Early-Time Extreme-UV Emission from OMEGA Plasmas

Time-gated spectral pre-emission in the 3-20 nm range, peaking as early as -11 ns before the main pulse at t=0, is recorded from OMEGA plasmas produced with SSD and DPP smoothing. The spectra suggest a prepulse irradiance ≈10^12 W/cm^2, the energy involved depending on any hot-spot sizes and pulse duration. For the main pulse, 60 beams of ≈20 kJ in 1 ns each at 351 nm were focused onto gas-filled CH microballoon targets of 920-μm diameter, coated with 0.03 μm of Al followed by 2 or 4 μm of Mg. Carbon spectra observed may arise from the CH base. Supporting data from an x-ray crystal streak spectrograph show an enlarged plasma at t=0, consistent with an early blowoff of magnesium expanding radially at ≈10^7 cm/sec to ≈4 mm by t=0. The x-ray data also show a brief presence of Al K-α emission and an intense dielectronic satellite to the Al XIII Ly-α line, both indicative of high energy electrons. A threshold for the xuv and x-ray observations near 10^14 W/cm^2 is observed. A search for a plausible explanation (e.g., a 1ω=1.05 μm prepulse) continues.