D2 and DT Liquid-Layer Target Shots on NIF

National Ignition Facility • Lawrence Livermore National Laboratory • Operated by the US Department of Energy

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Foam Lined Capsules give NIF the

Ability to Field D2 and DT Liquid-Layer

Targets

Target Fueling Qualification

Foam Layer Lining Capsule

Liquid wicks into and

conforms to the foam layer.

• NIF now fields two types of layered targets, ice-layer and liquid-layer.

• Ice layer growth requires tritium in the fuel, liquid layers do not.

• Liquid-layer formation is a simple, single, capsule fill that requires ~6 hours.

• Ice-layer formation is a complex process that requires ~62 hours.

• Ice-layer shot temperatures have an upper limit given by the fuel's melting

point.

• Liquid-layer targets may be shot at a wide range of temperatures which are

predominantly constrained only by the target design.

Critically Filled Capsule

Sub-Critical FillShowing First Strong

Contrast of Foam Edge and Scattering from Filled Foam

Pores

Capsule Overfilled for Monitoring

Thermal Switch Closed to Lock Liquid Inventory in

Capsule

Setting and Maintaining Fuel Inventory

Target Installed on CryoTARPOS Positioner

Liquid-Layer Target with Thermal

Switch on Cu Cooling Rod

70 mK

Target Surrounded by Cold

Shrouds with Clear Windows

• During fueling the inventory level is measured using X-ray imaging.

• Ice layer targets freeze the fuel in place when full inventory is reached.

• An Ice-plug in the fill tube then acts as a "valve" to maintain the fuel in the

capsule.

• Tritium B-decay gives a 70 mK difference between the capsule and hohlraum

wall, the ice-layer growth process can thus be performed with fuel in the fill

tube always frozen. • The standard ice-plug "valve" does

not exist in liquid-layer target fill

tubes.

• To be able to have an ice-plug, a

thermal switch is installed on the

cooling rod.

• When the switch is powered the fill

tube remains open for fueling the

capsule.

• When unpowered, an ice plug forms

in the fill tube at the switch location

and locks the inventory in the

capsule.

X-ray Images and Meniscus Heights During Fueling

• Qualification of the new thermal switch was performed in an off-line test-bed

where the long-term inventory level was seen to be unstable. It was

concluded that higher than normal IR heating of the fill tube is necessary to

avoid drift.

• The windows on the cold target shrouds are therefore changed from gold-

coated to clear when fielding liquid-layer targets on the CryoTARPOS

positioner at NIF.• During the initial stages of fueling, liquid wicks throughout the foam, filling

pores up until the time that the volume of fuel equals the volume of foam, i.e.,

a critically fill.

• Meniscus height tracking therefore only sees the foam edge up until the point

where the foam becomes overfilled.

• Inventory stability measurements are therefore performed with the foam

overfilled.

Long-term Stability of Inventory was observed in Fielded Target

FuelingStarts

Capsule Empty

LEH View Just

Before Critical Fill

Unwrapped Capsule Edge – Before Critical Fill

Unwrapped Capsule Edge – Slight OverfillVoids in Foam and Liquid Layer

A

A

B

B

Just beyond critical-fill, voids become filled. Additionally, defects beyond the foam edge that were partially filled become visible.

DC E F

X-ray Side View Image

During Fueling

Hohlraum

Cross section

Thermal

Gradient

Thermal Switch

- The switch must be able to maintain a temperature less than the MP of D2

~18.5K.

- At shot time the shrouds open and the switch temperature increases by

~4K.

- The Cu cooling rod to target base DT is 7K to 4K, depending on arm-heater

power.

- The target base must operate at a temperature of ~7.5K to 10.5K.

Hohlraum

- Shot temperature requirements: 21K to 26K, maintained by arm heaters.Target in Cold Shrouds Prior to Shot

Outside Shrouds

~300K IR

Requirements for Target Temperatures

and Arm Heater Power

Tshroud

~125K

Pre-Shot

- Cold shrouds ~125K IR

Shot – 8.5 seconds

- Shrouds open and fully retract.

