John Sethian Naval Research Laboratory Sep 24, 2003 Electra title pageElectra NRL J. Sethian M. Friedman M. Myers S. Obenschain R. Lehmberg J. Giuliani P. Kepple JAYCOR S. Swanekamp Commonwealth Tech F. Hegeler SAIC M. Wolford TITAN-PSD D. Weidenheimer D. Morton MRC-Albuquerque D. Rose D. Welch Work sponsored by DOE//NNSA/DP

Input Laser (Front end) Laser Gas Recirculator The Key Components of a KrF Laser Laser Cell (Kr + F 2 ) Pulsed Power System Electron Beam Foil Support (Hibachi) Amplifier Window Cathode BzBz ENERGY + (Kr + F 2 )  (KrF) * + F  (Kr + F 2 ) = h  ( =248 nm)

Input Laser (Front end) Topics This Time Output Optics Laser Physics Cell (Kr + F 2 ) Pulsed Power System Electron Beam Hibachi (Cooling) Amplifier Window Cathode BzBz Laser Gas Recirculator

Laser Physics: Last Meeting/ This time Previous: J rep 1 Hz / 10 sec burst..longer runs needs recirculator 2. Completed first round of laser physics experiments A.Higher laser output at lower pressures and Kr concentration, per Orestes code predictions. B.Laser output peaks at F 2 concentrations around 0.25%. Lower than code predictions, but good for KrF laser development This Time: 1. Measured small signal gain (5.3% / cm) Agrees with standard Rigrod formula 2. Measured power efficiency of 8.4% as an oscillator Based on this expect 12-14% as an amplifier 3. Now predict overall efficiency of 7-8%

Photodiode 248 nm Filter Configuration to measure Single Pass Small Signal Gain Parasitic Light Attenuator Polarizer ND Filter KrF Input (Commercial 1.5 J laser)

Single Pass Gain Measurements 700 kW/cc, 60%Ar, 39.75% Kr, 0.25% F 20 psi Input (Watts/cm 2 ) Output (Watts/cm 2 ) Raw data Average Rigrod Prediction* Results: I sat = 3.0 MW/cm 2 g 0 (gain) = 0.053/cm  (loss) = /cm IEEE J. Quant. Elec. QE 16, 12,1315 (1980) *

Flat Mirror Electra Configured as an oscillator Output Coupler

P e-beam /P laser = 5.3/63 = 8.4% Electra achieved  8% intrinsic efficiency as an oscillator...expect significantly more as an amplifier P e-beam (GW) t (nsec) P laser (GW) M. Wolford WPo4.36

How we project amplifier efficiency of 12% based on oscillator results of 8.4% A properly designed amp would have: No Output coupler (no 8% reflection losses) Good windows (>98% transmitting vs 70-80% in oscillator) Amplification from input laser, not "noise"

Based on our research, an IFE-sized KrF system is projected to have a wall plug efficiency of > 7% Pulsed PowerAdvanced Switch87% Hibachi StructureNo Anode, Pattern Beam80% KrFBased on Electra exp'ts12% OpticsEstimate95% AncillariesPumps, recirculator95% Total7.5% > 6 % is adequate for gains > and latest designs have 2D gains ~ 170

Recirculator/Hibachi Cooling: Last meeting/ This time Previous: 1. Recirculator installed 2. Louvers for cooling hibachi installed 3. Blower turned out to be a big pain in Louvers closed Louvers open This Time: 1. Blower bigger pain in the than originally thought But worked long enough to allow some foil cooling studies 2. Actuating louvers drops foil temperature from 360 to 140 deg 1 Hz Have over 1250 shots on foils Performance at 5Hz unresolved 3. Also pursuing alternate hibachi cooling techniques Spray cooling (Georgia Tech) Si/diamond hibachi (PPPL)

Recirculator to cool and quiet laser gas plus cool hibachi foil is under evaluation Homogenizers & Turning Vanes Blower Laser Cell Heat Exchanger Static Pressure Contours varies by 14 Pa (10 -4 ) over laser cell

