NRL J. Sethian M. Myers M. Wolford J. Giuliani J. Dubinger R. Lehmberg S. Obenschain Commonwealth Tech M. Friedman R. Jones K. Oakley T. Albert J. Parish K. Gunlicks RSI P. Burns S. Searles W. Webster SAIC R. Jaynes A. Mangassarian ECE F. Mora L-3 Com Pulse Sciences D. Weidenheimer D. Morton OptiSwitch Tech Corp D. Giorgi Naval Research Laboratory Plasma Physics Division Washington, DC presented by Frank Hegeler 14 th HAPL Meeting March 21-22, 2006 Oak Ridge National Laboratory Oak 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 cm seed osc 1cm x 3 cm

Recent accomplishments on the main amplifier (operating as an oscillator): Rep-rateTotal # of laser shotsE-beam pumping 5 Hz4,923double sided 2.5 Hz16,492double sided 2.5 Hz24,512single sided Continuous runs (# of shots) 5 Hz, double sided: (1,108 1,004 1, ) 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 hibachi laser cell 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 J with a strip cathode up to 300 J with a monolithic cathode Cathode size: 28 cm x 98 cm Electron beam emission is patterned to minimize electron loss on hibachi rib Cathode size: 35 cm x 105 cm Large fraction of the electron beam is lost to the hibachi rib Velvet cathode with iron bars Ceramic honeycomb cathode hibachi hibachi rib cathode shroud insulator (Z-stack) cathode ceramic honeycomb primary emitter: 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 temperature does not limit the laser run duration 2.5 Hz strip cathode 5 Hz monolithic 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 diode Construction techniques (joints, method of holding ceramic tiles) Robustness of cordierite ceramic honeycomb may decrease in time Metallic 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 feedthrough for 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 cathode cooling concept

Pre-amplifier results

40%Kr/60%Ar gas mixture Successfully tested electron beam pumping of the pre-amplifier Surveyed energy deposition characteristics with various pressure foils Laser experiments on the pre-amplifier will start in Summer 2006 Experimental results do not account for radiation losses 1-D ITS simulations for a ideal, rib-less hibachi Experimental results with 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 cathode and a low cost Al hibachi hibachi cathode Next generation hibachi will use a stronger material less ribs, shallower ribs higher 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 Lens BS M M Beam Block BS Cell WSBW Laser profile 8 x 3.8 mm 2 spot 1 J/cm 2 fluence Laser profile 6 x 3 mm 2 spot 2 J/cm 2 fluence UV/F 2 window testing apparatus

MgF 2 and CaF 2 windows show no damage with UV/F 2 SubstrateExposure Time Fluence J/cm 2 Fluorine Concentration (%) # of pulses % Change Transmission Change surface (nm) RMS>0.2 Damaged MgF 2 4 days NoneNo NoneNo NoneNo CaF 2 4 days NoneNo NoneNo NoneNo Fused Silica 11 days10.390,0000NoneNo ,0002%1.67Yes Fused silica with AR coating is preferred for cost and size considerations

Demonstrated advanced LGPT switch to > 10 7 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/cm 2 (macro), 14.7 kA/cm 2 (micro) … minimum requirement 13 kA/  sec-cm 2 (macro), 88 kA/  sec-cm 2 (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 Silicon Thyristor D Laser Diode Laser 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 MgF 2 and CaF 2 windows show no damage with UV/F 2 LGPT is undergoing life-cycle and durability testing with a new, improved switch configuration