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Polyimide Windows for MagLIF
Experiments
J. Sin - presenter
T. Bernat, C. Castro, J. Hund, A. Pastrnak, O. Stein, N. Petta
Target Fabrication Meeting
Las Vegas, Nevada
March 2017 Work supported under DOE
contract:
DE-NA0001430
Outline
• There is a long history of measurements on PI windows
- Burst pressures
- Deflection measurements
• We are testing higher pressure windows for MagLIF experiments
• Our higher burst pressures don’t follow a simple extrapolation from
earlier lower pressure data
• Analysis of deflection data following Bhandarkar et al. effectively
predicts window deflection with pressure
In 1997 Luxel published burst pressures for
thin PI windows below ~ 30 psi
This data fits a single interpolation formula over the entire range
Interpolation graphs
Bhandarkar et al.
LLNL-Luxel
TFM 2015
A couple more recent points can be added to the
1997 Luxel room temperature data
The new points fall very close to the interpolation
line for a 500 nm thick window
Bu
rst
pre
ssu
re -
psi
diameter - mm
0.964 microns
0.5 microns Interpolation
0.3435 microns
0.1024 microns
Film deflection and creep has been explored
by LLNL-Luxel (2015)
• 3.94mm ≤ window ID ≤ 5.5mm
• 0.208um ≤ thicknessfilm ≤ 2.94um
• 0 PSI ≤ pressure ≤ 90 PSI
Parameter Range
S. Bhandarkar et al., “Constitutive models for viscoelastic behavior of polyimide membranes”,
Fusion. Sci. Tech. 70, p332, Aug./Sept. 2016
Schafer has recently fabricated and tested
windows at higher pressures
• Magnetized Liner Inertial Fusion (MagLIF) experiments are being
conducted at SNL and LLE
• Windows are required to hold up to 14 atm (~205 PSI)
• Two issues
- Windows should be as thin as possible
- Knowing the window deflection may be important
• LEH windows use a 1.8um thick film
• Washer ID = 3mm
• Tested at pressures ≥ 60 psi;
- Window adhesive is in
“shear” mode
Sandia’s MagLIF targets operate at “medium”
pressures
SNL MagLIF LEH window PI Film
Washer
3 mm
bulge height
• Washer with 1.8um thick PI film caps parylene tube at one end
• Washer ID = 0.4mm
• Pressurized with 11 to 14 atm (165 - 205 psi);
- Window adhesive is in “peel” mode
LLE’s Small MagLIF Targets require higher
pressures
Small MagLIF assembly 400 um
Laser
PI Film
Washer
We tested various window parameters
Shear Orientation: Film
side facing down
Peel Orientation:
Film side facing up
• Size of ID
• Orientation
• Film Formulation
- Schafer PI “α”
- Schafer PI “β”
Ø3mm, 0.4mm
Computer
controlled
Regulator/
transducer
Pressure
source
Controller
Labview interface Window mount
m-e optical
displacement
transducer
Ballast volume
Data was taken with a computer controlled system
Our test procedure ramped pressure over ten
minutes and held it for 50 minutes.
Burst tests were carried out during a subsequent pressurization
• Typical bulge test ramp
Burst pressures of both films at 3mm IDs are
clustered closely together
A PI “β” window at 0.4 mm diameter did not burst up to 400 psi.
13 samples
5 samples
Our data at higher burst pressures deviates
substantially from the extrapolated values
Extrapolated values for
1.8 mm thick windows
Lower bound:
did not burst at 400 psi
Earlier data range
However, since the LLE 0.4 mm windows are required to go to only 200
psi, we could reduce the thickness with the PI “β” formulation
Bu
rst
pre
ssu
re -
psi
Diameter - mm
Bulge heights are also clustered closely together
3 mm window
0.4 mm window
6 samples
13 samples 6 samples
5 samples
• We used the method of Bhandarkar et al. with a modified
expression for high strains:
• Average deflection from 3mm windows is used to fit the data for
modulus curves
- The curves were fit to a
6th order polynomial rather
than a piece-wise fit
• The moduli of the two
formulations are similar
in shape but quite different
in magnitude
Data from 3mm ID windows was used to evaluate
strain and effective modulus
Instead of
The moduli shapes differ from the LLNL-Luxel data
particularly at low strains
• With the washer curved to prevent a stress riser, the effective
window ID changes with deflection
• This is especially pronounced at smaller deflections
A possible explanation for moduli differences
is the 3 mm window bonding
Effective window ID is
changing as film “wraps”
around the curve
• Used moduli from 3mm ID to predict bulge height of 0.4mm ID
windows pressurized at 200 psi
Bulge predictions using the effective modulus fit
quite well to the 0.4 mm window deflection data
Summary: Schafer is fabricating PI windows for
higher pressure MagLIF experiments
• Burst and deflection of PI films were tested at smaller ID’s and
higher pressures
• We fabricated and tested two different formulations of PI film
• Burst pressures are substantially different from earlier data in the
high-pressure regime
• Using the method of Bhandarkar et al., we measured and fit
effective moduli for two polyimides
• The moduli accurately predict window deflections with pressure