IFE-Level Electron-Beam Diode Design Study*

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IFE-Level Electron-Beam Diode Design Study

• D. V. Rose, D. R. Welch
• Mission Research Corp., Albuquerque, NM
• J. D. Sethian
• Naval Research Laboratory, Washington, DC
• F. Hegeler
• Commonwealth Technologies, Inc., Virginia

Presented at the HAPL Meeting, Wadison, WI,
September 24-25, 2003
Work funded by the Naval Research Laboratory
through the US DOE.
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Philosophy for producing a conceptual design for
the main amplifier in an IFE beam line
Objective Produce a self consistent
conceptual design for an IFE beam line
amp Arbitrarily select the following Approx.
60 kJ laser output Segmented design Optimize
design using pulsed power system that is on the
table 100 kJ, 800 kV, 175 kA, 600 nsec
modules (NB with 80 Hibachi 140 kA into
gas) Methodology RL/JG KrF/optics question
any reason not to segment cathodes
vertically? JDS Electrostatics cathode shape,
beam spacing, module architecture JDS Hibachi
based on airflow cooling (later versions mist
cooling) DR e-beam deposition profile, Hibachi
transmission efficiency (goal 80) MF/SS e-beam
deposition profile, beam stability for this
hibachi/cathode design RL/..? Laser output gtgt
Effects of axial segmentation, ASE, etc gtgt
Optimize design
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A Pre conceptual notion for a 60 kJ Amplifier
Estored 100 kJ x 8 800 kJ V, I, ? 800 keV, 84
kA x 16, 600 ns Energy in gas 544 kJ Laser
Input 4 kJ Laser Output 57.8 kJ (? 10.8)
Each module powers one vertical pair of diodes
100 kJ Pulsed Power Module
PFL
40 m
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Preconceptual Notion--pulsed power flow
Pulsed Power 100 kJ total
e-Beam Diodes 85 kJ each (85 eff)
e-Beam in gas 68 kJ each (80 eff)
800 kV 87.5 kA
720 kV 78 kA
800 kV 104 kA 600 nsec
800 kV 87.5 kA
720 kV 78 kA
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Pre-conceptual notion..details
160 cm X vertical aperture
X spacing

80 cm (cathode height)

Z spacing
50 cm cathode with
80 cm Horiz aperture
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Goal 3-D LSP simulations of a single diode in a
60 kJ Amplifier

• Provide energy deposition efficiency in KrF gas
  for realistic cathode/hibachi/foil design
• Baseline design uses 8 diode pairs with cathode
  (and hibachi) areas of 50 cm by 80 cm.
• Hibachi design identical to cooled hibachi but
  with 2.9 cm deep ribs to compensate for
  additional mechanical stress (compared with
  Electras 1.3 cm deep cooled hibachi ribs).

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Pathway to full 3-D simulation

• Find an AK gap setting and cathode width that
  gives (in the limit of no backscatter and no
  floating field shapers)
• 100 transmission efficiency between the ribs
• 84 kA total current (for 17 cathodes, 50 cm
  long)
• Acceptable current density on the foil
• Use 3-D simulations to optimize rotation angle
  and cathode-to-cathode spacing. First
  simulations will NOT include backscattering.
• Full 3-D simulations with gas fill test limits
  of gas deposition.
• Determine gas composition and gas pressure from
  J. Guiliani modeling for 800 kV operation.

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Step 1) 2-D simulation geometry
800 kV, static-field solve (simulations reach
equilibrium in 2 to 6 ns, depending on AK gap
spacing), NO ITS scattering all conductors are
perfect absorbers!
Bo 2.0 kG
Sample result with 2 cm cathode 4 cm AK gap
CathodeWidth
AK Gap
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Diode currents of 84 kA are required
simulations show a range of cathode widths and AK
gaps for achieving this.
Dashed line is desiredcurrent of 84 kA.
Differences in currentbefore and after ribsis
larger here thanwould be the case whenfloating
field shapersare used. (Becauseof the reduction
in high beam edge currents thatare scraped off
by the ribs.)
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Using ratios of currents from previous slide, a
transmission efficiency can be plotted for each
cathode width.
Why the drop at 7 cmAK gap for the 23mmcase?
See next slide.
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For a 23mm wide cathode, the beam just fits
inside the ribs at AK gaps of 6 cm (top) and 8
cm (bottom). The 7 cm AK gap (middle) has
somehalo that is scraped off by the ribs.
This might be improved IF a largerB-field can be
used. What are theconstraints on B?
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A reasonable parameter space exists for cathode
widths and AK gap distances for this hibachi
design
Using current densities from the center of the
foils provides a reasonable guess to the
average current density on the foil
when floating field shapers are used.
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For larger AK gaps, perturbation created by ribs
on potential structure is reduced and therefore
beam edge potential variations are reduced (i.e.,
green curves most similar)
23 mm widecathodes
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Optimal choice for AK gap spacing lies between 7
cm AK gap (top) and 8 cm AK gap (bottom) for a 20
mm wide cathode.
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Floating Field Shapers (FFS) reduce current
density enhancement at beam edges. A flatter
current density profile results.
50 kV contours
With FFS
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Step 2 3-D simulations (without gas transport or
backscattering) to determine basic 3-D geometry

