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High-energy ultraviolet (UV) sources are required to probe hot dense plasmas from fusion experiments by using Thomson scattering resulting from the lower self-generated background from the plasma in the 180- to 230-nm spectral region. Although recently we demonstrated efficient, joule-class fifth-harmonic conversion of 1053-nm pulses using a cesium lithium borate (CLBO) crystal, larger crystals are necessary for increased UV energy. This paper presents an alternative approach for fifth-harmonic generation of a large-aperture neodymium laser using ammonium dihydrogen phosphate (ADP). An Nd:YLF laser was optimized to produce square pulses with a flattop, square beam profile (1053 nm, 1 to 2.8 ns, 12 × 12 mm, ≤ 1.5 J, ≤ 5 Hz ). Noncritical phase matching in ADP suitable for sum–frequency mixing the fourth harmonic with residual fundamental light was achieved by cooling the crystal (65 × 65 × 10 mm) in a two-chamber cryostat to 200 K. The crystal chamber used helium (1 atm) as the thermally conductive medium between the crystal and the crystal chamber, which in turn was surrounded by a high-vacuum chamber that contained a liquid nitrogen reservoir to cool the crystal chamber. Temperature variation of 0.4 K across the crystal aperture was obtained using two 50-W heaters with active feedback. The total conversion efficiency from the fundamental to the fifth harmonic, including surface losses and absorption, was 26%.
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