Leopard
The Leopard laser is an ultra-intense, ultra-short hybrid Ti:Sapphire/Nd:glass laser system. The requirements for future experiments precipitated the Leopard design and construction: a peak irradiance on-target large enough to produce relativistic plasmas and generate Kα x-rays and protons; a pulse contrast large enough to prevent the formation of pre-plasmas on target; good pointing accuracy; and a time jitter small enough for the coupling of the laser with the Zebra z-pinch machine. The Leopard design was also tailored to a set of additional technical requirements: a spectral bandwidth sufficient for an approximate 400-fs laser pulse time duration, a minimal B-integral, a high-repetition-rate front-end ideal for laser chain alignment, and availability of commercial diffraction gratings with appropriate size, reliability, robustness and ease of maintenance.
The "Leopard" laser is a powerful and versatile laser system capable of delivering 10-20J of laser energy in 300 fs-long pulses at 1057 nm. Uncompressed pulses (pulses of temporal duration ~1 ns), can be amplified up to an energy of ~100J. The laser can attain an intensity of 1018 W/cm2 , but after implementation of an adaptive optics system, now reaches an intensity of 1019 W/cm2.
Front End
The front end of the Leopard laser consists of a commercial fs oscillator (Spectra Physics Tsunami), pumped by a cw 10-W, diode-pumped Nd:YVO3 laser (Spectra Physics Millenia Pro Xs), an Offner-type chirped pulse stretcher unit, and a commercial linear regenerative amplifier (Coherent). The regenerative amplifier is pumped by a frequency-doubled Nd:YLF laser operating a a 500 Hz repetition rate (Evolution-30, Coherent). The system provides frequency-chirped, stretched pulses with a 10-nm bandwidth and an energy of 1 mJ for the injection into the flash-lamp-pumped amplifier section. The contrast ration was improved to a level of 10-7 by adding an ultra-fast Pockels cell (UPC, Leysop Ltd) at the output of the regenerative amplifier. The complete Offner stretcher unit was built in-house. It converts 150 fs pulses from the fs oscillator into 1.2-ns-long chirped pulses, which are then amplified at an irradiance well below the self-focusing threshold.
Amplification chain
The Leopard amplification chain consists of rod and disk amplifiers, Faraday isolators and Pockels cells for additional contrast improvement and protection, and spatial-filter image relay systems. The Pockels cell's timing is set up in such a condition that the transmission time window is a bit longer than the duration time of the stretched pulse. The amplified spontaneous emission in front and back of the main pulse, as well as the pre-pulses from the regenerative amplifier, are cut-off to improve the contrast ratio and minimize target pre-heating. After expansion, the pulse from the regenerative amplifier is amplified in a 6-mm-diameter phosphate Nd:glass rod (135-mm long Shott's LG760 glass with 4.0 wt % of Nd) amplifier (refurbished Quantel head, model SF3120). This amplifier works in a double-pass configuration and each pass produces a gain of ~8-10 (depending on the HV applied to the flash lamps), thereby amplifying the <1 mJ input laser pulse to 70-90 mJ. After passing through a Faraday isolator and another expansion (to a diameter of 16 mm), the pulse is amplified in a 19-mm-diameter silicate Nd:glass rod amplifier (240-mm-long Shott's LG680 glass with 3.0 wt % of Nd) in a double-pass configuration. Each pass produces a gain if ~3 and after passing through isolating Pockels cell, the output energy from this stage is approx. 350-400 mJ. The Pockels cell and Faraday isolator introduce the majority of the losses, but the contrast ratio is improved, and the amplifiers are protected from back-reflections by their addition to the system. Next, the laser beam goes through a 1:1 telescope and vacuum spatial filter and is relay-imaged to a second 19-mm-diameter phosphate Nd:glass rod amplifier (240-mm long Schott's LG760 glass with 1.0 wt % of Nd). The gain factor here is similar to the first 6-mm phosphate amplifier and is ~8. The output beam is expanded to 43 mm in diameter and enters the 45-mm-diameter Faraday isolator and the 45-mm phosphate Nd:glass, single-pass amplifier (240-mm long Schott's LG760 glass with 1.0 wt % of Nd). The input pulse with an energy of approx. 1.7 J is amplified to an output energy of 6-7 J. Both the 19-mm-diameter and the 45-mm-diameter amplifiers are refurbished components from the decommissioned LLNL NOVA laser.
