Performance of a prototype for a large-aperture multipass Nd:glass laser for inertial confinement fusion

Abstract

The Beamlet is a single-beam prototype of future multibeam megajoule-class Nd:glass laser drivers for inertial confinement fusion. It uses a multipass main amplifier, adaptive optics, and efficient, high-fluence frequency conversion to the third harmonic. The Beamlet amplifier contains Brewster-angle glass slabs with a clear aperture of 39 cm x 39 cm and a full-aperture plasma-electrode Pockels cell switch. It has been successfully tested over a range of pulse lengths from 1-10 ns up to energies at 1.053 mum of 5.8 kJ at 1 ns and 17.3 kJ at 10 ns. A 39-actuator deformable mirror corrects the beam quality to a Strehl ratio of as much as 0.4. The 1.053-mum output has been converted to the third harmonic at efficiencies as high as 80% and fluences as high as 8.7 J/cm(2) for 3-ns pulses.

Petawatt class lasers worldwide

“…The facility uses Beamlet [80] , which was the original prototype facility for NIF at LLNL that was decommissioned in 1998 before being transferred to Sandia. Z-Beamlet [81] provides x-ray radiographic capability to the Z-pinch facility.…”

Z-beamlet (ZBL) multi-frame back-lighter (MFB) system for ICF/plasma diagnostics

“…This class of laser is able to generate bright x-ray sources in the 1-10 keV x-ray spectral range that are ideally suited for x-ray radiography of a wide variety of high-energy-density experiments conducted at the Z-facility [3]. Many of the components of Z-Beamlet were originally developed and tested as part of the Beamlet laser at Lawrence Livermore National Laboratory where it was built as a scientific prototype for the National Ignition Facility [4]. The laser is housed in a separate 900-m 2 building with a telescope to transport its beam to the adjacent Z-facility as shown in Figure 1.…”

Wavefront and divergence of the Beamlet prototype laser

“…In this case, performance is primarily limited by the fraction of prompt and static phase errors that remain uncorrected by the wavefront control adaptive optic system. Recent wavefront characterization experiments conducted on the Beamlet prototype laser [9] have played an important role in ensuring that the NIF will meet its focal spot requirements. As a result of this work we have quantified the beam quality at the injection point to the main amplifier; static phase errors in the main amplifier with the system cold; prompt pump-induced phase errors in the main amplifier; limits of phase error correction with the adaptive optics system; and the best attainable output beam divergence at both low and high power.…”