A Ti:sapphire laser system has been constructed with two synchronized main beams of 110 TW and 13 TW, and a 5-TW wavelength-tunable synchronized auxiliary beam for versatile control of laser-plasma interaction. The first main beam provides 3.3-J, 30-fs, 810-nm pulses, and the second 450-mJ, 34-fs, 805-nm pulses. The auxiliary beam comes from amplified spectral windows selected from a supercontinuum of high spatial coherence and provides 38-fs pulses with tunable wavelengths (870-920 nm). The two main beams can be focused down to M 2 = 1.2 and 1.1, with 77 and 81 % energy enclosed in the focal spots, respectively. The energy fluctuations are 1.1 and 1.8 %, and the pointing fluctuations are 4.5 and 4.8 lrad, respectively. By using a preamplifier and saturable absorber before the pulse stretcher to suppress amplified spontaneous emission, the temporal contrast of the 110-TW main beam reaches 4 Â 10 À10 at the -100-ps timescale. Even though the auxiliary beam is generated from a highly nonlinear process, by confining the supercontinuum generation in a single self-trapping filament, a spatial coherence close to the main beams can be achieved. It can be focused down to M 2 = 1.3, with 72 % energy enclosed in the focal spot. The energy fluctuation is 2.6 %, and the pointing fluctuation is 4.7 lrad. The versatility of synchronized multiple-beams with tunable wavelengths, good energy and pointing stability, and the spatiotemporal quality of the laser system has been essential to our experiments in high-harmonic generation, extreme-UV lasers, and laser-wakefield accelerators in which precision control of laser-plasma interaction is facilitated by a concerted sequence of driving pulses.
Single-shot ultrashort extreme-UV(EUV) pulse waveform measurement is demonstrated by utilizing strong field ionization of H2 gas for transmission gating. A cross-propagating intense near-IR gate pulse ionizes the EUV absorbing H2 molecules into EUV-non-absorbing H2++ (two protons) and creates a time sweep of transmission encoded spatially across the EUV pulse. The temporal envelope is then retrieved from the lopsided spatial profile of the transmitted pulse. This method not only measures EUV temporal envelope for each single shot, but also determines timing jitter and envelope fluctuation statistically, thus is particularly useful for characterizing low-repetition-rate fluctuating EUV/soft x-ray sources.
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