Electron Beam Deposition Into the KrF Laser Gas

1
Electron Beam Deposition Into the KrF Laser Gas
F. Hegelera), M.C. Myers, M. Friedman, J.D.
Sethian, S.B. Swanekampb), D.V. Rosec), D.R.
Welchc), and M. Wolfordd) Naval Research
Laboratory Plasma Physics Division Washington, DC
20375

• a) Commonwealth Technology, Inc., Alexandria, VA
  22315
• b) TITAN/JAYCOR, McLean, VA 22102
• c) Mission Research Corporation, Albuquerque, NM
  87110
• d) SAIC, McLean, VA 22102

Work supported by the U.S. Department of Energy
High Average Power Laser Program Workshop, Naval
Research Laboratory, December 5, 2002
2
Summary

• Should be able to meet ultimate goal of 80
  e-beam energy deposition efficiency during the
  flat-top portion of the pulse at
  750 kV.
• Experimental measurements, plus 1-D and 3-D
  simulations that have been benchmarked with
  experiments, point the way
• No anode foil is needed (Demonstrated)
• Pattern and counter-rotate e-beam to miss ribs
  (Demonstrated)
• Reduce pressure foil from 2 mil to 1 mil
  (Demonstrated)
• Shallower hibachi ribs (To be evaluated with new
  hibachi
• in early Spring 2003)
• 750 keV beam (as in ultimate application)
• Eliminated electron beam halo with floating
  electric field shapers.

3
First Generation system can run at 5 Hz for 5
hoursExcellent test bed for developing laser
components
The Electra Laser Facility Two beams 30 cm x
100 cm, 500 keV, 90 kA, 100 nsec _at_ 5 Hz
4
The key components of a Krypton Fluoride (KrF)
Laser
Laser Input
Laser Gas Recirculator
Pulsed Power System
Cathode
Electron Beam
Foil Support(Hibachi)
Laser Cell (Kr F2)
ENERGY (Kr F2) ? (KrF) F ? (Kr
F2) h? (248 nm)
5
Opposite electron beams pump the laser cell
Top view
120 cm
30 cm
1.8 m
Cathode
Hibachi
Side view
Hibachi
6
Diode Energy Distribution of a 6.1 kJ Pulse
risetime 0.7 kJ
fall time 1.2 kJ
100 ns flat-top 4.2 kJ
Voltage and current waveforms from a single diode.
7
Goal Maximize e-beam energy deposition in the
laser cell (gt 80 at 750 kV during the flat-top)
Two innovations have allowed high hibachi
transmission1. Eliminate the anode foil2.
Pattern the electron beam to miss the hibachi
ribs
Anode foil

• Issues
• High Current edge from each strip
• Non-uniform electric field at anode causes beam
  spreading
• Beam rotates and skews between cathode and anode

8
3-D LSP simulation show that an anode foil can be
eliminated
E-beam beam spreading in the non-uniform electric
field is controlled by the cathode strip width.
cathode
field shaper
ribs
foil
Simulations by D. Rose D. Welch MRC Albuquerque
9
Floating Field Shapers on perimeter of cathode
strips eliminate enhanced edge emission
A/cm2
WITHOUT
WITH FIELD SHAPER
Line out of Radiachromic film image of beam at
anode
F. Hegeler, et al., Physics of Plasmas, vol. 9,
October 2002, pp. 4309-4315.
10
The emitter strips are counter-rotated so that
the beam goes straight through the hibachi ribs
Cathode strips rotated 6 degrees
27 cm
Position of the hibachi ribs
Radiachromic Film Time integrated current
profile at the pressure foil
11
Measured Electron Beam Deposition Profile
12
Strip cathode increases energy deposition by 26
over monolithic cathode performance close to
rib-less hibachi
Electron beam energy deposition efficiency at 500
kV, 1.2 atm. of Kr, and a 50 mm (2 mil) thick Ti
pressure foil.
Entire pulse Flat-top portion
Simulation 67 72 (1-D Tiger)
93 of max
Experiment 47 53
gt26 Increase
Experiment 62 67
13
Flat-top energy deposition efficiency of up to
75 is achieved for 500 keV electrons, with a 1
mil Ti pressure foil
Flat-top e-beam energy deposition efficiency with
a strip cathode at 400-500 kV and 25-50 mm thick
Ti pressure foils.
Required laser cell gas pressure to stop the
e-beam (1) 1 atm. at 40 Kr and 60 Ar (2) 1
atm. at 60 Kr and 40 Ar (3) 1.2 atm. at 100
Kr (4) 1.3 atm. at 100 Kr
14
3-D LSP simulations and experimental
measurementsagree well for the counter-rotated
strip cathode
E-beam deposition efficiency for the flat-top
portion of the e-beam (500 kV)
80
with 2 mil Ti pressure foil 6 degree counter
rotation 1.2 atm Kr in laser cell 2 mil Ti
foil Simulations 66 efficiency Experiments
67 efficiency
70
60
50
deposited energy
40
30
20
10
0
laser gas
pressure foil
hibachi ribs
Deposition efficiency Energy deposited in
laser gas/energy in diode (for flat top
portion of beam)
Simulations by D. Rose D. Welch, MRC Albuquerque
15
3-D simulation of a single cathode strip and ribs
confirms the 1-D results
Static fields simulation with periodic boundaries
in X 2-cm wide cathode, 4-cm rib spacing, 30-cm
of 1.2 atm Kr, 500 kV, 1 mil Ti foil.
Foil
Kr Gas
Rib
Number Density
Periodic Boundary
Energy deposition fractions (for a 2-cm wide
cathode) Gas 76.6 Ribs 12.6 Foil
10.3 Other lt1
16
New Hibachi with shallower ribs will increase the
e-beam deposition efficiency

• E-beam spreading is minimized
• Electric field in A-K gap is more uniform
  compared to deep rib Hibachi

1 mil Ti pressure foil
Hibachi ribs
Preliminary simulation using new hibachi gives
74 energy deposition into gas at 430 kV.
17
1-D Tiger simulations at 750 kV
1-D codes predict a maximum energy deposition
efficiency of 81 for a 750keV electron beam.