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. The Electra Laser Facility
First Generation system can run at 5 Hz for 5 hours Excellent test bed for developing laser components
The Electra Laser Facility
Two beams: 30 cm x 100 cm, 500 keV, 90 kA, Hz

4. The key components of a Krypton Fluoride (KrF) Laser
Input
Laser Gas
Recirculator
Pulsed
Power
System
Cathode
Amplifier
Window
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 (> 80% at 750 kV during the flat-top)
Two innovations have allowed high hibachi transmission: 1. Eliminate the anode foil 2. Pattern the electron beam to “miss” the hibachi ribs
Rib
Laser Gas
Kr + Ar
Vacuum
Pressure
Foil
.001”Ti
Emitter
e-beam
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.
e-beam hits
hibachi ribs
Non-uniform foil heat loading.
100 90 80 70 60 50 40 30 20 10
cathode strip width
% beam energy deposited in gas
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
Emitter
Floating Field Shaper
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

10. Position of the hibachi ribs
The emitter strips are counter-rotated so that the beam goes straight through the hibachi ribs
Cathode strips
rotated 6 degrees
27 cm
96 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.
Emitter
Pressure
Foil
e-beam
Solid cathode, no ribs
Entire pulse Flat-top portion
Simulation 67% %
(1-D Tiger)
Ribs
Solid cathode, with ribs
93%
of max
Experiment 47% %
>26%
Increase
Strip cathode, with ribs
Experiment 62% %

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 kV and mm thick Ti pressure foils.
Diode voltage
Flat-top efficiency with 50 mm thick Ti pressure foil
Flat-top efficiency with 25 mm thick Ti pressure foil
400 kV
57%(1)
71%(2)
500 kV
67%(3)
75%(4)
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 measurements agree 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
Rib
Kr Gas
Periodic Boundary
Number Density
Energy deposition fractions:
(for a 2-cm wide cathode)
Gas: 76.6% Ribs: 12.6%
Foil: 10.3%
Other: <1%

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.