- IR load on target increases to

~300K.

- Power load increases by ~2.4W.

Si Arm Heaters

- Power output decreases to

compensate for increased IR load.

- Target temperature control at shot-

time requires an initial heater power

> 2.4W.

Exploded View of Target Showing:

A) Clamp to Si Arm Interface Foil

B) Si Arm to Cooling Rod Interface

Foil

Thermal Switch Temperature at Shot-

Time

Switch on low-temperature target starts at higher temperature due to greater arm heater power.

Shrouds Open

Shot

Target Temperature and Arm Heater

Output at Shot-Time

A

B

Hohlraum Cu Cooling Rod with Thermal Switch

Si Armsto Target Base

and CryoCooler

• Foils inserted between components

provide mechanical compliance and a

reproducible thermal connection.

• Standard Low-Temperature Targets use In

foils, which result in a hohlraum to cooling

rod DT of ~3.5K with full power on the Si-

arm heaters.

• To achieve shot temperatures of greater

than ~21K the In foil must be replaced by a

material with a greater thermal resistance.

• For the 24.7K shot temperature targets the

In foil was replaced by kapton.

Shot

After shrouds open the additional IR load exceeds the initial heater power. The heaters turn off and the target temperature rises prior to the shot.

Shrouds Open

Shot

Shrouds Open

After shrouds open the additional IR load is less then the initial heater power. The heaters drop to compensate and the target temperature stays in control till the shot.

CryoIDPre-Shot

Temperature(K)

ShotTime

Temperature(K)

Deviationfrom

ShotTemperature(K)

Temperature

ControlledAfter

ShroudOpening?

C160315-AA 24.70 26.45 1.75 No

C160420-AA 24.70 26.10 1.40 No

C160624-AA 21.00 24.59 3.59 No

C161203-AB 21.00 21.00 0.00 Yes

C160620-AA* 21.00 21.00 0.00 Yes

CryoIDPre-Shot

Temperature(K)

ShotTime

Temperature(K)

SwitchOpening

Temperature(K)

SwitchMargin

atShotTime

C160315-AA 11.00 15.25 23.00 7.75

C160420-AA 12.00 18.45 25.47 7.02

C160624-AA 12.65 20.85 20.83 -0.02

C161203-AB 17.60 20.90 21.32 0.42

C160620-AA* 18.32* 20.29 20.33 0.04

*Targetthermalswitchfailed-targetnotshot.Replacedwithhightemperaturedesignbackuptarget.

LowTemperature

TargetDesign

ThermalSwitchPerformance

TargetTemperaturePerformance

HighTemperature

TargetDesign

LowTemperature

TargetDesign

HighTemperature

TargetDesign

Conclusions and Future Work

Foam lined capsule's give NIF the ability to shoot liquid-layer targets.

- Improving foam layer quality is the subject of an ongoing investigation.

An acceptable target design exists for a shot temperature of 21K.

A target design exists for shot temperatures ~25K; however, the target

temperature is uncontrolled at shot time.

- Investigations with the aim of developing targets that have controllable

shot temperatures between 21K and 26K are in progress.

Thermal switch temperature performance at shot-time is marginally

acceptable.

- Development of an improved thermal switch design is in progress.

High and Low Temperature Target Design

Next generation thermal

Switch

Cu pipe connects to fuel line

Proposed IR shield for thermal

switch to reduce temperature rise at

shot time

Curtis Walters1, Ethan Alger2, Suhas Bhandarkar1, Kurt Boehm2, Francisco Espinosaloza1, Benjamin Haid1, Ricardo Heredia1, John Kline3, Bernard Kozioziemski1, Jeremy Kroll1, Abbas Nikroo1, Patrick Opsahl1, James Sater1, Alex Zylstra3

1Lawrence Livermore National Laboratory, Livermore, CA 945502General Atomics, San Diego, CA 92121 3Los Alamos National Laboratory, Los Alamos, NM 87545