KrF Laser Development

KrF Laser Development
1

Opening Remarks on KrF Laser Development
Many institutions have had programs in
e-beam pumped KrF lasers
Los Alamos National Laboratory
Livermore National Laboratory
Avco Textron
Rutherford Appeleton Labs, UK
ETL, Ministry of Technology and Industry, Japan
Lebedev Institute, Russia
The Naval Research Laboratory is the first to develop
routine, high energy, efficient, and repetitive KrF capability
2

NRL Progress in KrF Laser Development
Repetition rate
(pulses/second)
Predicted Efficiency
(%)
Durability
(continuous shots)
2001
.00056
1.9
200
2010
2.5 to 5
7.1
90,000
IFE
5
> 6.0
300 M
Most all of these advances have been made through
understanding and controlling the relevant physics
3

Elements of a Krypton Fluoride (KrF)
electron beam pumped gas laser
Energy + ( Kr+ F 2) ⇒ ( KrF)* + F ⇒ Kr + F 2 + hν (λ = 248 nm)
Pulsed
power
Cathode
Input Laser
(Front end)
electron
beam
e-beam
window
(hibachi)
Window
Laser cell
(Kr+F 2 +Ar)
Typical e-beam: 500 -700 keV, 100 - 500 kA, 3000 – 12,000 cm 2
Laser Gas
Recirculator
KrF laser
Physics
4

KrF developed with the NRL Electra and Nike Lasers
Electra: (5 Hz)
400-700 J laser light
500 keV/100 kA/100 nsec
30 cm x 100 cm e-beam
Develop technologies for:
Rep-Rate,
Durability,
Efficiency,
Cost
Nike: (Single Shot)
3-5 kJ laser light
750 keV, 500 kA, 240 nsec
60 cm x 200 cm e-beam
E-beam physics on full scale diode
Laser-target physics
5

Pulsed Power
Pulsed
power
Cathode
Input Laser
(Front end)
electron
beam
e-beam
window
(hibachi)
Window
Laser cell
(Kr+F 2 +Ar)
Laser Gas
Recirculator
KrF laser
Physics
6

Existing “spark gap based” pulsed power system:
Limited to 50,000 – 100,000 shots continuous
Spark Gap
Primary
Switch
Primary
Energy
Store
(Capacitor
Bank)
NEW
Pulse Forming Lines
Discharge through the
1:12 step-up transformer
92,000 shots
Laser
triggered
Spark Gap
main switch
Lifetime
limit is
Electrode
Erosion
7

Solid State system should be durable and efficient
♦ Concept for Electra, scalable to IFE size system
♦ Basic elements tested to > 300 M shots
Primary Energy Store
Fast Marx
Long Life capacitors
Primary Switches
New fast thyristors
(Modified commercial units)
Output Switch
Saturable Magnetic Inductor
Modeled system for Electra:
Predict Efficiency > 82%
(Wall plug to flat top pulsed power)
PLEX, LLC
8

All Solid State Pulsed Power demonstrator
Continuous run: 11.5 M shots @ 10 Hz (319 hrs)
Load
200 kV, 4.5 kA, 250 ns,
Marx > 80% efficiency
Pulse Forming Line
Marx
Magnetic
Switch
PLEX, LLC
9

Electron Beam -- Generation and Transport
Pulsed
power
Cathode
Input Laser
(Front end)
electron
beam
e-beam
window
(hibachi)
Window
Laser cell
(Kr+F 2 +Ar)
Laser Gas
Recirculator
KrF laser
Physics
10

ELECTRON BEAM GENERATION
Need cold cathodes: simplicity, efficiency, durability
Best so far: Carbon Fibers pyrolized to aluminum
Robust (> 250,000 shots)
Low cost ($15 k Electra)
Easy to make to patterned
cathodes
Cathode dimensions:
30 cm x 100 cm
11

ELECTRON BEAM TRANSPORT
Two innovations gave high hibachi transmission:
1. Eliminate anode foil
2. Pattern the beam to “miss” the ribs
Emitter
Vacuum
Anode
Foil
(.001” Ti)
e-beam
Rib
Laser Gas
Kr + F 2 + Ar
Pressure
Foil
(.001” SS)
Energy
Deposited in gas
Before: ∼ 35%
After: ∼ 76 - 81%
12

