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

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

M.S. Tillack and J.E. Pulsifer

University of California, San Diego

D.T. Goodin and R.W. Petzoldt

General Atomics

- Purpose for in-hohlraum layering:
- Layering with capsule already assembled in the hohlraum is advantageous
- Eliminates the need for separate layering device
- Eliminates the need for rapid, precision cryogenic assembly

- Requires highly uniform DT surface temperature (~100mK) for up to several hours
- Need a well controlled temperature profile on hohlraum

- Layering with capsule already assembled in the hohlraum is advantageous
- Objective of our research:
- Determine required temperature profile and suggest method(s) for implementation
- Chillers on staging tubes
- Tailoring of target material properties

- Determine required temperature profile and suggest method(s) for implementation

Overview

- DT temperature profile with constant T at hohlraum surface: BeBr shell
- DT profile with constant T at hohlraum surface: polystyrene shell
- Relaxed temperature requirements for high-yield targets
- Effect of segmenting the Au layer
- Hohlraum surface temperature profile needed for uniform DT
- Passive control system for obtaining proper surface temperature
- Sensitivity to small variations

Target Design

- Close-coupled, distributed radiator heavy ion target
- Materials:

A: AuGd <1% denseB: AuGd 100% denseC: Fe 0.2% denseD: (CD2)AuE: AuGd <1% denseF: Al <3% denseG: AuGd <2% denseH: CD2I: Al 2% denseJ: AuGd 4% denseK, L: DTM: BeBr or PolystyreneN: (CD2)Au

From Nuclear Fusion 39(11)D. A. Callahan-Miller and M. Tabak

Axisymmetric ANSYS Model of Target

FliBe

AuGd

He (used to model all low-density materials)

DT

BeBr or Polystyrene

1. Benchmark with BeBr shell agrees with earlier work.

- The BeBr shell around the capsule creates a spherical isotherm
- Temperature of the DT outer surface is small and agrees with prior work (3 mK variation at DT surface)

Constant surface temperature = 19.2 K on right boundary

DT ice outer surface temperature

2. Polystyrene shell exhibits much larger DT temp. variations

- Change to polystyrene shell results in 10 mK temperature variation at the DT outer surface
- Is it too much?

Constant surface temperature = 19.2 K on right boundary

DT ice outer surface temperature

3. High-yield capsules can tolerate significantly rougher

surface finishes than ignition capsules

Standards used for NIF targets

Results for plastic ablator capsule

Ref: Mark Hermann, Indirect Drive Target Workshop, GA (1 May 2001).

The required degree of temperature symmetry is derived

from the surface “roughness” requirement

In the limit L /a<<1,

L ~ sqrt(2k DT/q’’’)

For small perturbations,

d ~ (k/Lq’’’)x

x=temperature perturbation

d=thickness perturbation

DT thickness variation is ~200 mm with hohlraum temperature fixed at 19.2 K

• Too much!

k = 0.33 W/m-k

L = 0.33 mm

q’’’ = 48,700 W/m3

4. The AuGd layer must be altered to allow external control

- 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

5. An “inverse problem” is solved to determine the correct temperature profile to apply on the hohlraum surface

- An artificial block of material is added to remove the RHS boundary condition.
- The temperature at the 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 approximate temperature distribution to apply to the target model

Flibe surface temperatures

Results of applying temperature profile to the hohlraum surface

DT outer surface temperatures

- Variation at DT surface is reduced from 10 mK to ~200 mK
- Subsequent iterations further improve the result (<100 mK achieved)

Applied nodal temperatures

DT thickness corresponding to the modified surface temperature

Variation of DT thickness is ~5 mm

6. Use of a passive thermal control system to

establish boundary temperature profile

- Varying thickness insulator used to map constant temperature cooled surface to desired hohlraum temperature profile
- Estimate of thickness profile made using T(y) and q(y) from original solution

Temperature variation at DT for 0.5-cm thick scallop and 17 K boundary temperature (k=0.0125 W/m-K)

DT outer surface temperatures

Design of a cooling system for the scalloped tube

Concept = Using the heat flux profile, provide a uniform T boundary and a varying radial conductance - to result in desired T profile

Design Data

Cu rods 4.68 m long

234 hohlraums/rod

∆T top/bottom = 0.1K

2 - 1/4” cooling tubes/rod

0.3 g/s He at 200 psi/rod

∆P = ~10 psi

Concept = stack of hohlraums in cooled tubes

Hohlraum delivery system

~1 m

Design Data

3 hr layering + 0.5 h backlog

18 rods per bundle

18 bundles total

75,600 hohlraums

Total He cooling flow = 97 g/s

54 s between movements

3 s between movements

Six per second

7. Sensitivity studies

- Sensitivity of DT temperature profile to 1-10% changes in properties and applied temperature was explored
- Case shown is 1-10% change in k for region D
- 1% changes roughly double the nonuniformity; 10% changes cause an order of magnitude increase

Input temperature profile error

A 10% error in the peak temperature of the inverse problem profile was used as input.

A 900mK temperature difference results at the surface of the DT as opposed to 200mK reported from the original.

Conclusions

- Benchmark using BeBr shell agrees well with earlier work.
- Target using polystyrene in place of BeBr does not provide adequately smooth DT surface temperature distribution.
- Temperature requirements are relaxed with high-yield targets, but not enough to avoid external temperature control.
- AuGd layer must be modified to minimize conduction along the length of the target and allow hohlraum outer surface to “communicate” with the capsule.
- The required applied temperature variation was determined and shown to reduce the temperature variation from 10mK to ~200mK.
- The passive control scheme requires optimization, but appears feasible.
- Variations in properties and applied temperatures must be kept below a few percent.