Niobium-Base Alloys for Space Nuclear Applications

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Pulsed E-beam Thermal Fatigue System
Chad E. Duty and Lance L. Snead Materials Science
Technology Division Oak Ridge National
Laboratory
High Average Power Laser Program Workshop Naval
Research Laboratory, Washington DC October 30,
2007
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Current HAPL Capabilities
Goal Reproduce thermomechanical behavior of IFE
first wall. Solution Various systems approach
behavior, each with unique advantages.

• Pulse width
• -- Long pulses underestimate
• elastic wave strength
• -- Short pulses underestimate
• depth of temperature penetration
• Thermal phenomena
• -- Experiments do not keep material
• at temperature long enough
• RHEPP is the only facility for
• pulsed ion effects.
• Dragonfire is best for large cycles
• IR facility is only option for reproducing
• interfacial stresses.
• Using x-rays, Xapper allows comparison between
  laser ions.

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New Capability Electron Beam
An electron beam system better simulates the
thermo-mechanical threat to the IFE first wall.
Calculations courtesy of Ion deposition
J. Blanchard Electron depo F. Hegeler
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Pulsed E-beam Thermal Fatigue System (PETS)

Electron Gun Chamber
High Voltage Shield
E-beam Remote Control
Pulser Unit
Vacuum System Controls
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Pulsed E-beam Thermal Fatigue System (PETS)


Cathode Stalk Assembly
Faraday Cup
Pump Port
Cathode
Anode
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Pulsed E-beam Thermal Fatigue System (PETS)

• Peak Voltage 70 kV (variable)
• Peak Current 74 Amps
• Pulse Width 0.5 to 1.5 µsec (variable)
• Pulse Rise/Fall Time 800 ns
• Pulse Frequency Single shot to 100 Hz
• Duration gt 10 million shots
• Usable Beam Waist 0.565 cm (1 cm2 area)
• Current Density Variation (at UBW) 31

Cathode
Anode
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E-Beam Heating Cooling
Electron Beam Power

• Time averaged power 820 W
• Peak density 34 W/mm2

E-beam Penetration Depth (S)

• S 5.2 µm for W substrate
• Maximum heating at 1.7 µm

Coolant Flow

• Need 0.5 gpm to remove 820 W

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Sample Holder / Cooling Design

• Functions
• -- Hold sample securely
• -- Permit optical access
• (from both top sides)
• -- Versatile clamping design
• (allow for various sample dims)
• -- Cool sample
• (provide heat sink for e-beam energy)
• -- Heat sample
• (low temp thermal processing / aging)
• -- Electrically ground sample
• (prevents sample from charging)
• -- Measure temperature
• (either fast TC or melt blocks)

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Sample Holder / Cooling Design
Ceramic Insulator for 8 Conflat (prevents e-
from returning to copper anode )
Sample Holder (previous slide)
Thermocouple / Electrical Feedthrough
Resistive Heater Thermocouple Leads
Path to ground for electrons
Rated 1250W at 1.5 gpm
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E-beam chamber supplied by HeatWave Inc.
Use oscilloscope to measure voltage drop across a
1 W resistor (1 V 1 A through e-beam)
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Sample Holder / Cooling Design
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Finite Element Thermal Analysis
Developed preliminary finite element model of
sample holder in ABAQUS.
E-beam Heating Options
Example Case

• Radial Symmetry
• Steady State
• Resistive Heater (250 W)
• Coolant Flow
• Tinf 10-20dC
• hoff 50 W/mK
• hon 5,000 W/mK
• Radiation
• Tinf 20dC
• e 0.2 (free surfaces)
• Conduction
• W 174 W/mK
• Mo 138 W/mK
• Steel 26 W/mK
• Insul 1 W/mK

Equal Inside 738 W/cm2 Outside 738 W/cm2
21 Split Inside 428 W/cm2 Outside 856 W/cm2
E-beam 21 Split Heater 250 W Coolant 5,000
W/mK
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Steady State Heat Transfer
Heater Off (0 W)
Heater On (250 W)
E-beam Equal (738738)
E-beam Split (428856)

• Due to radial symmetry, natural tendency is for
  center of sample to get hotter.
• Splitting beam produced more uniform surface
  temperature than using the resistive heater.
• Predict 2.61 beam distribution will not be
  problematic (steady state).

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Transient Thermal Model
Maximum surface temperature profile mimics 31
variation in e-beam power.
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Power Adjustments
PETS can deliver enough power to destroy sample.
How do we adjust?

• Pulse width
• Variable from 0.5 to 1.5 µs
• Can reduce peak temperature by 30
• Input voltage
• Adjustable from 0 to 74 kV
• Input power scales linearly
• Sample position / beam waist
• Adjust incrementally (-1/4 to 2)
• Increases beam waist
• Reduces power density

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Acceptance Testing Round 1
Date April 13, 2007
Location HeatWave Labs, CA

• Focused beam
• Faraday cup target (1 cm2)
• Nearly full voltage (66 kV)
• Low rep rate (1 Hz)
• Constant perveance (K 4.4µp)
• Imax K V3/2

Toroid (generated) current Faraday (received)
current Confirm Focus e-beam to 1 cm2 area
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PETS Shipped to ORNL
Arrived July 16, 2007
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Acceptance Testing Round 2
Date August 3, 2007
Location Oak Ridge, TN

• Diffuse beam
• No Target (anode tube)
• Full voltage (70 kV)
• High rep rate (100 Hz)
• Long dwell time (1.5 µs)
• Constant Perveance (4.4µp)

Remaining Issues Pulse signal generator
drifts System losing vacuum (10-7 T)
PASS
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Pulse Signal Generator
At high rep rates, pulse signal generator
unstable

• Occasional extra pulses
• Frequency ( 5 Hz)
• High voltage ( 5 kV)

New unit delivered September 27, 2007
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System Vacuum
Pressure interlocks trip frequently, suspect
leak.

• Cathode heating interlock (10-5 T)
• Pulsing interlock (10-7 T)
• Under ion pump, pressure ? 10-4 T
• Chased leak for 2 months
• Largest problems
• Ion pump feed-through
• Flanges exposed to e-beam

Leaks fixed! Currently holding at 6x10-8 T
Gaskets heavily oxidized
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Radiation Shielding
PETS is a Radiation Generating Device (X-rays)

• Initial testing without target
• Peak reading through view port
• Radiation increases with all variables

Voltage Freq Width Contact 30 cm
(kV) (Hz) (µs) (mR/h) (mR/h)
62 10 1.0 650 75
62 20 1.0 1,200 120
60 100 1.0 4,900 560
67 100 1.0 7,800 870
70 100 1.5 12,500 1,300

• Effective shielding with ¼ steel plate
• Maximum shielded reading 0.1 mR/h

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Future Directions

• Install transformer (240 VAC)
• Test new pulse signal generator
• Install sample holder / cooling
• Radiation with target in place
• Shielding, interlocks, and safety
• Ready for testing Jan 2008
• Design for sample exchange