat (1-5 m) 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

The reference process for overcoating

has 2 fluid exchangesOvercoating fills in the 1-3 m foam pores, to make a smooth diffusion barrier

DEP filledfoam shell

DEP to IPA solvent exchange

IPA to liquid CO2 exchange, then warm to

remove CO2(g)

PVP IPADEP DVB

A fundamental problem arises with the DEP-IPA exchange

We estimate the osmotic force at the DEP-IPA exchange, leads to large pressure drop across the overcoat

- We propose some potential solutions

DEP to IPA solvent exchange

We are seeing imploded shells, or broken overcoat

Consistently see imploded shellsor spheres which don’t hold gas

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

An imploded shell during exchange may be evidence of continuous overcoat

No Ar signal observedin permeation test of dry spherical target

The good overcoated shell fails to surviveThe leaking overcoat seems to survive!

DEP coming out creates a pressure drop across wall

At start, P =0 across wall P >0 across wall

•Nucleate a bubble–Damages foam

•Crack the overcoat–No longer continuous

•Shrink or implode the shell

P causes 3 bad features:

What is the magnitude of thebuckling pressures for IFE shell?P = 2E (w/r)2/(3(1-0.342))1/2

~ 0.365 [C2] (w/r)2

Assume foam wall bears the P - Schwendt et al, Fusion Science & Technology,

43, 217,2003.

• P = 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.

What Is the osmotic pressure?

= (RT/V*)(moles IPA/moles DEP)

• = 0.0643 atm– Shells immersed in 0.020 vol % IPA in DEP

• = 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

If IPA diffuses in (slowly), P is relieved

Immerse DEP filled shells into 0.030 vol% IPA/DEPk = Perm(IPA)/Perm(DEP)

If IPA is impermeableor very slowtarget buckles

If IPA diffuses in atrate similar to DEP out,P is relieved andtarget does not buckle if you wait!

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 50 100 150 200 250 300

time (arb)

( )P atm

=0k =0.25k =0.125k =0.05k

foam crush

0.030% IPA 0.060% IPA

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?

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 P issue in performing the CO2 extraction. Permeation rates of DEP coming out and liquid CO2 going in across overcoat will be important.

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?

GDP

PVP DVB foam

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