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