1. Full Anode Insert Pressure Cycle Simulation Device
DEVELOPMENT AND TESTING OF A SILICON (Si) BASED ELECTRON BEAM TRANSMISSION WINDOW
Evan K. Friis, UCSD
Mentor: Charles A. Gentile, PPPL
Abstract
A silicon (Si) based electron beam transmission window is being developed for use in the Electra KrF laser system in support of Inertial Fusion Energy. A 150 μm thick Si foil separates the KrF lasing medium from the electron beam source which is maintained at vacuum. Si has emerged as an excellent candidate due to its high strength, low Z, desirable thermal properties, and high electron transmission. The device must withstand cyclic (5 Hz) pressure differentials (2.3 atm), thermal load (300°C), and a highly corrosive environment with a minimum 80% e-beam transmission. To prevent corrosive degradation, one face of the window is coated with a 1.2μm diamond passivation barrier. This hostile environment presents a challenge in simulation. Empirical bench top simulation methods have been developed and results are discussed.
Full Anode Insert Pressure Cycle Simulation Device
The individual hibachi window panes are sandwiched in between two 3/16” titanium sheets cut to the to the specified pattern with a precision waterjet. Several possible anode insert mounting configurations are being fabricated for testing.
A high excursion voice coil actuated diaphragm constitutes one of the walls the gas side simulation chamber. Applying a current to through the voice coil decreases chamber volume and increases pressure accordingly.
Below from left to right – Diaphragm flange, gas chamber, window frame shim, hibachi window, window mount/vacuum chamber, vacuum backing plate. Modular steel plate design allows for different mounting schemes to be tested without retooling entire device. A thermal load can be applied to the speaker by applying a heat source to the vacuum plate.
NRL KrF Laser Facility
Hibachi Frame Pattern
To form a good vacuum seal and minimize the possibility of damage to the window during installation a mold has been designed and fabricated which will be used to pot the silicon wafer edges with a integral silicone sealing surface.
Sample 5 Hz Pressure Waveform
Diaphragm mounts to front face of device. 9 1
Design
Diaphragm at Equilibrium
Pulsed Diaphragm
Cooling Tube
Si Window
Titanium Window Frame
Potted Silicone Si Edge
Resistive Heating Element
Thermocouple
Pressure Gauge
Gas Side Simulation Chamber
Vacuum Side Simulation Chamber
Window heating is managed by tubing attached to the gas side of the window frame. Chilled water is pumped through the frame and through a heat exchanger. 2 10 3 4 5
After promising results regarding the single silicon-nanocrystalline diamond window’s performance, the design is being extended to a multi-window anode insert that is suitable for full-scale testing in the Electra laser. Twenty individual Si wafers held in metal grid that forms the anode. The e-beam is shaped to minimize interaction with the ribs and supporting structure.
Si Window 6 11 8
Titanium Frame 7
Anode Insert
Testing
Summary
To support continued successful development, a benchtop method of testing prototypes has been developed. The initial single window pane test apparatus utilized a cam driven bellows to simulate the pressure cycle of the laser. In designing an apparatus to cycle the
Silicon/nanocrystalline diamond wafers have been shown to be an effective e-beam transmission window material for use in KrF laser systems. Multi-window anode inserts have been fabricated and are currently undergoing extended cycle benchtesting- at PPPL. New techniques have been developed and deployed in simulating the lasing medium chamber conditions at the benchtop level.
pressure against the entire window frame assembly an electromechanical approach was taken to allow variation of the pressure waveform without modifying the device.
Hibachi Frame in Anode Insert
First Generation Test Apparatus
Acknowledgements: R. Parsells, L. Ciebiera, S. Langish, C. Priniski, R. Yager, C. Jun, J. Dong, R. Camp