ELECTRA:

1
NRL J. Sethian M. Myers J. Giuliani P.
Kepple R. Lehmberg S. Obenschain SAIC M.
Wolford Commonwealth Tech F. Hegeler M.
Friedman RSI T. Jones S. Searles TITAN/JAYCOR
S. Swanekamp MRC Albuquerque D. Rose D.
Welch Titan PSD, Inc. D. Weidenheimer D. Morton
ELECTRA A REPETITIVELY PULSED, 700 J, 120 ns,
KrF LASER
Work sponsored by U.S. Department of Energy
NNSA/DP
2
Electra Configuration as an Oscillator
Laser Gas
Laser Gas
Laser Gas
Laser Gas
Recirculator
Recirculator
Recirculator
Recirculator
Flat Mirror
Flat Mirror
Flat Mirror
Flat Mirror
Pulsed
Pulsed
Pulsed
Pulsed
Power
Power
Power
Power
System
System
System
System
B
B
B
B
z
z
z
z
Cathode
Cathode
Cathode
Cathode
Output
Output
Output
Output
Electron
Electron
Electron
Electron
Coupler
Coupler
Coupler
Coupler
Beam
Beam
Beam
Beam
Foil
Foil
Foil
Foil
Baratron
Laser Cell
Laser Cell
Laser Cell
Laser Cell
Support
Support
Support
Support
Window
(
?
P)
(Kr F

Ar
)
Window
(Kr F

Ar
)
Window
(Kr F

Ar
)
Window
(Kr F

Ar
)
(Hibachi)
(Hibachi)
(Hibachi)
(Hibachi)
2
2
2
2
Þ
Þ
Þ
Þ
Þ
Þ
Þ
Þ
ENERGY (Kr F
(
KrF
)

F
(Kr F
) h
(248 nm)
ENERGY (Kr F
(
KrF
)
F
(Kr F
) h
n
(248 nm)
ENERGY (Kr F
(
KrF
)

F
(Kr F
) h
(248 nm)
ENERGY (Kr F
(
KrF
)
F
(Kr F
) h
n
(248 nm)
)
)
2
2
2
2
3
625 Joules Single Shot
Photodiode Response
Conditions 39.75 Kr, 60 Ar, 0.25 F2 Laser
Cell Pressure 1.36 atm Recirculator flow rate 7.2
m/s
Intensity (arb.)
Time (Nanoseconds)
Calorimeter Measurement
Energy (Joules)
Time (s)
4
Time Dependence of Energy Deposition and
Oscillator
E-Beam Power
PLaser (GW)
PE-beam (GW)
Oscillator Jan. 04 Tw 89
Oscillator Feb. 03 Tw 7580
Time (ns)
5
How we project an amplifier intrinsic efficiency
of 12 based on oscillator results of 8.4
How we project an amplifier intrinsic efficiency
of 12 based on oscillator results of 9.9

• No Output coupler (no 8 reflection losses)

• Lower laser light absorption due to fluorine,
  less passes through e-beam unpumped regions

A properly designed amp would have

• Good windows (gt98 transmitting vs. 89 measured
  transmission in oscillator)

• Amplification from input laser, no oscillator
  build-up time

6
Calorimeter Specifications
Calorimeter Response for 100 shots at 1 Hz

• Maximum Energy 700 Joules
• Maximum Continuous Power 3.5kW
• Maximum Energy Density 1.1J/cm2
• Maximum Power Density 50W/cm2

7
Photodiode Response 100 shots at 1Hz Rep-Rate,
625 J - 700 J per pulse (except shot 1) 39.75
Kr, 60 Ar, 0.25 F2 _at_1.36 atm, Recirculator flow
rate 7.2 m/s
Intensity (arb.)
Time (Nanoseconds)
8
Photodiode Integrated Response for 100 shots at 1
Hz shows Energy Increases for first 50 shots, 
then remains constant
Average Energy 677 J /- 40.9 J Last 50 shots
Average Energy 696 J /- 8.62 J
Due to changing Tg of recirculator? Window
transmission?
9
Photodiode Response at 5Hz Rep-Rate,
640 J per pulse (shots 1-6, 3.2 kW) 39.75 Kr,
60 Ar, 0.25 F2 _at_1.36 atm, Recirculator flow
rate 7.2 m/s
Intensity (arb.)
Time (Nanoseconds)
10
Previous Photodiode Response at 5Hz Rep-Rate
without recirculator or louvers
11
Recirculator both cools and quiets the laser gas
provides cooling for the foils
Louvers
Static Pressure Contours varies by 14 Pa (10-4)
over laser cell
12
Louvers provide cooling for the foil
Louvers Open
Louvers closed
gas flow
gas flow
Foils
Rib
Contours of Stream Function-- flow is quiescent
for next shot
Foil Temperature below required 650?F
0
Cell Entrance
louvers
10
gas flow
cm along foil
20
After 1st shot After 1st cycle After 2nd shot
Cell Exit
200?F
400?F
600?F
Concept Modeling A.Banka J.Mansfield,
Airflow Sciences, Inc
13
The Louvers Significantly Lower the Foil
Temperature
Also Run 1250 shots continuous at 1 Hz
(limit not reached) Run 169 shots
continuous _at_ 5 Hz (cathode failure)
14
Velvet Strip Cathode may be Limiting Foil
Lifetime _at_ 5 Hz

• Random Stochastic Breaking of foil (6-170 shots
  at 5Hz), Foil Temperature is
• constant within 50 shots
• Location of Punctures are
• Random in Foil

