materials science and technology: condensed matter and thermal physics simulation of direct drive...

Materials Science and Technology: Condensed Matter and Thermal Physics

Simulation of Direct Drive Target Injection into ‘Hot’ Chambers

James K. Hoffer & Timothy R. Gosnell

Condensed Matter and Thermal Physics Group, Material Sciences Division

Los Alamos National Laboratory

Presented at the Laser IFE Workshop

U. S. Naval Research Laboratory

February 6, 2001


Overall Objective…………. Response of target materials to injection stresses

FY 01 Deliverables……….. 1. Start experiments to measure DT yield strength and modulus.

2. Start experiments to measure effect of rapid thermal load on layered target

PI Experience………………. Extensive experience with DT and DT layering (POC: A. Nobile)Proposed Amount………….. $ 400 k

Relevance of Deliverables [X] NIF…………………… Research in materials in NIF targets [ ] Laser RR Facility…. [ ] Other DP/NNSA…… [X] Energy……………… Needed for injection into chamber

Related OFES activities…… Design equipment for thermal stress ($ 50 k)

Target Injection-2: Target Materials Response - LANL


There are two major building blocks to produce a ‘simulation’:

• ‘Simulating’ a DT filled target:– there are two routes:

• spherical target filled & layered with solid DT (not really a surrogate at all, but an actual target!)

• cylindrical surrogates that permit direct heating

• both the above are routinely done at LANL

• Simulating a hot chamber:– again there are two main routes:

• heating of a spherical or cylindrical shroud to high temperatures– (difficult to do quickly and the temperature will be ramping)

• illumination of a cold spherical shroud or ‘integrating sphere’ to duplicate black-body radiation from a hot shroud

– easy to do quickly but difficult to match the IR spectrum– choice of window materials critical to IR spectrum on target


By using a spherical beta-layered target, rapid heating of a metallic shroud can simulate ballistic injection of a prototype beta-layered direct drive target:


Both shroud heating and IR illumination concepts have practical limitations:

• The highest power that can be practically applied to a shroud in a conventional cryostat is only a few kW– thermometry is difficult if the mass is low– high heating rates imply high thermal ramping rates and the

shroud temperature will significantly overshoot the desired value, leading to target runaway and loss of primary containment (an important ES&H concern!)

• With IR illumination, a true black-body spectrum will be difficult to achieve– the source spectrum may not be black-body– window materials will cut off the far IR


Heat input to a layered D2 target due to Stefan (black-body) radiation from a hot surface:(as a function of the emissivity ecold of the cold surface)

Thot (K)

0.02 0.05 0.1 0.2 0.4 0.6 0.8 1

300 0.001 0.002 0.005 0.009 0.018 0.028 0.037 0.046600 0.015 0.037 0.073 0.147 0.294 0.441 0.588 0.735900 0.074 0.186 0.372 0.744 1.488 2.232 2.976 3.7201200 0.235 0.588 1.176 2.351 4.703 7.054 9.406 11.7571500 0.574 1.435 2.870 5.741 11.482 17.223 22.963 28.7041800 1.190 2.976 5.952 11.904 23.809 35.713 47.617 59.521

Thot (K)

300 90000 36000 18000 9000 4500 3000 2250 1800600 5625 2250 1125 562 281 187 141 112900 1111 444 222 111 56 37 28 221200 352 141 70 35 18 12 9 71500 144 58 29 14 7 5 4 31800 69 28 14 7 3 2 2 1

Q (watt/cm2)

__________________________ ecold __________________________

ts (ms)

Survival time (based on the Omega result:10 seconds @ 300 K, ecold = 0.18)

Tcold = 18.0 K; hot surface area >> target surface area

times in red will not survive a 50-ms-long injection process!


100

101

102

103

104 specific intensity (erg s

-1 sr -1 cm-1)

102 4 6 8

1002 4 6 8

10002 4

frequency (cm-1

)

5

4

3

2

1

0

abso

rptio

n co

effic

ient

(cm

-1)

estimated

infrared spectrum, solid nD2

~100 m to melt @ 300 K~4 s to melt @ 1000 KT = 1 K, 100% of ice meltedshell diameter = 0.93 mmice thickness = 96 m

300 K Planck

1000 K Planck

The solid D2 fuel by itself is transparent to much of the black-body radiation, especially at 300 K:


Solid DT is equally transparent to black-body radiation, especially at 300 K:

2.0

1.5

1.0

0.5

0.0

abso

rptio

n co

effic

ient

(cm

-1)

102 4 6 8

1002 4 6 8

10002 4

frequency (cm-1

)