The recirculating laser gas can be used to cool the Hibachi Foils Rib t = 0 ms e-beam gas flow Louvers Modeling: A.Banka & J.Mansfield, Airflow Sciences, Inc Gas Velocity t = 25 ms Foils Rib t = 0 ms e-beam t = 75 ms t = 125 ms t = 0 ms (200 ms) e-beam t = 25 ms

The louvers significantly lower the foil temperature Also: Run 1250 shots continuous at 1 Hz (limit not reached) Run 169 shots 5 Hz (cathode failure) 1 Hz, with louvers 5 1 atm 210  C 1 Hz, with louvers atm 140  C 1 Hz, no louvers 360  C

50 shot burst at 5 Hz with louvers 5 1 atm of Ar At 5 Hz operation, the pressure foil reaches 520°C with the recirculator operating at 50% of its cooling capacity

We are evaluating two alternate foil cooling techniques for smaller (front end) & larger (full scale beam line) systems Silicon + diamond window panes with Ti mullions Spray Cooling Advantages high T c of Si may allow rib cooling for small, low voltage system See C. Gentile et al (PPPL) Poster Advantages Heat removal very high for large, high voltage system See S. Abdel-Kahlik et al (GT) Poster Anode foil Pressure foil Coolant Injector(s)  1 cm He/air Flow Turbulent He/air + atomized coolant LASER GAS e-beam Pump

We are refining our design for a full scale beam line amplifier 60 kJ Laser Amplifier e-beam 80 cm 50 cm 20 cm See David Rose Poster on e-beam propagation E stored 100 kJ x 8 = 800 kJ V, I,  :800 keV, 84 kA x 16, 600 ns Energy in gas:544 kJ Laser Input:4 kJ Laser Output:57.8 kJ Laser

SUMMARY-progress in last five months 1.Measured small signal gain..in agreement with Rigrod analysis 2.Measured intrinsic power efficiency (P laser /P e-beam ) 1.8.4% as oscillator 2.based on this predict 12% as amplifier 3.Deflecting laser gas to cool hibachi foil looks good…so far 1.More than adequate cooling at 1 Hz shots on foil and counting 2.5 Hz operation and gas quieting still needs to be established 4.Other foil cooling techniques are under development

Status of KrF Laser Development for IFE... a "qualitative self assessment " Efficiency Durability Rep-Rate Beam quality Cost Just starting Ready for Phase II 1.Demo as amplifier 2.Pulse shaping k shots cont on system 2.Pulsed power switch demo 3.Windows Consistent performance at rep-rate

Previous progress-1 1. Pulsed Power Prototype demo of solid state laser triggered switch Basis for long life pulsed power system with 87% efficiency 2. Electron beam: Completely eliminated Transit Time instability BEFORE AFTER

Previous progress-2 3. Hibachi: Demonstrated high transmission hibachi Up to 75% e-beam energy deposited into gas Agrees with LSP modeling Expect >80% on full scale (750 keV) system

Single Pass Gain Measurements all in close agreement

Laser Energy is emitted during constant power region allows steady-state approximation (Rigrod) W.W. Rigrod J. Appl. Phys. 36, 2487 (1965) Application of Rigrod to a single pass gain amplifier is (J. Appl. Phys. 70, 15, 4073 (1991) In a single pass case the windows are assumed to be 100% transmissive as well as no absorption in the unpumped region of the amplifier. The parameters are the small signal gain (g 0 ), length (L), nonsaturable absorption (α), input intensity (I in ), output intensity (I out ), saturation intensity (I s ) and gamma (γ = g 0 / α) Application of Rigrod to an Oscillator yields the following equation (IEEE J. Quant. Elect. QE- 16, 12, 1315 (1980) R c is output coupler reflectivity 8%. T w is window transmission of 80%. Under the conditions of 60% Ar, 39.75% Kr, 0.25% F 2 at 20 psi with 700 kW/cc e-beam deposition, I s was 2.7 MW/cm 2 which agrees with single pass data. Steady-State Analysis of Measured Energy