Transcript Is a 1-5m thick & gas tight PVP overcoat feasible? Barry McQuillan

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 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
The reference process for overcoating
has 2 fluid exchanges
Overcoating fills in the 1-3 mm foam pores,
to make a smooth diffusion barrier
DEP DVB
DEP filled
foam shell
PVP
IPA
DEP to IPA
solvent exchange
IPA to liquid CO2
exchange,
then warm to
remove CO2(g)
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
Consistently see imploded shells
or 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
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
The good overcoated shell fails to survive
The leaking overcoat seems to survive!
DEP coming out creates a
pressure drop across wall
At start,
DP =0 across wall
DP >0 across wall
DP causes 3 bad features: •Nucleate a bubble
–Damages foam
•Crack the overcoat
–No longer continuous
•Shrink or implode the shell
What is the magnitude of the
buckling 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.
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), 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
k=0.25
k=0.125
k=0.05
foam crush
0.14
0.060% IPA
0.030% IPA
0.12
DP (atm)
If IPA diffuses in at
rate similar to DEP out,
DP is relieved and
target does not
buckle if you wait!
k=0
0.1
0.08
0.06
0.04
0.02
0
0
50
100
150
200
time (arb)
250
300
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 DP 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?
PVP
DVB foam
GDP
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?
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