Simulations and experiments show iron bars can
efficiently guide electron beam past the hibachi ribs
B z
foil
iron
rib
laser gas
emitter
13

3-D LSP Simulations (Voss Scientific/Albuquerque)
♦ Prescribe the emitter topology
♦ Predict observed electron beam deposition into the gas
Efficiency ≡ Energy deposited in laser gas/energy flat portion of e- beam
Deposition Efficiency
Efficiency ≡ Energy deposited in laser gas/energy flat top portion of beam)
E-beam Voltage
Pressure Foil
Experiments
Simulation
500 keV
2 mil (Ti)
67%
66%
500 keV
1 mil (Ti)
75%
76%
800 keV
1 mil (SS)
N/A
> 81%*
*1-D modeling for
800 keV full size
IFE system
14

Hibachi Foil: Cooling and Durability
Pulsed
power
Cathode
Input Laser
(Front end)
electron
beam
e-beam
window
(hibachi)
Window
Laser cell
(Kr+F 2 +Ar)
Laser Gas
Recirculator
KrF laser
Physics
15

FOIL COOLING
Needed to avoid metal fatigue (470 °C for SS 304)
and minimize unwanted chemistry
Two previous concepts we evaluated:
“V” plate
Foil Temp = 220 °C @ 2.5 Hz
Predict 440 °C @ 5 Hz. (A bit warm)
E-beam
“V” plate
Foils
E-beam
gas flow
“Mist Cooling”
Foil Temp < 140 °C @ 5Hz
Electron
Emitter
e-beam
Rib
Rib
Laser Gas
Kr + Ar + F 2
Anode
Foil
He (air) water
mist + film on
foils
Pressure
Foil
16

"Jet" cooling technique developed by Georgia Tech:
Effective, efficient, and scalable to large apertures
TOP VIEW
VIEW FROM GAS
INTO THE DIODE
Main
Gas
Flow
Rib
foil
Tube
Jets
foil
Tube Jets
e-beam
Laser Gas
17

tem p (C )
Jets cool foil while maintaining laser quality
with minimal power consumption
450
400
350
300
250
200
150
100
50
0
Laser
average foil temperature < 370° C
(Fatigue Reprate=5Hz, limit Blower=30 ∼480 Hz, ° GT=80 C) cfm
0 2 4 6 8 10 12 14
time (s)
Intenstiy (Counts)
100000
90000
80000
70000
60000
50000
40000
30000
20000
10000
0
Jets cool foil
Do not perturb laser gas
no decline in energy in focus
at 5 Hz for 50 shots
0 10 20 30 40 50
Shot #
9 x 9 pixel
18
Georgia Institute of Technology & Matt Wolford

FOIL DURABILITY
One key: Control late time voltage in e - beam diode.
Increasing diode impedance 10%, lowering charge voltage 15%:
Eliminates voltage reversal, and hence damaging foil emission
300
200
100
0
-100
-200
-300
-400
-500
-600
Diode Voltage (kV) vs time
time (nsec)
Voltage reverses
< 10,000 shot runs
Foil fails due to pinhole.
(“Cathode spots”)
No Voltage reversal
∼ 100,000 shot runs
80 µm
pinhole
19

Electra now capable of ∼100 k shot runs
90,000 laser shots (10 hrs) continuous @ 2.5 Hz
150,000 laser shots on same foils @ 2.5 Hz
50,000 laser shots on same foils @ 5 Hz
300,000 laser shots in 8 days of operation
Electra Cell after 30,000 shot continuous laser run
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A video starring Electra
21

KrF Physics: Simulations and Modeling
Pulsed
power
Cathode
Input Laser
(Front end)
electron
beam
e-beam
window
(hibachi)
Window
Laser cell
(Kr+F 2 +Ar)
Laser Gas
Recirculator
KrF laser
Physics
22

