NRLJ. SethianM. Myers M. WolfordJ. GiulianiJ. DubingerR. LehmbergS. Obenschain
Commonwealth TechM. Friedman
R. JonesK. OakleyT. AlbertJ. ParishK. Gunlicks
RSIP. BurnsS. SearlesW. Webster
SAICR. JaynesA. Mangassarian
ECEF. Mora
L-3 Com Pulse SciencesD. WeidenheimerD. Morton
OptiSwitch Tech CorpD. Giorgi
Naval Research LaboratoryPlasma Physics DivisionWashington, DC 20375
presented by
Frank Hegeler
14th HAPL MeetingMarch 21-22, 2006
Oak Ridge National LaboratoryOak Ridge, TN
Work supported by DOE/NNSA/DP
KrF Laser Development
Electra KrF Laser Layout
main amp 30 cm x 30 cm pre-amp 10 cm x 10 cmseed osc 1cm x 3 cm
Recent accomplishments on the main amplifier(operating as an oscillator):
Rep-rate Total # of laser shots E-beam pumping5 Hz 4,923 double sided
2.5 Hz 16,492 double sided2.5 Hz 24,512 single sided
Continuous runs (# of shots)5 Hz, double sided: (1,108 1,004 1,001 922 888)2.5 Hz, double sided: (2,607 2,404 2,001 1,999 1,497 …)2.5 Hz, single sided: (10,004 4,176 3,089 3,007 1,500 …)
Since the last HAPL meeting we have been concentrating on durability (foil lifetime)
vacuum diode
cathode
hibachilasercell laser window opening
Laser gas is pumped by electron beams that are generated by cathodes in the vacuum diodes
Electron beam pumping of the main amplifier
Laser energy of “main amplifier” (operated as an oscillator)
up to 750 Jwith a strip cathode
up to 300 Jwith a monolithic cathode
Cathode size: 28 cm x 98 cmElectron beam emission is patterned to minimize electron loss on hibachi rib
Cathode size: 35 cm x 105 cmLarge fraction of the electron beam
is lost to the hibachi rib
Velvet cathodewith iron bars
Ceramic honeycomb
cathode
hibachi
hibachi rib
cathode shroud
insulator (Z-stack)
cathode
ceramic honeycomb
primaryemitter:velvet
The foil heat load of the strip cathode is approximately twice the heat load produced by the monolithic cathode (for the same rep-rate).
At a rep-rate of 2.5 Hz, global foil temperaturedoes not limit the laser run duration
0
50
100
150
200
250
300
0 5 10 15 20 25 30 35 40 45
time (s)fo
il te
mpe
ratu
re (d
eg C
)
0
50
100
150
200
250
300
0 50 100 150 200
time (s)
foil
tem
pera
ture
(deg
C)
2.5 Hzstrip cathode
5 Hzmonolithic cathode
Data obtained from new infrared thermal diagnostic
Possible cause for continuous shot limit (gas buildup): Velvet emitter heats up during rep-rate runs, releases gas
Secondary emission from honeycomb cathode releases gas
Presently, the laser durability is limited by the ceramic honeycomb cathode performance
Possible causes for the cathode failure (end of run): High current density hot spots develop due to gas buildup in the diodeConstruction techniques (joints, method of holding ceramic tiles)Robustness of cordierite ceramic honeycomb may decrease in timeMetallic support structures (cathode shroud) may generate debris
• The continuous shot number is limited by gas buildup in the vacuum diode (forces us to briefly break up long runs)
• Laser run duration is ultimately limited by foil failure.(due to cathode debris and/or excess plasma in the A-K gap)
high voltage feedthroughfor water cooling
copper cooling plate
primary emitter:graphite cathode
Cathode improvements are ongoing• Conditioning of the cathode• Replace velvet primary emitter with graphite emitter• Cool the cathode• Modify the cathode construction to eliminate weak spots• Replace cordierite ceramic with more robust SiC or YZA (yttria
stabilized zirconia-alumina)• Eliminate cathode shroud
