Kinematic Mount Design of Line Replaceable Units at the National Ignition Facility ME Precision Machine Design - Dr. Furman April 8, 2003 Dennis Pak Behrouz Sadrabadi

National Ignition Facility Located at Lawrence Livermore National Lab $3.9 Billion DOE Defense Programs project 192 laser beams will all converge onto a BB-sized capsule Power output 1000 times the electric generating power of the U.S.

NIF Missions Primary mission: support the Stockpile Stewardship Program Maintain safety and reliability of U.S.’s nuclear arsenal 85% of experiments: nuclear weapons physics Will provide experimental data necessary to complete computer simulations Inertial fusion energy (IFE) Determine feasibility of IFE as an energy source Basic scientific research Experiments in high-energy-density physics for astrophysics, hydrodynamics, plasma physics, material properties, radiation physics

NIF Laser System - LRUs Each beamline contains 40 large optical elements Light path must have clean room atmosphere - beamlines are sealed Optics needed to be removable for repair and cleaning Line Replaceable Unit (LRU) concept was thus adopted - can be installed and removed with a semi-automated robotic unit

Line Replaceable Units (LRUs) Kinematic mount is essentially a three-vee coupling Two vee grooves at bottom of LRU resting on retractable pins on laser structure Upper mount consists of two pin-slot constraints, effectively creating a wide vee

Kinematic Coupling Evolution The LRU kinematic coupling evolved from the basic three-vee coupling Coupling was rotated to better accommodate tall LRU geometry Lower vees were rotated to carry gravity load Upper vee was spread wide to decrease rotational inertia and increase torsional stiffness

Mount Points in Vertical Plane Placing mount points into vertical plane provides a more favorable aspect ratio. Provides for smallest footprint - dense packing of LRUs in beamline

Wide Upper Vee - Inertia Upper vee spread with pin-slot constraints Instant center of rotation is thus brought closer to the principal axis of LRU Rotational inertia reduced, vibrational frequency increased

Wide Upper Vee - Stiffness Primary motivation for wide vee is to maximize torsional stiffness. Torsional stiffness of pin- slot constraint is an order of magnitude greater than LRU structure Maximum potential static twist due to friction remains well below the maximum allowable

Modeling of Coupling Kinematic mount modeled as parallel combination of 6 springs Load vector: 3 forces, 3 moments Deflection vector: 3 displacements, 3 rotations 6 X 6 stiffness matrix for a mount point determined in local coordinate system Stiffness matrices transformed into global coordinates, combined by addition

Optimization: Variable Parameters Locations of mounts already established by overall geometry of LRU The remaining geometric attributes were set as variable parameters subject to optimization Lower mount pin angle Lower mount outside vee angle Lower mount inside vee angle Upper mount slot angle

Optimization: Centering Ability LRUs optimized for maximum limiting coefficient of friction -> maximum centering ability As LRU contacts engage, minimum limiting C.O.F. occurs when 5 contacts are engaged Limiting C.O.F. for a given set of variable parameters taken to be minimum of six values Although optimization algorithm could have been used, graphical approach was taken instead

Summary LRUs needed to be easily installed and removed with a high degree of repeatability by a robotic system Basic configuration of three-vee coupling was determined by overall geometry of LRU The remaining, unspecified parameters were determined by optimizing for maximum centering ability In general, the performance of a kinematic mount can be maximized through analysis and optimization