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