cathodecoolingconcept
Pre-amplifier results
0
50
100
150
200
250
300
350
400
450
6 8 10 12 14 16 18 20laser cell gas pressure (psia)
elec
tron
ener
gy d
epos
ited
in la
ser c
ell (
J)
1 mil Kapton 1 mil Al5052
0.5 mil SS 1 mil Ti
40%Kr/60%Ar gas mixture
Successfully tested electron beam pumping of the pre-amplifierSurveyed energy deposition characteristics with various pressure foils
Laser experiments on the pre-amplifier will start in Summer 2006
0100200300400500600700800900
10001100
6 8 10 12 14 16 18 20laser cell gas pressure (psia)
elec
tron
ener
gy d
epos
ited
in la
ser c
ell (
J)
1 mil Kapton 1 mil Al
0.5 mil SS 1 mil Ti
Experimental results do not account for radiation losses
1-D ITS simulations fora ideal, rib-less hibachi
Experimental resultswith a low cost Al hibachi
Difference due to inefficient hibachi and monolithic cathode
Pre-amplifier hibachi issues and future design improvements
Current system uses a monolithic cathodeand a low cost Al hibachi
hibachi
cathode
Next generation hibachi will use a stronger material
less ribs, shallower ribshigher electron energy deposition into the laser gas
Al hibachi has - many ribs (57)- deep ribs (1 cm)- narrow rib opening (1.3 cm)
This hibachi will be used for initial laser experiments
Existing Al hibachi
Aperture
1 J discharge laser
Calorimeter Calorimeter
Lens BS
M
M Beam Block
BSCell
WS BW
Laser profile 8 x 3.8 mm2 spot1 J/cm2 fluence
Laser profile 6 x 3 mm2 spot2 J/cm2 fluence
UV/F2 window testing apparatus
MgF2 and CaF2 windows show no damage with UV/F2
Substrate ExposureTime
FluenceJ/cm2
FluorineConcentration
(%)
# of pulses
% Change Transmission
Change surface (nm)
RMS>0.2
Damaged
MgF2 4 days 1 0.3 45000 0 None No
1 0.3 90000 0 None No
1 0.3 180000 0 None No
CaF2 4 days 2 5 5400 0 None No
2 5 60000 0 None No
2 5 80000 0 None No
Fused Silica
11 days 1 0.3 90,000 0 None No
1 0.3 250,000 2% 1.67 Yes
Fused silica with AR coating is preferred for cost and size considerations
Demonstrated advanced LGPT switch to > 107 shots in the low inductance rectangular geometry required for the Marx
anode
cathode
PROGRESS:• > 15 M shots, 5-7 Hz• 16.4 kV (meets specs)• 1.5 kA/cm2 (macro), 14.7 kA/cm2 (micro) … minimum requirement• 13 kA/sec-cm2 (macro), 88 kA/sec-cm2 (micro) … 3x requirement
LGPT: Laser gated and pumped thyristor
Pulse Sciences Division
PROGRESS:• >15 M shots, 5-7 Hz • Active area ~15% of the Silicon
PROGRESS:• 4.5 M shots, 5-7 Hz • 14 kV (meets specs)• Active area: > 60% of the Silicon
Developed a new, more efficient, illumination geometry for the LGPT
Old:Lasers illuminate switch
through holes in electrodes
New:Lasers illuminate switch
through sides
p
n+
n-
n++
p++
Diode Laser
SiliconThyristor
D Laser
Diode LaserDio
de L
aser
Pulse Sciences Division 4 times more of the Si is participating in the switching
Summary
• Fired more than 45,000 total laser shots at rep-rates of 2.5 and 5 Hz• Achieved 10,000 continuous shots at 2.5 Hz• At present, cathode performance is the limiting factor in laser durability• We have a clear path towards solving the cathode issue• Foil temperature is not the limiting factor under these conditions
• Electron beam pumping of the pre-amplifier has been tested• Laser experiments will start in the Summer of 2006
• MgF2 and CaF2 windows show no damage with UV/F2
• LGPT is undergoing life-cycle and durability testing with a new, improved switch configuration
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