High Power Microwave Breakdown

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High Power Microwave Breakdown of Gas-Dielectric
Interface at 90 to 760 Torr
Gregory Edmiston Advisor Dr. Neuber
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Objectives
A major limiting factor in the transportation of
high power microwaves is breakdown phenomena at
the transition boundary between the origin of HPM
and the transmitting medium - surface
breakdown at the vacuum/dielectric/gas
interfaces Physical mechanisms leading to
breakdown are unclear - generation of free
electrons - amplification/avalanche
mechanisms - interface breakdown ? volume
breakdown Clarify basic mechanisms and find ways
to increase breakdown thresholds - real-time
high speed diagnostics (electrical, optical,
x-rays, ...) - select materials, coatings,
geometries - apply external electric/magnetic
fields
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Project Objectives

• Study mechanics of dielectric surface breakdown
  in high power microwave fields
• Influential Variables Studied
• Pressure (Vacuum to Atmosphere)
• Relative Humidity
• Temperature (-30 C to 25C)
• Gas Composition
• Power Levels (up to 50 MW)

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Interface Breakdown Models
Dielectric/Vacuum Interface Electron Emission
from triple point Saturated Secondary Electron
Avalanche Outgassing due to electron
bombardment Paschen Breakdown in desorbed
gas Dielectric/Gas Interface Electron Emission
from triple point/field enhancment point Gaseous
ionization Surface Effects Photoemission Elec
tron absorption
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Test facility S-Band Traveling Wave Resonance
Ring (TWRR)
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Magnetron Pulse into TWRR
FWHM 2µs Risetime 550 ns
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Slab in Waveguide Setup
Graphic from D. Hemmert 98
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Diagnostics
MKS Pressure Transducers
Master Trigger
Magnetron Power
Delay Generator
Reverse Power
Forward Power
Luminosity
2 Gs/s, 500 MHz Oscilloscopes
Imaging
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Dual Camera Setup
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Typical Breakdown Sequence (Al2O3 _at_ 200 torr air)
Field Enhancement Points
Camera Gate 1
Camera Gate 2
Camera Gate 3
Reference
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Typical Breakdown Sequence (Al2O3 _at_ 200 torr air)
Field Enhancement Points
Camera Gate 1
Camera Gate 2
Camera Gate 3
Reference
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Breakdown Field Strength for Selected Materials
at Atmospheric Pressures
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RF - Field Component Normal to Surface
In air Arc path follows the surface (expected
from unipolar flashover)
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MURI Question

• Could the field enhancement points be
  overwhelming the surface with electrons?
• INVESTIGATION
• Maxwell 3-D Simulations
• Opposite plates at 40kV
• Separation of FEP 1.14 or 2.89cm
• Unperturbed field at 13.8 kV/cm

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Simulation Results Side
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Simulation Results Front
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(No Transcript)
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New ApproachLaunch a Wave from WR284
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Launching a Wave
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Focusing into a Horn
S110.06 Less than 1 reflected
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Couple into a Horn
S11.42 S12.45 S21.42 S22.15
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From Horn to WR284
S11.20 S12.48 S21.48 S22.52
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WR284?WR650 (Custom Flange)
S11.40 S12.92 S21.92 S22.40
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New Setup
Isolator
F/R Coupler
Magnetron
Dielectric Window
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Conclusions/Continuing Work

• Successful Operation of TWRR
• Breakdown field strength studies
• Various Materials
• Various Pressures
• Variety of Flange adapters for TWRR
• For the Future
• Re-Test data from TWRR (verify lexan
  conditioning)
• Surface breakdown at exit of waveguide

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Gantt Chart
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Budget
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References

• Dr. Neuber
• Dr. Dickens
• John Krile
• Dr. David Hemmert
• http//cp.literature.agilent.com/litweb/pdf/5952-0
  801.pdf
• Experimental Studies of High Power Microwave
  Breakdown at a Dielectric Surface, Hemmert,
  David, TTU, 2002