50 shot burst at 5 Hz
Possible Causes for Cathode limiting foil
lifetime ) Hot Spots (F. Hegeler) ) Floating
Edge Reducers (M. Myers) ) Random Plasma
Formation (J. Sethian) ) Macroscopic Debris
(M. Friedman) ) Something we have not
thought of (M. Wolford)

• Current status
• Working on Foil and Cathode Diagnostics
• Working on new cathodes (Ceramic- secondary
  electron emission)

15
Orestes (KrF Kinetics Code) Contour Plot (2002)
Rosc 10 Pbeam800 kW/cc T(t0) 300 oK F2
0.5 30 x 30 x 100 cc
Higher Laser Output for 1. Lower absolute
pressure 2. Lower Kr concentration
16
Kinetics Change Explains Fluorine Concentration
dissociative attachment for KrF, ArF, Kr2F, Ar2F,
ArKrF e.g. KrF e ? Kr F- Tw 75
17
KrF Kinetics Change in Products for 5 reactions
Reactants Super-Elastic Dissociative Attachment
eKrF KrFe KrF-
eArF Ar Fe ArF-
eArKrF ArKrFe ArKrF-
eKr2F KrKrFe KrKrF-
eAr2F ArArFe ArArF-
e
e
e
e
e
e
e
e
e
e
Peters et al. Appl. Phys. B 43, 253 (1987)
18
Single Pass Gain Set-Up
248 nm bandpass filter
Parasitic Light Attenuators
Neutral Density Filter
KrF output
1
12
Photodiode (1 ns risetime)
Iin
Iout
PD2
PD3
ND
E-Beam Pumped Region
KrF input
PD1
PD4
Beam Cube Polarizer
19
Single Pass Gain Measurements
Agrees with Previous and Current Oscillator
Output Measurements
20
Summary

• Extended Rep-Rate Run output constant after 50
  shots, indicates nothing is changing
• Recirculator and Louvers did not adversely
  effect output energy
• No Fluorine passivation is observed
• 89 Window Transmission before and after shots

• 100 shots at 1Hz with Laser energy pulse
  asymptotes to 700 J

• 5Hz Rep-Rate Energy constant (3.2 kW)

• Suspect, Durability Limitation is Velvet Strip
  Cathode

• Better Understanding of Fluorine Kinetics

• Single Pass Gain Measurements are Consistent with
  Oscillator Data

21
Steady-State Analysis of Measured Energy

• Laser Energy is emitted during constant power
  region allows steady-state approximation (Rigrod)
• W.W. Rigrod J. Appl. Phys. 36, 2487 (1965)
• Application of Rigrod to a single pass gain
  amplifier is (J. Appl. Phys. 70, 15, 4073 (1991)
• In a single pass case the windows are assumed to
  be 100 transmissive as well as no absorption in
  the unpumped region of the amplifier. The
  parameters are the small signal gain (g0), length
  (L), nonsaturable absorption (a), input intensity
  (Iin), output intensity (Iout), saturation
  intensity (Is) and gamma ( ? g0/a)
• Application of Rigrod to an Oscillator yields the
  following equation (IEEE J. Quant. Elect. QE-16,
  12, 1315 (1980)
• Rc is output coupler reflectivity 8. Tw is
  window transmission of 80.
• Under the conditions of 60 Ar, 39.75 Kr , 0.25
  F2 at 20 psi with 700 kW/cc e-beam deposition, Is
  was 2.7 MW/cm2 which agrees with single pass
  data.

New Oscillator Measurements are consistent with
2.5 MW/cm2 for Saturation Intensity
22
Target gain and power plant studies define the
laser requirements. Key issues and challenges are
High Gain Target Design (G gt100) 1
Laser IFE Requirements
IFE NIKE Beam quality (high mode)
0.2 0.2 Beam quality (low mode) 2
N/A(4) Optical bandwidth 1-2 THz 3 THz Beam
Power Balance 2 N/A(4) Rep-Rate
5 Hz .0005 Laser Energy (amplifier) 30-150
kJ 5 kJ Cost of pulsed power(1) 5-10/J(e-beam)
N/A3 Cost of entire laser(1) 225/J(laser)
3600/J System efficiency 6-7
1.4 Durability (shots) (2) 3 x 108
20 Lifetime (shots) 1010 104
Easy
Power Plant Study 2
Hard
1. 1999 . Sombrero (1992) gave 180/J and
4.00/J 2. Shots between major maintenance (2.0
years) 3. Not Applicable Different technology 4.
Not Applicable Nike shoots planar targets
1. 1-D gain. S.E. Bodner et al, .Direct drive
laser fusion status and prospects, Physics of
Plasmas 5, 1901, (1998). 2. Sombrero 1000 MWe,
3.4 MJ Laser, Gain 110 Cost of Electricity
0.04-0.08/kWh Fusion Technology, 21,1470,
(1992)
23
Oscillator Energy Dependence on Laser Cell
Pressure
24
Time Response of Oscillator at Various Fluorine
Concentrations
0.25F2, 39.75 Kr, 60 Ar
0.7F2 39.3 Kr 60 Ar
0.1F2, 39.9 Kr, 60 Ar
25
Temperature Rise for a 11 Shot burst at 1
Hz (Oscillator Energy Remains Constant)
Laser Cell
E-beam pumped region
505K
487K
463K
427K
Temperature of the e-beam pumped laser
gas (after e-beam deposition)