Physics and technology of electron beam-pumped wide-aperture excimer laser systems

Physics and technology of electron beam-pumped wide-aperture excimer laser systems

To investigate processes of the interaction of femtosecond-duration radiation with matter, we at the LGL developed a hybrid laser system (i.e., the one consisting of gas and solid-state lasers) based on  a titanium-sapphire laser starting complex and earlier developed electron-beam KrF laser amplifiers. A distinctive feature of the hybrid system is the direct amplification of femtosecond pulses, generated by the solid-state laser system and passed through a prism stretcher with negative dispersion, in gaseous active media without using complex and expensive optical compressors based on diffraction gratings, with subsequent compression by plane-parallel plates with positive dispersion. The hybrid laser system consists of a solid-state starting complex generating femtosecond radiation pulses, a KrF laser preamplifier and an end stage amplifier.
The solid state starting complex emitting UV femtosecond pulses was developed and fabricated by the Russian company Avesta Project  jointly with the Lebedev Physical Institute of the Russian Academy of Sciences. This complex, consisting of a Ti:sapphire laser (radiation wavelength, 744 nm; radiation pulse duration, 30 fs), an optical stretcher, a regenerative amplifier, multi-pass amplifiers, an optical compressor and an optical third-harmonic radiation converter emits 60–100-fs pulses at a wavelength of 248 nm with emission energy of 0.5 mJ at recurrence frequency of 10 Hz. The laser beam diameter, 8 mm. The maximal radiation energy at the basic wavelength is 8 mJ. The complex can also generate second-harmonic femtosecond pulses of a wavelength of 372 nm and energy of 2 mJ. The Berdysh KrF laser preamplifier pumped by an electron beam has an active volume 10×10×100 cm3.  The Garpun end stage KrF laser amplifier of an active volume of 16×18×100 cm3 is pumped by two electron beams propagating towards each other. These beams are generated by two electron guns and are guided into the laser chamber through titanium foil.
This hybrid laser setup was used to study the amplification of UV ultrashort pulses in two cascades of electron beam-pumped wide-aperture KrF amplifiers. Total energy at the output of the preamplifier reached 23 mJ; herewith, its beam area of 38.5 cm2 was about 60% of preamplifier’s aperture. The energy of UV ultrashort pulses at the output of the end stage amplifier in the saturation mode was 0.62 J in the beam of an area of 92.5 cm2, which was 43% of the aperture. A radiation pulse of ~100 fs in duration was fed to preamplifier’s input, that is, the stretcher was not used after the exit from the starting complex. The duration at the output of the end stage amplifier did not exceed 1 ps (as measured by an electron-optical camera) and was no less than ~330 fs (the coherence length measurement). Thus, the peak power was ~1 TW. The angular divergence of radiation was 20 μrad. Estimates show that at an aperture filling factor of 1.0 the energy of an amplified pulse will reach ~1.5 J. The use of the stretcher for a pulse of ~60  fs at the exit of the starting complex will enable a radiation pulse of the peak power of ~20–30 TW at the output of the system.
For this multi-stage hybrid Ti:Sapphire–KrF laser facility, the LGL, within the framework of ISTC partner project 4073, developed a new optical scheme to enable trains of UV ultrashort pulses of high peak power of 0.2–0.3 TW in combination with 100-ns high-energy pulses. Single (or a train of) UV ultrashort pulses were injected at recurrence intervals of ~5 ns into the instable confocal cavity of the basic wide-aperture KrF laser amplifier through the semi-transparent meniscus lens, were amplified in multiple passages of the cavity and were overlapped with  the free generation pulse. The injected pulses of a radiation wavelength of 248.4 nm, energy of 0.5 mJ and duration of 60 fs were generated by a Ti:Sapphire starting complex and amplified up to ~20 mJ in the KrF laser preamplifier. Amplification of single (or a train of) ultrashort pulses produced (at the exit of the system) pulses of energy of ~1 J and duration of ~1 ps (peak power, 1 TW) or a train of picosecond pulses of 0.1–0.2 TW power, of total energy ~2 J, spaced several nanoseconds apart. Combined (amplitude modulated) pulses of energy ~30 J, consisting of an amplified train of UV ultrashort pulses and a quasi-continuous generation pulse of 100-ns duration, were produced. Thereby, experimental evidence was found to support the earlier proposed concept of using a KrF laser in schemes of laser fusion (LF) with fast ignition or shock ignition. Degradation of the transmission of various optical materials for windows of KrF amplifiers under the action of high-power UV, X-ray and electron radiations was measured; and it was shown that induced short-lived nonstationary and long-lived absorption is one of the critical factors for implementation of a reactor based on LF.
We measured the spectra of fluorescence and nonstationary absorption in UV and visible ranges for pure noble gases and their mixtures with fluorine and fluorine-containing gases, enabling a judgement of the kinetic processes proceeding in the active media of excimer lasers.  Mixtures of Ar/Kr/NF3 and Ar/Kr/F2 with minor additions of nitrogen were found to have a weak laser amplification of radiation (of the gain factor, ~0.1±0.05 m-1) at a transition 42G → 1,2 2G of the triple excimer molecule of Kr2F of the wavelength of 460 nm and bandwidth of ~60 nm, which is of interest for the amplification of UV ultrashort pulses of high energy density.

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