ELECTRA PRE-AMPLIFIER: A REPETITIVELY PULSED, ELECTRON BEAM PUMPED, KrF LASER*

1. ELECTRA PRE-AMPLIFIER: A REPETITIVELY PULSED, ELECTRON BEAM PUMPED, KrF LASER* • Naval Research Lab • Matt Myers • John Sethian • John Giuliani • CTI • Frank Hegeler • Tom Albert • Moshe Friedman • James Parrish • RSI • Warren Webster • SAIC • Matt Wolford • Areg Mangassarian • Titan/PSD • Dave Morton • Doug Weidenheimer * Work sponsored by DOE/NNSA/DP

2. The Electra Laser System Main Amp Osc. Pre-amp • A - Oscillator bench (LPX-305i) - 20 ns pulse at 248 nm ( ~ 1 J) • B, C, D - Split beam into 2 pulses and line up end to end (40 ns) • L - Single pass through pre-amplifier ( ~ 30 J) • E, F - Turning mirrors • G, H, I - Multiplex: 6 x 20 ns beams in pulse train • K - double pass through main amplifier ( ~ 700 J)

3. Electra Pre-Amplifier: Design Constraints • ORESTES KrF physics code estimates Electra Main Amp requires ~30J input to develop ~700 J output energy. • Allowing for losses, pre-amp must supply ~36 J. • Oscillator input to pre-amp will be 0.5 J, 20 ns FWHM pulse from commercial discharge • KrF (Lambda Physik LPX305i). • A double-sided, e-beam pumped system is less technologically risky than pure discharge or e-beam assisted discharge systems. • Incorporate advanced pulsed power architecture • fast marx/PFL/magnetic switch/TTI • marx initially uses conventional gas switches • marx retrofitted w/ advanced solid state switch • Demonstrate pulse shaping • necessitates single pass system • requires low system jitter (< 1 ns 1s) • requires zero jitter between e-beams

4. Opt for 10 cm x 10 cm Square Aperture • Why 10 cm x 10 cm square? • beam relaying to main amp easier • can use off-the-shelf, durable windows • flexible multiplexing • more compact pulsed power • less x-ray shielding • smaller magnets • good optical size for other applications (i.e. lithography) • Implications: • dictates 150-175 kV e-beams for efficient deposition at ~1 Atmosphere • low voltage means thin/transparent hibachi foils • reinforced or diamond coated aluminum maximize performance • initially use aluminum foil for acceptable performance • estimated hibachi efficiency 63-87%; pump power ~ 1200 kW/cc • need ~80 kA/side at ~ 175 kV • design for 20 ns rise/40 ns flat-top/20 ns fall pulse • minimize foil heating by slow electrons • establish a well-defined pump before amplification • flexible timing of pulse-train • ASE not thought to be a problem

5. 100 cm 10 cm 120 cm 10 cm Ar (Torr) Orestes: Predicted Laser Yield vs. Pressure and Composition for Electra Pre-Amp

6. Pulse Sciences Division Advanced Pulsed Power Architecture • High efficiency • Low $/E-beam joule cost • Excellent durability Pulse Forming Line Magnetic Switch Electron Beam Diodes Fast Marx (LGPT switching) Transit Time Isolator

7. Pulse Sciences Division Electra Pre-Amp: Conceptual Design Transit Time Isolators: water sections Transit Time Isolators: oil sections Top View Magnets Marx (gas switches) Mag Switch PFL Nominal: 175 kV, 80 kA, Z = 2.2  Low V: 150 kV, 68 kA, Z = 2.2  PRF: 5 Hz 20 ns rise, 40 ns flat-top, 20 ns fall Laser Cell 10cm x 10 cm x 100 cm Water Joints Cathode: 10 cm x 100 cm Side View Optical path 60” beam height Scale (100 cm)

8. Electra Pre-Amp: Performance DEMONSTRATED Pulsed Power Performance With Resistive Load Output: 175 kV, 160 kA, 40 nsec flat pulse (< 20 nsec rise) Rep Rate: Single shot to 5 Hz Durability: >100,000 shots before maintenance (Marx switches) 1  Jitter (Single Shot): 600 ps 1  Jitter (5 Hz, 10k Shots): 850-1200 ps

9. Electra Pre-Amp: Performance With Electron Beam Diode voltage • Diode performance with 1200 cm2 velvet cathode, 2.1 cm AK gap. • No electron beam rotation or shear with Bext=2.1 kG. • 50 shot, 5 Hz jitter ~ 800 ps 1s. • Diode voltage and current very reproducible.

