Numerical Modeling of the Electra Electron-Beam Diode*

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Numerical Modeling of the Electra Electron-Beam
Diode

• D. V. Rose1, D. R. Welch1, S. B. Swanekamp2, M.
  Friedman3, M. C. Myers3, J. D. Sethian3, and F.
  Hegeler4.
• 1Mission Research Corp., Albuquerque, NM 87110
• 2TITAN/JAYCOR, McLean, VA 22102
• 3Naval Research Laboratory, Washington, DC 20375
• 4Commmonwealth Technology Inc., Alexandria, VA
  22315
• High Average Power Laser Program Workshop
• Naval Research Laboratory
• Washington, DC
• December 5 6, 2002

Work supported by U.S. DOE through the Naval
Research Laboratory.
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Goal is to minimize electron energy losses in
order to maximize energy deposition in the KrF
gas on Electra. Detailed comparisons between
measurements and numerical simulations provide a
benchmark and develop confidence in the
simulations as a design tool.
Diodes and Laser Cell
Electra Facility
Applied B1.4 kG
Anode
Anode
Electron Emitter
Hibachi
Cathode
Cathode
Laser Cell
Laser Window
Anode Foil
Anode Foil
Pressure Foils
Laser Beam
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Basic Diode Physics

• Static field solutions using LSP in 1-D, 2-D, and
  3-D simplified geometries.
• No beam rotation or shearing accounted for
• Scattering and energy loss of beam in foil and
  gas using ITS 1 Monte Carlo algorithms
  incorporated into LSP.
• Study role of backscatter on diode operation,
  especially losses to hibachi and foil.

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Sample trajectory from 1-D diode simulation
illustrates multiple passes through a 2-mil Ti
pressure foil.
Eo 500 keV 1.2 atm Kr 2-mil Ti pressure foil
1-D LSP simulations of a parallel plate diode
operated at 500 kV with backscattering from
foil/gas define an upper limit on energy
deposited into the foil
Energy fraction deposited in foil for case with
perfect absorber behind foil 11 Energy fraction
deposited in foil for case with 30-cm KrF gas
cell 27
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1-D diode simulations show optimal energy
deposition in gas cell for 500 keV operation at
1.2 atm of Kr.
1.2 atm Kr 2-mil pressure foils on both sides of
gas cell
Gas pressure not high enoughto stop all of the
beam for600 keV operation
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2-D periodic simulations illustrate the role of
the floating field shapers (FFS) in reducing
losses to the ribs.
Eo 500 keV 1.2 atm Kr 2-mil Ti pressure
foil Slotted cathode 2.5 cm wide
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Three cathode configurations considered, along
with Hibachi ribs
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Current density across a single beam is more
uniform with the floating field shapers.
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Experimental Configuration
With an anode
Front view
Without an anode
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Good Agreement with Measured Current Density
Found in LSP Simulations

• Flat Cathode w/Mark I hibachi
• 2-mil Ti Pressure Foil
• 1.2 atm Kr
• Faraday cups filtered with 1-mil Ti foil

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Full 3-D EM Simulations

• Detailed simulations of the Electra diode
  including the Mark I hibachi and newly designed
  cooled hibachi.
• Comparisons with measured data highlighted for
  both flat and slotted cathodes.

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Sample 3-D LSP Simulation with a slotted cathode
Cathode face, emission surfaces are 2-cm
wide,with 4-cm center-to-center spacing, 7 deg.
tilt (SCL emission from shaded regions only)
Ribs (0.5 cm wide, 4-cm apart)
(y0)
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Electron Beam Density Patterns Show Illustrate
Beam Rotation in AK Gap
550 keV operation
Entering Ribs
Inside Ribs
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Electron beamlets are merged within 1-2 cm after
entering gas
Just After Foil
1 cm Past Foil
550 keV, 1,3 atm Kr
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No Rib Shadowing After 2-cm of Gas Transport
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3-D LSP simulations use a drive pulse that
approximates the Electra voltage and current
waveforms.
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3-D LSP simulations of the full Electra pulse in
good agreement with measured energy deposition
efficiencies.
Solid Cathode
Strip Cathode
Experiment 47 (entire pulse)
Experiment 62 (entire pulse)
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Summary

• Detailed comparisons of Electra diode data in
  good agreement with LSP simulations.
• Benchmarked LSP code a powerful design tool for
  large-area electron beam diode/hibachi designs
  for ICF systems.

• Ongoing Work
• Adding conductivity evolution model to laser gas
• Exporting 3-D energy electron energy deposition
  profiles from LSP to Guiliani et al. laser
  kinetics model