Quick Shot

High-energy ultraviolet sources are required to probe dense, hot plasma from fusion experiments using Thomson scattering because there is less self-generated background from the plasma in the 180- to 230-nm spectral region. The fifth harmonic of neodymium glass lasers has a wavelength of 211 nm, which matches this spectral “window.” This high-energy application requires a large beam (~10 × 10 cm) and only a few nonlinear crystals suitable for frequency conversion can be grown sufficiently large. Although scientists working at LLE and LLNL have recently demonstrated 30% conversion efficiency in cesium lithium borate (CLBO), such crystals are extremely expensive and must be heated in an oven to avoid hygroscopic damage. Recent work has examined ammonium dihydrogen phosphate (ADP), which can be grown inexpensively in large sizes, but must be cooled to –70°C (or 200 K) for fifth-harmonic generation, with a precision of 0.1 K. LLE has developed a two-chamber cryostat that uses liquid nitrogen to cool an internal chamber that contains the crystal along with helium gas at a pressure of 1 atm for uniform cooling. Shown in the Laser Technology Development Laboratory at the cryostat chamber is Research Engineer Ildar Begishev, along with the insert that shows a view of the square crystal through the cryostat.