10. Advanced Components In addition to demonstrating the advanced pulsed power architecture the Electra pre- amplifier will serve as a test bed for advanced KrF laser components. • The gas switches used in the Marx will be replaced by solid state LGPTs (laser gated and pumped thyristors.* This retrofit will significantly improve reliability, efficiency, and durability. • Ceramic honeycomb cathodes are used with 1010 steel focusing bars to segment the electron beams so they pass through the foil support structure (hibachi) with minimal attenuation allowing high efficiency deposition in the laser gas.** • A new hibachi that minimizes foil clamping stress and maximizes conduction cooling of the foil is being designed.*** *D. Weidenheimer, et al., “Advanced pulsed power concept and component development for KrF laser IFE”, Conference Record of the 25th International Power Modulator Symposium, 2002, p. 165. ** M. Friedman, et al., Jour. Appl. Phys. 96, 7714, 2004. *** J. Giuliani, see poster this HAPL Conference.

11. Diode Laser n++ p n- Silicon Thyristor n+ p++ D Laser Advanced Components: LGPT (Laser Gated and Pumped Thyristor) • CONCEPT: • All solid state • Diode lasers flood entire thyristor with photons • Fast switching times (< 100 nsec) • Continuous laser pumping reduces losses Power and Energy • PROGRESS: • > 1,200,000 shots (multiple runs) • Required specs: 16.4 kV, 5 Hz, 1 kA/cm2 • Switch has run @ 50 Hz shot 220,000 vs shot 1,000,000

12. Advanced Components: LGPT (Laser Gated and Pumped Thyristor) Application- Ultra Fast Marx (+/- 16.4 kV stage) Configuration with Conventional Switches 16.4 kV LGPT Switch Laser Drive Electronics Laser 1/2 Capacitor 1/2 Capacitor 7 cm 1/2 Capacitor 1/2 Capacitor Oil Insulation

13. Advanced Components: Ceramic Honeycomb - Iron Bar Cathode • Tests of monolithic ceramic honeycomb cathodes show promising durability without compromising rise time, gap closure, and uniformity. • Conversion of the full-sized ceramic honeycomb cathode to a strip geometry may be accomplished using soft iron bars. • Tests using iron bars with velvet emitter have been very successful up to 1 Hz.The next step is to adapt iron bars to ceramic honeycomb geometry and test at 5 Hz. • Strategically placed iron bars interact with the external magnetic guide field to produce local focussing fields along the emitter surface. The overall guide field is not perturbed. • The local concentration of magnetic field focuses the emitted beam electrons into vertical strips that propagate across the AK gap and through the openings in the hibachi.

14. Advanced Components: Ceramic Honeycomb - Iron Bar Cathode • Use 2.54 cm thick, 325 ppi cordierite ceramic honeycomb with g-alumina wash coat. • Use 1010 low-carbon steel bars for high mH (10 - 100). • Cathode is 10 cm x 100 cm.

15. H = 0.9 H = 0.9 - - 2.3 2.3 W/cm W/cm 2 2 Advanced Components: Conduction Cooled Hibachi • Simply cool foil by conduction to ribs using materials with high thermal conductivity and properly managed fluid flow. (see poster by John Giuliani) • Scalloped foil clamping design reduces tensile stress significantly. • Combination of cooler foil with less clamping stress allows use of thinner foils of lower Z material thus improving efficiency and durability.

16. Electra Pre-Amp: Status • Resistive load testing complete • 10,000 shot run w/ 1s jitter at 800 – 1200 ps • Marx gas switches require maintenance after ~ 50k – 100 k shots • Experimental diode performance nicely follows simulations. • E-beam testing into cooled anodes going well • 50 shot bursts at 5 Hz with velvet cathodes • diode voltages and currents very reproducible and consistent • 1s jitter at 800 ps • no beam shearing or rotation at Bext = 2.1 kG • longevity testing at 5 Hz will commence with Health Physics approval. • Advanced component development • LGPT has met design goals and pre amp Marx is due for retro-fit circa 2006. This will improve lifetime to 106 – 107 shot range. • Longevity testing of velvet-iron bar cathodes in progress. Ceramic honeycomb – iron bar cathode design complete. Mock up being built . • Conduction cooled hibachi design complete. Drawings finalized next week.