The 45-mm diameter amplifier is the last rod amplifier in the Leopard system. Afterwards, the laser beam is expanded to a 90-mm diameter, and image-relayed to the two NOVA 94-mm-diameter mixed-glass disk amplifiers. A pulsed 100-mm-diameter Faraday isolator is located upstream of the disk amplifiers for protection from back reflections. Each amplifier consists of six Nd:glass disks mounted at the Brewster angle. To yield broader output bandwidth and also shorter pulses, we use 4 phosphate glass disks, and 2 silicate glass disks inside the first 94-mm amplifier. The second amplifier consist phosphate glass disks only. The amplified pulse has an energy of ~100 J. Due to the damage threshold of the diffraction gratings inside the compressor, the energy of the pulse entering the compressor is limited to a 30 J maximum. Thus, only the long, uncompressed pulse (which bypasses the compressor) can have an energy as high as 100 J.
After amplification, the beam is spatially filtered and relay-imaged to a vacuum pulse compressor. Image relay is used to obtain a uniform spatial beam profile. An image of the hard serrated aperture installed at the exit of the Ti:Sapphire regenerative amplifier is relayed by vacuum spatial filters through the amplifier chain. The final output spatial filter relays the image of the hard aperture to the diffraction grating inside the vacuum compressor (the vacuum spatial filter is simply an evacuated telescope with aperture in the focal plane). The function of the vacuum spatial filters is to expand the laser beam between the amplifiers to increase its diameter, and to keep the fluence below the damage threshold of the optics. They also image the laser beam so that the beam spatial profile is uniform inside the amplifiers. Finally, the aperture removes the spatial, high-frequency components of the laser beam so as to avoid damage to other laser system components. The Leopard laser has a long propagation path in glass amplifiers and the fluence is high inside the chain, so five sequential vacuum spatial filters are used inside the laser chain.
Falcon Optical Compressor
The laser pulse is compressed in a double-pass, two-grating, vacuum pulse compressor (see next figure). The compressor chamber is a refurbished LLNL chamber originally built for the "Falcon" laser. A half-waveplate in the front of the compressor is used to fine-tune the direction of the polarization of the laser beam.
The compression chamber is 4 meters long, 1.5 meter wide and 1 meter high, and can be evacuated to 10-6 Torr by a dedicated vacuum system. The diffraction gratings are gold-coated Horiba-JobinYvon-brand pulse compression gratings with 1740 grooves per millimeter. The size of first grating is 190x350x50 mm and the size of the second is 210x420x50 mm. The gratings have an absolute efficiency of 90% at 1 micron and better than 1/8 wave surface figure at the same wavelength. The grating separation distance is approx. 80 cm and the diffraction angle is approx. 78 deg. These two parameters are fine-tuned during the pulse-duration time optimization. The second pass through the compressor is achieved by a vertical separation of the beams by the use of a large high-quality roof mirror made by PCX, Inc. All crucial components inside the compressor are mounted on remotely-controlled rotational or translational stages, so that the compressor alignment process can be performed under vacuum. Approximately 60% of the energy input into the compressor arrives onto the target. After compression, a diagnostic pickoff mirror reflects ~ 99% of the compressed pulse energy toward the target chamber and transmits the remainder as a sample beam to the laser diagnostics system. The beam diagnostics include pulse duration measurement, energy measurement, a far-field camera and a Shack-Hartmann wavefront sensor.
After exiting the Falcon compressor, the final (compressed) beam is steered via an optical switching apparatus to either the Phoenix or Zebra experimental chambers.