Damage testing of partial reflectors for 157-nm laser calorimeters

We determined the damage thresholds and lifetimes of several materials using 157- and 193-nm excimer lasers and a beam profile technique similar to that described in ISO 11254-2. We made these measurements to select an appropriate absorbing material for use in our primary standard laser calorimeter for 157-nm excimer laser energy measurements. The materials we tested were nickel-plated sapphire, chemically-vapor-deposited silicon carbide (CVD SiC), nickel-plated copper, and polished copper. Applied pulse energy densities (or dose) ranged from 80 to 840 mJ/cm². We determined the applied dose from a series of laser beam profile measurements. Silicon carbide had the highest damage threshold: 730 mJ/cm² per pulse. For this reason, and because of its high thermal and electrical conductivities, we have chosen silicon carbide as the absorber material for the 157-nm calorimeter. We also conducted long-term exposure studies in cooperation with MIT Lincoln Laboratory at a pulse energy density of 5 mJ/cm² to simulate typical calorimeter operating conditions. No aging effects or other surface changes were observed at these dose levels after 500 million pulses, corresponding to a projected calorimeter lifetime of 50 years.