Zhao Junpu

Beam wavefront control of a thermal inertia laser for inertial confinement fusion application.

A novel scheme to correct aberration of each beam from the front-end to the target point in a thermal inertia laser (TIL) is presented. Each beam contains a deformable mirror (DM) with an aperture of 70 mm x 70 mm at the injection of the main amplifier and a Hartman-Shack (HS) sensor in a parameter diagnostic unit (PDU). A temporary HS sensor for measuring the static aberration of each beam with 1 Hz source is placed at the target point. The sensor will be removed from the target point during the main single shot, so we transfer the results measured at the target point to the sensors in the PDU. Dynamic aberration can also be measured by the HS sensor in the PDU during the single shot. In this way, we need not calibrate the aberration of the PDU, and aberration of each beam can be corrected by the DM with the HS sensor in the PDU. We demonstrate that with this scheme the divergence angle of the TIL pulses can be improved from 100 to less than 60 murad with a focal length of 2200 mm and beam size of 290 mm x 290 mm, which meets the requirement of a TIL.

Characteristics of Laser-Induced Surface Damage on Large-Aperture KDP Crystals at 351 nm

Laser-induced damage often determines the effective lifetime of an optic in large high-energy laser facilities. We present the damage performance on the rear surface of a large-aperture KDP crystal for 351 nm, 5 ns laser pulses. Surface damage shows a lower threshold than bulk damage after conditioning. Craters initiated on the scratch are found to increase with the shot number before filling the scratch. The experimental results reveal that damage initiation is mainly caused by extrinsic nanoabsorbers buried in the surface during the large-aperture laser operation.