ELECTRON BEAM TRANSMISSION WINDOW EMPLOYING SINGLE-CRYSTAL Si AND NANOCRYSTALLINE DIAMOND Charles Gentile 1, Charles Gentile 1, J. Butler 2, L. Ciebiera 1, H. M. Fan 1, R. J. Hawryluk 1, F. Hegeler 1, P. Heitzenroeder 1, C. Jun 1, L. P. Ku 1, P. LaMarche 1, S. Langish 1, M. Myers 2, R. Parsells 1, S. Raftopoulos 1, J.D. Sethian 2, M. Viola 1 (1) Princeton University PPPL (2) Naval Research Laboratory Abstract A silicon (Si) based electron beam transmission window with a 2 µm nanocrystal diamond coating is to be produced for use in a KrF laser system in support of inertial fusion energy technology. The window separates the lasing medium from an electron beam source while allowing the electron beam to pass through. The window must meet requirements set by the characteristics of the laser and must be able to withstand the hostile environment presented by the lasing medium and electron beam source. This environment includes KrF gas,  1500 gauss magnetic fields, large exposure to x-rays, and a ΔP of 2 atm. Materials, structures, and cooling systems for the window are determined theoretically based on requirements of the system. Empirical tests for the window are designed to simulate the actual environment it will operate in. Results of these empirical tests are discussed. Various design changes are considered to enhance the structural integrity of the windows and relieve stresses at the edges due to thermal expansion. This work is supported by the Naval Research Laboratory (NRL) in collaboration with the Princeton Plasma Physics Laboratory (PPPL). Motivation "Theoretical and experimental studies of KrF laser systems." [Naval Research Laboratory Broad Agency Announcement No , BAA 45] “Titanium foils typically used...” may not be “... suitable because of low heat conductivity, corrosion with fluorine and... allotropic transformation at elevated temperatures.” [Zvorykin, IFSA184] “Kapton ° C deformation temperature... under such conditions, the Kapton... partially lost its flexibility.” [cond-mat/ ] “Single-crystal silicon... advanced micro fabrication technology... high- precision high-strength high-reliability mechanical material... Young’s modulus of silicon... has a value approaching that of stainless steel... tough, hard, corrosion-resistant, thin-film coatings such as CVD... prevent direct contact of the silicon to fluorine gas” [Petersen, Proceedings of the IEEE, Vol 70, No. 5 (1982)] The electron transmission window in the Electra KrF laser must allow greater than 80% beam energy transmission while withstanding a harsh environment that includes fluorine gas, 5 Hz cyclic pressure (ΔP ABS  2.0 to 2.5 atm), and thermal load. Pulsed Power System Electron Beam Electron Transmission Window (Hibachi) Laser Cell Output Optics Laser Output Laser Input Laser Gas Recirculation Theory Single crystal silicon provided with a 2 µm nanocrystal diamond coating may provide for a robust window with high transmission and good thermal properties. Yield Strength:7000 MPa53000 MPa Young’s Modulus:190 GPa7000 GPa Thermal Conductivity:157 w/m °C2,000 w/m °C Thermal Expansion:2.33 E-06 / °C1.0 E-06 / °C Properties Silicon (SCS) Nanocrystalline Diamond ParameterValue repetition rate5 Hz voltage peak500 kV current peak100 kA pulse length (rise)40 ns pulse length (flat-top)100 ns pulse length (fall)80 ns cathode area30 cm x 100 cm current density33.3 A/cm 2 flow rate 1.1 m 3 /s inlet temperature 35 °C LASER CELL / GAS PULSED POWER / E-BEAM composition Kr, Ar, F 2 After 88 hours of exposure to 0.05% fluorine gas, silicon nitride passivation layer was etched away. Key Components of a KrF Excimer Laser Electra Laser Parameters Summary Empirical testing methods were developed to simulate the environment present in the Electra KrF laser. Si wafers coated with nanocrystalline diamond were subjected to extended exposures to fluorine gas with no degradation of the diamond foil. Delta P testing with Si/nanocrystalline diamond hybrid wafers indicated that material integrity remained within design parameter considerations. Various cooling schemes are under investigation. Si and nanocrystalline diamond have physical properties that provide good materials for use as an electron beam transmission window in KrF lasers. The (Generation II) window stiffened membrane configuration allows for high electron beam transmission and structural integrity. Protection from fluorine gas is accomplished with ~ 2 micron layer of nanocrystal diamond. Modeling of the Generation III window configuration indicates increased structural stability. The Generation III window is being investigated for prototype fabrication. Si window spray nozzles + POSITIVE - NEGATIVE hibachi frame (electrical conductor) TCTC - Forced Convection Etched Micro-channels Evaporative Cooling Concept - Prior to each pulse, low vapor liquid will be sprayed uniformly on the surface of the Si windows. Flow tubes within the support ribs of the hibachi frame will deliver this coolant to the spray nozzles. Conductive Heat Transfer is achieved through the pumping of water through channels within the support frame. Forced Convection allows for laser gas flow across the entire hibachi surface area. Cryogenic coolant is pumped through etched micro-channels within the rib structures. Thermoelectric Cooling Concept - Incorporating the Peltier effect to move heat from the bulk section of the window to the edges. The hibachi frame will serve as a sink for facilitating heat rejection. A second-generation configured (pre-etched) window configured foil during a routine pressure deflection test under static differential pressure. Window Design Evolution To maximize fatigue life, various heat rejection methods are considered: Mechanical Load (monotonic and cyclic) sustained monotonic pressure differential to 1.5 atm before failure survived cyclic pressurization between 1.0 atm and 1.3 atm (5 Hz) Electron Bombardment (500 keV, 33.3 A/cm2) survived 43 electron beam shots in Electra permitted electron transmission efficiency of 69% ± 5% Results of Empirical Testing Performed (First-Generation Window) With Si 3 N 4 Coating Generation I Single window - 25 pane configuration With Diamond Coating Empirical Testing A pressure cycling apparatus was designed to mechanically model the KrF Laser System. A low vacuum environment is applied to the etched region of the window. An initial pressure of approximately 1.3 to 2 atm (absolute) is placed on the other surface and cycled at 5 rev/sec. Generation I Window Generation II Window Single foil (cryo-spray) cooling concept With Diamond Coating Diamond foil on silicon wafer at 5 mtorr Cryo Panel PivotCooling Tube Silicon coated with nanocrystalline diamond foil Convex side faces KrFAr gas Generation II Single window - 81 pane configuration Generation III Single hemispherical configuration Generation III Window Multiple hemispherical configuration