ELECTRA PRE-AMPLIFIER: A REPETITIVELY PULSED, ELECTRON BEAM PUMPED, KrF LASER* * Work sponsored by DOE/NNSA/DP 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

The Electra Laser System  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) Main Amp Pre-amp Osc.

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 1  )  requires zero jitter between e-beams

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 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

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

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

Electra Pre-Amp: Conceptual Design Top View Side View Transit Time Isolators: water sections Magnets Laser Cell 10cm x 10 cm x 100 cm Optical path Cathode: 10 cm x 100 cm Water Joints 60” beam height Marx (gas switches) PFL Mag Switch Scale (100 cm) 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 Pulse Sciences Division Transit Time Isolators: oil sections

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): ps

Electra Pre-Amp: Performance With Electron Beam Diode  Diode performance with 1200 cm 2 velvet cathode, 2.1 cm AK gap.  No electron beam rotation or shear with B ext =2.1 kG.  50 shot, 5 Hz jitter ~ 800 ps 1 .  Diode voltage and current very reproducible. voltage

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 25 th International Power Modulator Symposium, 2002, p ** M. Friedman, et al., Jour. Appl. Phys. 96, 7714, *** J. Giuliani, see poster this HAPL Conference.

Advanced Components: LGPT (Laser Gated and Pumped Thyristor) p n+ n- n++ p++ Diode Laser D Laser Silicon Thyristor CONCEPT: All solid state Diode lasers flood entire thyristor with photons Fast switching times (< 100 nsec) Continuous laser pumping reduces losses PROGRESS: > 1,200,000 shots (multiple runs) Required specs: 16.4 kV, 5 Hz, 1 kA/cm 2 Switch has 50 Hz shot 220,000 vs shot 1,000,000 Power and Energy

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

 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. Advanced Components: Ceramic Honeycomb - Iron Bar Cathode

 Use 2.54 cm thick, 325 ppi cordierite ceramic honeycomb with  -alumina wash coat.  Use 1010 low-carbon steel bars for high  H ( ).  Cathode is 10 cm x 100 cm.

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. H = W/cm 2 H = W/cm 2

 Resistive load testing complete  10,000 shot run w/ 1  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  1  jitter at 800 ps  no beam shearing or rotation at B ext = 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 This will improve lifetime to 10 6 – 10 7 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. Electra Pre-Amp: Status