John Sethian Naval Research Laboratory April 4, 2002 Electra title pageElectra NRL J. Sethian M. Friedman M. Myers S. Obenschain R. Lehmberg J. Giuliani JAYCOR S. Swanekamp Commonwealth Tech F. Hegeler SAIC M. Wolford Titan PSD, Inc D. Weidenheimer Work sponsored by DOE//NNSA/DP The Electra Laser Facility 500 keV, 100 kA, Hz (x 2 sides = 50 kW) The Electra Laser Facility 500 keV, 100 kA, Hz (x 2 sides = 50 kW)

Laser Gas Recirculator Topics This Time Rear Mirror Output Optics Laser Cell (Kr + F 2 ) Pulsed Power System Electron Beam Foil Support (Hibachi) Amplifier Window Cathode BzBz

Front runner for Advanced Pulsed Power should meet IFE requirements ( 80% eff) Notes: Cost: $ / e-beam Joule, for 100 kJ systems in quantities, NOT Electra; Efficiency: Flat top e-beam/wall plug TTI DIODE Marx/PFN Marx / Pulse Forming Network too new to evaluate cost and efficiency XFMRS1 DIODE PFL Laser-gated switch stack/array Transformer + PFL + HV laser output switch $ 8.35/J 87% eff TTI DIODE MC-1 Fast Marx Fast Marx w/ laser gated switches + 1 stage Magnetic Compressor $ 7.15/J 85% eff

 Burst 5pps – 10 4 shots  Shots to failure of chip under test – 1.36 x 10 5  Max voltage – 3.2 kV (limited by external insulation)  Max current density – 2.7 kA/cm 2 (121% of IFE requirement)  Max di/dt – 1.36 x A/sec/cm 2 (154% of IFE requirement, limited by circuit) > 10 x higher than non laser triggered devices  Max charge transfer per pulse – 1.7 mCb/cm 2 (152% of IFE requirement)  Laser output ~40% of maximum (tested at >10 7 shots)  Successful demo of thyristor/ laser diode integration Advanced Pulse Power Switch Development-1 RESULTS TO DATE LGPT (Laser Gated and Pumped Thyristor) Flood entire switch volume & junction with laser light… Gives high turn-on (dI/dt) *Photon  1105 nm = 1.36 eV I.E. Just above the band edge of the PN junction Diode Laser* Si Switch p n+ n n++ p++ CONCEPT Prototype has demonstrated concept Switch Lasers (hidden) Feed Electrodes Pulse Sciences Division

Advanced Pulse Power Switch Development-2 Full required performance (16.7 kV) NEXT GENERATION Generation 2 Forecasts  Maximum voltage > 16.7 kV (IFE requirements) Thicker device with two sided laser pumping Design allows use of advanced encapsulants (insulation) (soldered or alloyed interface of silicon and electrodes)  Action (I 2 t) in excess of IFE requirements  Integration of larger area reverse parallel co-planar diode to meet 100% of IFE reverse current requirement  Extrapolates to “rail gap” aspect ratio easily (full IFE scale) EXPECT TO BE READY FOR TESTING WITHIN 6 MONTHS Pulse Sciences Division 1 cm 2 silicon thyristor and co-planar diode Laser array in electrode (anode and/or cathode) (Single sided system shown)

Advanced Pulse Power Switch Development-3 Rail Gap Geometry FINAL CONFIGURATION Pulse Sciences Division

Test bed is evaluating lifetime of capacitor dielectric systems Capacitor/Switch Test Bed Results · 2 x 10 8 shots on CSI capacitors (~0.03 J/cm 3 ) – no failures · 1 x 10 8 shots on GA capacitors ( ~0.015 J/cm 3 ) – no failures · 2 x 10 8 shots on ABB thyristor in service comparable to 3 stage MPC for IFE Rep-rate: 55 Hz

Modified SLIA Rep Pre-Compression Stage HV Transformer Parallel Plate Test Volume Coaxial Test Volume Liquid Breakdown Dielectric Test facility nearing completion Pulsed Power Systems use liquids to store electrical energy V = V 0 V =0 Oil or water Electrodes, area A d E = V 0.d Electrical Breakdown strength determines size of the system: Oil (+): E =.48/(  1/3 A.075 ) Oil (-): E =.72/(  1/3 A.075 ) Water (+): E =.23/(  1/2 A.058 ) Water (-): E =.56/(  1/3 A.070 ) (  is time voltage is above 63% of peak) IFE driver..A(oil) = 10 8 cm 2, A (water) = 3 x 10 8 cm 2 Above formula derived for single shot events, not long term repetitive operation.

E = V o /d d Cathode (V =V o ) Anode (V = 0) Cathode development: Edge effect current density (A/cm 2 ) PredictionElimination Field- Shaper Problems with this solution 1. Field shaper emits in rep-mode or longer pulse 3 cm x 30 cm “Strip” Cathode 2. Tough to do with strip cathode Cumulative charge (AU) Verification Radiachromic film E = C-L e-beam Base Emitter

“Floating Edge Removers” eliminate edge, fit strip geometry, and don’t emit (over 3000 shots) (a) (b) Cathode base Cathode Emitter (velvet) Edge removers Anode current density (A/cm 2 ) (a) (b) Radiachromic Film See Frank Hegeler Poster

Expts & theory show beam subject to “transit time” instability. 2D PIC simulations predict resistive slots stabilize beam dN/dE (Arb. units) E (MeV) Not stopped by gas stopped by foils Cathode Anode e-e- Before Time (ns) V (MV) f=2.5 GHz Experiment (Nike 60 cm AMPLIFIER_ Simulation Time (ns) X (m) /4 Slot with Resistive Wire Z (cm) Cathode After dN/dE (Arb. units) E (MeV) Not stopped by gas stopped by foils Simulation

7 current density A/cm 2 time (100 ns/div) 0 14 frequency (GHz) amplitude time (100 ns/div) current density A/cm amplitude frequency (GHz) e-e- e-e- The slotted cathode suppresses the transit-time instability on NIKE 60 cm Amplifier Slotting cathode in other direction may completely eliminate instability

e-e- Nike-Solid Cathode cathode anode ribs pressure foil e-e- Nike-Slotted cathode Reducing instability increases hibachi transmission efficiency See Matt Myers Poster e-e- Electra/IFE: No anode Pattern, rotate beam Shallow Ribs

10 ms60 ms100 ms200 ms Concept & Modeling: A.Banka & J.Mansfield, Airflow Sciences, Inc Cell Exit Cell Entrance Contours of Stream Function-- flow is quiescent for next shot gas flow louvers After 1 st shot After 1 st cycle After 2 nd shot 200  F400  F600  F Foil Temperature below required 650  F cm along foil The recirculating laser gas can be used to cool the Hibachi Foils e-beam Rib Louvers Open gas flow Louvers closed gas flow FOR REFERENCE ONLY, DOES NOT VIOLATE “NO OLD VIEWGRAPH” RULE

Recirculator to both cool and quiet laser gas plus provide hibachi cooling has been designed Heat Exchanger Blower Laser Cell Homogenizers & Turning Vanes Static Pressure Contours varies by 14 Pa (10 -4 ) over laser cell Louvers

Other accomplishments Turn Electra into a laser: Start with oscillator, then add discharged pump commercial input Laser on order, optics awaiting in-house damage testing Rear Mirror stand here: Front end for Electra: Prescription from Orestes KrF kinetics code 30 cm x 10 cm aperture, 100 cm long 1.0 J input J output (enough to give 700 J output on main amp) e-beam pumped May be first demonstration of advanced switch technology Big issue: We still don’t have a cathode with the require durability