NRL J. Sethian M. Myers M. Wolford J. Giuliani J. Dubinger R. Lehmberg S. Obenschain

1. 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 KrF Laser Development Naval Research Laboratory Plasma Physics Division Washington, DC 20375 presented by Frank Hegeler 14th HAPL Meeting March 21-22, 2006 Oak Ridge National Laboratory Oak Ridge, TN Work supported by DOE/NNSA/DP

2. Electra KrF Laser Layout pre-amp 10 cm x 10 cm main amp 30 cm x 30 cm seed osc 1cm x 3 cm

3. Since the last HAPL meeting we have been concentrating on durability (foil lifetime) Recent accomplishments on the main amplifier (operating as an oscillator): 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 …)

4. Electron beam pumping of the main amplifier cathode vacuum diode Laser gas is pumped by electron beams that are generated by cathodes in the vacuum diodes laser cell laser window opening hibachi

5. 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 Velvet cathode with iron bars Ceramic honeycomb cathode insulator (Z-stack) primary emitter: velvet cathode hibachi rib hibachi cathode shroud ceramic honeycomb 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

6. At a rep-rate of 2.5 Hz, global foil temperature does not limit the laser run duration 5 Hz monolithic cathode 2.5 Hz strip cathode Data obtained from new infrared thermal diagnostic The foil heat load of the strip cathode is approximately twice the heat load produced by the monolithic cathode (for the same rep-rate).

7. Presently, the laser durability is limited by the ceramic honeycomb cathode performance • 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) 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 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

8. 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 high voltage feedthrough for water cooling primary emitter: graphite cathode cathode cooling concept copper cooling plate

9. Pre-amplifier results

10. 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 with a low cost Al hibachi 1-D ITS simulations for a ideal, rib-less hibachi Experimental results do not account for radiation losses 40%Kr/60%Ar gas mixture Difference due to inefficient hibachi and monolithic cathode

11. Pre-amplifier hibachi issues and future design improvements Current system uses a monolithic cathode and a low cost Al hibachi • Al hibachi has • many ribs (57) • deep ribs (1 cm) • narrow rib opening (1.3 cm) Existing Al hibachi cathode This hibachi will be used for initial laser experiments Next generation hibachi will use a stronger material hibachi less ribs, shallower ribs higher electron energy deposition into the laser gas

12. Cell Lens BS BS M Beam Block WS BW Calorimeter Calorimeter 1 J discharge laser M UV/F2 window testing apparatus Aperture Laser profile 8 x 3.8 mm2 spot 1 J/cm2 fluence Laser profile 6 x 3 mm2 spot 2 J/cm2 fluence

13. MgF2 and CaF2 windows show no damage with UV/F2 Fused silica with AR coating is preferred for cost and size considerations

14. Pulse Sciences Division Demonstrated advanced LGPT switch to > 107 shots in the low inductance rectangular geometry required for the Marx LGPT: Laser gated and pumped thyristor 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

15. Diode Laser Diode Laser Pulse Sciences Division Developed a new, more efficient, illumination geometry for the LGPT Old: Lasers illuminate switch through holes in electrodes New: Lasers illuminate switch through sides Diode Laser n++ p n- Silicon Thyristor n+ p++ D Laser • PROGRESS: • 4.5 M shots, 5-7 Hz • 14 kV (meets specs) • Active area: > 60% of the Silicon • PROGRESS: • >15 M shots, 5-7 Hz • Active area ~15% of the Silicon 4 times more of the Si is participating in the switching

16. 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