• Determine rotation angle for a given AK gap
  setting
• Estimate zeroth order beam energy transport
  through ribs to foil
• Set cathode-to-cathode spacing

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Basic 3-D simulation uses floating field shapers
(FFS) and rotated hibachi ribs (rather than
rotated cathode strips). Coordinate convention
is
80 cm
x
y
z
50 cm
Anode (foil) Plane
Cathode Plane
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Design Parameters

• 2-cm wide cathode strips, 50 cm long, 17 strips
• AK gap (cathode plane to rib entrance plane)
  7.0 cm
• 2.5mm wide FFS, with 2.5 mm floater-to-cathode
  gap on all 4 sides of cathodes
• FFS not allowed to emit, but they charge (float)
  dynamically during the rise of the voltage
  (previous 2-D simulations used pre-charged FFS)
• 2.9 cm deep, 1-cm wide ribs, rotated 4.65 deg.

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Cathode plane (1 quadrant) showing floating field
shaper connections.
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Slice view of power feed, cathode, and ribs
y0 plane
For this AK gap (7 cm),diode voltage andcurrent
are 809 kV,and 84 kA, respectively.
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Particle plot inside rib space shows that proper
beam rotation achieved, but cathode-to-cathode
spacing can be improved for outermost beamlets.
Particles plotted lie between z9.0 and 9.9 cm
(z9.9 cmis the foil plane)
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Step 3) 3-D simulation with floating
field-shapers and gas transport

• Beam patterned sothat gt98 of the beamenergy at
  peak power passes between ribs and strikes
  foil.
• Floating field shapersimprove uniformity
  ofcurrent density onfoil.

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Uniform electron beam densityimpinging on foil

• Low density component due to back-scattered
  electrons

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Time-integrated energy deposition on foil and
ribs is uniform.
Energy deposited on foil showsrib-image
Energy deposition on ribs (near foil)is uniform.
Mostly due to backscatteredelectrons striking
ribs.
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Uniform energy deposition in gas within 3 cm of
the foil
Slices at the following distances pastthe foil
0.3, 1.3, 2.3, 3.3 cm
Rib shadowing in gas deposition
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First diode/hibachi design is on track with IFE
goals

• 100 Kr, 1.3 Atm
• 73.6 in gas
• 9.1 in foil
• 11.3 in ribs
• 2.8 in hibachi frame
• 1.6 in laser cell walls
• 0.4 in 2nd foil

• 100 Ar, 2.6 Atm
• 82.6 in gas
• 7.4 in foil
• 6.2 in ribs
• 1.5 in hibachi frame
• 0.5 in laser cell walls
• 1.7 in 2nd foil

• Further optimization may be possible.
• Optimal gas mixture/pressure for lasing needs to
  used.

Lower atomic-number gas (Ar) yields less
backscattering, reducing energy deposition
fractions in hibachi and foil. Pressures chosen
to stop beam in 60-cm of gas.
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Status of work

• 2D simulations at 800 kV suggest 20-mm wide
  cathodes with a 7.0 cm AK gap. This AK gap gives
  a total current of 84 kA from 17 cathode strips.
  
• 3D simulations without backscattering show gt98
  of the beam reaching the foil for this this
  particular combination of cathode and hibachi
  design. Other designs with this transmission
  efficiency may be possible.
• 3D simulations with backscattering and gas
  transport result in energy deposition in the gas
  between 73.6 and 82.6, depending on choice of
  gas composition and pressure.