100

101

102

103


-1 sr -1 cm-1)

estimated infrared spectrum, solid DT(25% D2, 50% DT, 25% T2)

~160 m to melt @ 300 K~4.5 s to melt @ 1000 KT = 1 K, 100% of ice meltedshell diameter = 3.4 mmice thickness = 360 m

300 K Planck

1000 K Planck


If all heating is solely due to IR absorption of DT ice, (i.e., none due to the shell wall) very long survival times are expected (for the NRL IFE target):

10-1

100

101

102

103

104

105

time

(s)

200015001000500

radiation temperature (K)

time to melt (absorption by DT ice only)T = 1 K, 100% of ice meltedshell diameter = 3.4 mm, ice thickness = 360 m


We have just measured an IR spectrum on ‘Kapton’ brand polyimide, using a 13 micron-thick-film:

6000

5000

4000

3000

2000

1000

0

abso

rptio

n co

effic

ient

(cm

-1)

50004000300020001000

frequency (cm-1

)

infrared spectrum, Kapton @ 300 K


6000

5000

4000

3000

2000

1000

0

abso

rptio

n co

effic

ient

(cm

-1)

102 4 6 8

1002 4 6 8

10002 4

frequency (cm-1

)

100

101

102

103


-1 sr -1 cm-1)

infrared spectrum, Kapton

350 cm-1

to 4000 cm-1

~4.5 s to melt @ 300 K~56 ms to melt @ 1000 KT = 1 K, 100% of D2 ice melted

shell diameter = 0.93 mmshell thickness = 9 mice thickness = 96 m

1000 K Planck

300 K Planck

Now we may calculate a ‘lifetime’ based on absorption only in a Kapton plastic shell and compare that to the Omega experiment:


0.01

0.1

1

10

100

1000

time

(s)

200015001000500


time to melt (absorption by Kapton only)T = 1 K, 100% of DT ice meltedshell diameter = 3.4 mmshell thickness = 10 mice thickness = 360 m

For the NRL IFE target, the times are longer:


10-2

10-1

100

101

102

103

104

105

time

(s)

200015001000500


time to meltT = 1 K, 100% of ice meltedshell diameter = 3.4 mmshell thickness = 10 mice thickness = 360 m

absorption by DT ice only absorption by Kapton only

Here we compare DT to Kapton absorption for the NRL IFE target:


What is the appropriate experiment to investigate target ‘lifetimes’?

• Question: Does the IR absorption of the DT layer play a significant role?– target lifetimes based on DT absorption are ~ 1000 times

longer than observed at 300 K at Omega!! – lifetimes based on polyimide absorption are ‘about right’– hence, the total IR absorption appears to be dominated by

the plastic shell

• Answer: No!– therefore, heating of the DT layer and consequent behavior

can be adequately simulated by adding heat directly to the outside surface

– our present apparatus, with an appropriately designed cylindrical or toroidal layering chamber, is well suited for this type of study


The information necessary for more accurate calculations is the infrared absorption spectra of plastics, but not that of DT.

• We need to measure the IR absorption spectrum of actual plastics at low temperatures:

12x103

10

8

6

4

2

0

abso

rptio

n co

effic

ient

(cm

-1)

4 5 6 7 8 91000

2

frequency (cm-1

)

infrared spectrum, Kapton

350 cm-1

to 2000 cm-1

300 K low T?

– IR absorption in plastics is known to be strongly temperature dependent:

– we must accurately measure the near-IR spectrum, less accurately the far-IR, and not neglect the visible

– we can use simple slabs of material

– we may need additional operating funds (far-IR optics are expensive!)


If we know the effects of heating to near the triple point of DT, then we can adequately calculate ‘lifetimes.’

• Longer lifetimes (up to ~10 x) are possible if we allow for a portion of the DT ice to melt.

• Do we know if the act of melting will affect the shape of a DT layer in free fall?– the most accurate experiment may have to be done on the

space shuttle or the ‘vomit comet.’

• We cannot simulate melting in zero-g, but we can rapidly heat DT layers and look for stress-induced irregularities in the solid.– With the addition of a high-speed, high-precision camera,

our present apparatus will be sufficient for this type of study


Using direct heating, we need to assume a value for the surface emissivity, but thereafter, both opaque shells and foam filled shells can be investigated.

Empty torusside view

(windows not shown)

Filled with foam, bored out to yield a 75 micron-thick layer at the waist

Filled with DT and beta-layered to yield a solid

layer 100 microns thick.

Using an offset foam bore, the thickness of DT

above the foam varies.

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