Platinum Particles in the Nd:doped Disks of Phosphate Glass in the Nova Laser

The disks of Nd:doped phosphate glass in the amplifiers of the Nova laser contain platinum particles with sizes ranging from <5μm (detection limit) to about lOOμm. The particle density varies from about 0.01 to 1.0 cm^-3. These particles cause fractures when irradiated at fluences >2.5 J/cm² delivered in 1-ns, 1064-nm pulses. Under repeated irradiation at 5–7 J/cm², damage from small (< 5μm) particles asymptotically approaches a limiting size, but damage surrounding the larger particles grows steadily. The damage threshold fluence, 2.5 J/cm², corresponds to operation of Nova at one-half the desired output for pulse durations longer than 1 nsec. Operation at higher fluences causes accumulation of damage in the output amplifiers and requires replacement of the disks in those amplifiers on an accelerated schedule.

Large-area damage tests have been used to characterize samples from recent melts made to reduce the number of platinum particles. The 500-J, 1-ns pulses available at the midpoint of an arm were used to produce fluences of 7–10 J/cm² over test areas 6–9 cm in diameter. Similar irradiations of areas 25–30 cm in diameter were accomplished using 5-kJ, 1-ns pulses available at the output of an arm. Preliminary results from these experiments indicate that a significant reduction of the inclusion density can be obtained through changes in the vendor's glass melting conditions.