Thermal Control Techniques for Improved DT Layering of Indirect Drive IFE Targets

John E. Pulsifer and Mark S. TillackUniversity of California, San Diego

Dan T. Goodin and Ron W. PetzoldtGeneral Atomics

IFE Target Layering: Indirect Drive • Layering with capsule already assembled in the hohlraum is

advantageous– Eliminates necessity of “layering spheres”

– Eliminates rapid assembly of hohlraum after layering

• Requires highly uniform DT surface temperature (~100K) for up to several hours

– Need a tightly controlled temperature profile on hohlraum

Determine required temperature profile and suggest method(s) for implementation• Heater array on staging tubes

• Tailor target material properties in our favor

Statement of Work

Target Design

• Close-coupled, distributed radiator heavy ion target

• Materials:

From Nuclear Fusion, Vol. 39, No. 11D.A. Callahan-Miller and M. Tabak (LLNL)

A: AuGd <1% denseB: AuGd 100% denseC: Fe 0.2% denseD: (CD2)AuE: AuGd <1% denseF: Al <3% denseG: AuGd <2% denseH: CD2

I: Al 2% denseJ: AuGd 4% denseK, L: DTM: BeBr or PolystyreneN: (CD2)Au

Axisymmetric ANSYS Model of Target

FliBe

DT

AuGd He (used to model all low-density materials)

BeBr or Polystyrene

We explored the effectiveness of various thermal control techniques for BeBr and polystyrene shells.

Begin with benchmark of previous work by Nathan Siegel (constant T at hohlraum surface and BeBr shell)

Repeat benchmark problem using polystyrene in place of BeBr

Analyzed effect of “B” layer conductivity

Determine the hohlraum surface temperature profile necessary for a given DT heat generation

Apply hohlraum surface temperature profile and check uniformity of DT surface temperature

Add low density material properties and experiment with anisotropy in interior regions

Benchmark with BeBr shell agreed with prior work.

• BeBr shell around capsule acts as an “integrating sphere”

• Constant surface temperature of 19.2 K on right boundary

• Agrees with prior work (3 K variation at DT surface)

Polystyrene shell does not smooth DT surface temperature.

• Constant surface temperature of 19.2 K results in 10 mK variation at DT surface

Temperature profile at hohlraum surface does not matter when the AuGd layer “B” is present.

• To affect the variation of temperature at the DT surface, we must eliminate conduction along the “B” layer

• Thermal conductivity of the “B” layer in the y-direction is modeled with helium properties

Determining the correct Temperature profile to Apply

• Attach a block of material that is 10% of the conductivity of FliBe to the target model

• Temperature at DT surface is fixed at 18 K

• Heat Flux applied at the right boundary is calculated based on 48,700 W/m3 volumetric heat generation in DT layer

• Solution gives temperature distribution to apply to the target model

• Nodal temperatures at the FliBe surface are recorded

• The FliBe surface nodal temperatures are applied to the target model

Results of Applying Temperature Profile to hohlraum surface

•Variation at DT surface is 500 K over /2 radians (800 K over radians)

•Automatic mesh is not symmetric about the x-axis and will need refinement

Applied Nodal Temperatures

Material tailoring could relax the requirements on the varying surface temperature.

Ideally, a sphere of highly conductive material concentric with the capsule (like the BeBr shell) will smooth the DT surface temperature variation.

• Anisotropic properties (such as in insulating foils) may be used to approximate a sphere through regions I, J, F, G, E, D, and A.

• With a perfect sphere, the DT surface temp is not as sensitive to temperature at the hohlraum surface (constant T at hohlraum would be acceptable)

Observations

• Benchmark using BeBr shell agreed well with prior work.

• Target model using polystyrene in place of BeBr does not provide a smooth DT surface temperature distribution.

• AuGd layer “B” must be modified in order to minimize conduction along the length of the target and allow hohlraum outer surface to “communicate” with the capsule.

• Application of a calculated temperature profile at hohlraum surface reduces the temperature variation from 10mK to ~500K.

• Tailoring of materials inside the hohlraum may relax outer surface temperature profile requirements.

Future Work

• Test sensitivity of the DT surface temperature to changes in the applied temperature profile

• Model coupling of energy from an outside source to the surface of the hohlraum

Model of energy coupling to the hohlraum while in a transport tube

• Experiment with anisotropic material properties inside the hohlraum

• Modify the target’s internal geometry to control heat flow