KrF PHYSICS
“Orestes” Code includes all relevant phenomena
1-D & 2-D Electron Deposition Plasma Chemistry
3-D Laser Transport 3-D Amplified Spontaneous Emission
24 species, 146 reactions, 53 vibrational states
Good
Bad
Neutral
Channel
Kr *
e-beam
Kr
F 2
e -
F -
e-beam
harpoon Kr ion-ion rec
F 2
γ, Ar, Kr, F 2 , e -
Kr,Ar,F 2
Ar* Ar +
exchange
ArF *
KrF*
F -
Kr
GAIN, go 2Ar 2Kr
γ, Ar, Kr, F 2 , e -
ArKrF * Kr 2F *
Kr,Ar,F
γ, F 2 , e -
Kr +
Ion
Channel
absorption, σ = σ F2 η F2 + σ F- η F- + σ KrF2 η KrF2 + σ ArF2 η ArF2
23

Orestes accurately predicts Electra Main Amplifier
Laser Pulse
9
8
7
6
5
4
3
2
1
0
0 50 100 150 200 250
P e-beam (expt)
I Laser (expt)
P e-beam (Orestes)
I Laser (Orestes)
Shot OSC080404_11
E Laser = 731 Joules
24

Orestes predicts performance of many KrF Lasers
operating over a wide range of parameters
(a) Keio Univ.'s
single pass amp
I out -I in (MW/cm 2 )
P beam
=1730 kW/cc
Orestes
(b) NRL's NIKE
double pass amp
E(laser) out (kJ)
no ASE
Orestes
I
in
=52.5 kW/cm 2
(c) Lebedev's Garpun
single pass amp
P beam =1.3 MW/ cc
no ASE
I out (MW/cm 2 )
Orestes
0.62 MW/cc
no ASE
[Suda, et.al., Appl.Phys. Lett., 51, 218 (1987)]. [McGeoch, et.al., Fusion Tech., 32, 610 (1997)]. [Zvorykin, et.al., Final Report,
Lebedev Institute (2002)].
25

Electra: ∼ 10% intrinsic efficiency as oscillator
expect ∼ 12 % as an amplifier
P E-beam (GW)
Laser energy
Intrinsic Efficiency ≡ (in flat part of pulse)
e-beam energy
70
60
50
40
30
20
10
0
Efficiency (9.8%)=
E Laser (71.5 J)/
E e-Beam (730 J)
Total laser energy 730 Joules
0 50 100 150 200
7
6
5
4
3
2
1
0
P Laser (GW)
26

Based on our research, an IFE-sized KrF system
should have a wall plug efficiency > 7%
Pulsed Power All solid state 82%
(wall plug- flat top e-beam)
Hibachi No Anode, Pattern Beam 81%
(e-beam in diode into gas)
KrF Electra Experiments 12%
(e-beam to laser) (literature ∼ 14%)
Optics to target Estimate 95%
Ancillaries Pumps, recirculator 95%
Total 7.1%
For fusion energy want ηG > 10.
with KrF and advanced targets: ηG = 7.1% x 300 ~ 21
27

18 kJ, 5 Hz, KrF laser amplifier:
Extrapolate Nike, using Electra Technologies
Window
Laser
Nike - 5000 J
650 keV, 500 kA, 240 nsec
60 cm x 60 cm window
60 cm x 200 cm each beam
Monolithic Cathode
15
10
5
250 nsec
Iout [MW/cm2 Iout [MW/cm ] 2 ]
electron beam
P Pe-beam e-beam [MW/cc x 10]
Iin [MW/cm2 Iin [MW/cm ] 2 ]
0 0 100 200 300 400
time (nsec)
Mirror
FTF - 18,000 J
800 keV, 500 kA, 250 nsec
60 cm x 100 cm window
60 cm x 250 cm each beam
Strip cathode
efficiency ∼ 12%
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Summary of Achievements.
• Durable, efficient, all solid state pulsed power
• Generation, transport, efficient deposition of electron beam
• Jet foil cooling
• Should meet efficiency, based on experiments
• Meets pulse shaping, zooming, uniformity requirements
• Orestes KrF physics code to design future systems
29

Main outstanding issue: >> 1M shot durability
SHORT TERM
• Windows (now quartz):
• Exploring degradation mechanisms
• Can use Calcium Fluoride, as in commercial units
•Foils:
• Improve pulsed power durability
• Mitigate all late time voltage
LONG TERM
• Long term integrated demonstration at IFE class size
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