The L4n is a nanosecond-kilojoule laser beamline that delivers temporally shapeable nanosecond pulses at a maximum energy of 1.2 kJ. It was recently commissioned at ELI Beamlines and offers unique opportunities for high-pressure, high-energy-density physics, and laser-plasma interaction experiments, particularly due to its high repetition rate of up to 1 shot per minute. Compared to other kJ-class laser systems worldwide, which offer much lower shot rates, the L4n driven experiments will enable significant improvements in collecting data statistics. The results gathered during the first L4n commissioning campaigns, demonstrate the laser capability to deliver hundreds of joules every three minutes with excellent repeatability and clearly show its potential to make significant contributions to the field of high-energy density physics in the coming decades.
The limited aperture and damage threshold of the compressor gratings remains one of the bottlenecks in reaching higher peak powers for the current state-of-the-art laser systems. Object-image-grating self-tiling method provides a way how to double the effective aperture of compressor gratings by phasing them with perpendicularly positioned mirrors. This method is planned to be used in the main compressor for the L4 beamline in ELI Beamlines. A subaperture version of the main compressor was designed to test the feasibility of the objectimage- grating self-tiling method and to measure the temporal profile of the pulse throughout the amplification stages during the operation. The subaperture compressor was successfully implemented and temporal profile of the amplified pulse close to its transform limit was retrieved. The grating-mirror alignment was secured through the online measurement using an in-house developed Fizeau interferometer.
We report on the status of the re-commissioning of a high energy OPCPA laser system with programmable spectrum that serves as a frontend for a 10 PW laser at ELI-Beamlines. The OPCPA chain was developed by a consortium of National Energetics and Ekspla along with scientists of ELI-Beamlines.1 The laser system, consisting of three picosecond OPCPA stages, pulse cleaner, Offner stretcher, and 5 nanosecond OPCPA stages pumped by Nd:YAG lasers with programmable pulse shape (NL944, Ekspla), allows for precise spectral shaping while achieving high nonlinear conversion efficiency. Employing a subsequent Nd:glass power amplifiers (PA), the system was demonstrated to yield>1 kJ of energy, while maintaining broad spectrum of > 13 nm (FWHM). After recommissioning the OPCPA frontend in Dolní Břežany, an output energy of 4.3 J, flat beam-profile and good far-field quality has been demonstrated. The spectral shape has been optimized to support > 15 nm bandwidth and >1.5 kJ, consistent with 10 PW operation of the fully integrated laser system after compression.
State-of-the-art physics experiments are pushing the development of lasers with ultra-high peak power pulses. 4 PW pulses have been produced with TiSa [1] and 10 PW with the same gain medium is scheduled at LULI (Apollon) and at ELI-NP.
The other approach is to use Nd-doped glass as gain medium, whose interest is in its capability of delivering higher energy at the expense of a longer pulse duration. Based on this gain material combined with an OPCPA based front-end, a kJ-10 PW class laser has been designed and built.
The front-end, consisting of picosecond OPCPA, temporal pulse cleaning and nanosecond OPCPA, delivers pulses with excess of 4 Joules at 5 Hz with a shaped spectrum to pre-compensate for gain distortions in Nd:glass power amplifiers. Two liquid-cooled, mixed glass power amplifiers, namely PA1 and PA2, are used for further amplification. Up to now, they have been activated demonstrating 70 J at 1 shot a minute after PA1 and 1 kJ at 1 shot every 7 minutes for PA2. The Fourier limit of the spectrum is 150 fs meaning 6 PW capability after compression.
This energy level has been obtained with only 3 Joules seed energy, from the OPCPA and partial activation of PA2. Scaling of this result suggests that more than 1.7 kJ should be obtained leading to 10 PW after compression while the output spectrum will remain compatible with 150 Fs thanks to the OPCPA spectral tailoring capability.
Overview of progress in construction and testing of the laser systems of ELI-Beamlines, accomplished since 2015, is presented. Good progress has been achieved in construction of all four lasers based largely on the technology of diode-pumped solid state lasers (DPSSL). The first part of the L1 laser, designed to provide 200 mJ <15 fs pulses at 1 kHz repetition rate, is up and running. The L2 is a development line employing a 10 J / 10 Hz cryogenic gas-cooled pump laser which has recently been equipped with an advanced cryogenic engine. Operation of the L3-HAPLS system, using a gas-cooled DPSSL pump laser and a Ti:sapphire broadband amplifier, was recently demonstrated at 16 J / 28 fs, at 3.33 Hz rep rate. Finally, the 5 Hz OPCPA front end of the L4 kJ laser is up running and amplification in the Nd:glass large-aperture power amplifiers was demonstrated.
Tiled-grating compressors of ultra-short pulse multi-petawatt lasers are currently the only viable way how to meet beam size requirements and stay within the damage threshold of the largest available gratings. Recently, a method how to double the effective aperture of compressor gratings by phasing them with perpendicularly positioned mirrors has been proposed, providing simplification to the traditional grating-grating tiling scheme by reducing the number of alignment degrees of freedom. The drawback of the method lies in tighter requirements on adjustment precision and stability of the system making the alignment and monitoring a challenging task. Here we propose and analyze different approaches to precision control of mirror-grating phasing and present a comparative experimental verification of the alignment systems on a small-scale test bench.
A comparison of various pulse stretcher designs accommodating material dispersion for a <; 150 fs 10 PW Nd:glass laser system using low dispersion diffraction gratings is presented. Since the pulse amplification demands a high stretch ratio of the stretcher to suppress non-linear effects and a high temporal contrast of the pulse is required to avoid ionization of the experimental targets, the design of the stretcher is a critical part for dispersion management. Here, we compare several designs using only one diffraction grating based on either a Perry-Banks or an Offner stretcher types, mostly at the Littrow angle. The target spectral phase profile is achieved through the tuning of the grating position, the angle of incidence on the grating, the radii of curvature of curved mirrors and the line density of the grating.
The development of chirped pulse amplification lasers toward multi-PetaWatt power imposes more demands on laser system elements. To make the spectral band of pulse compressors wider, laser designers began to consider Littrow mounted grating setups. In this study we investigate two Littrow type configurations. The first one is roll - a grating is rotated in the grating plane by a small angle. The second configuration is pitch - a grating is rotated by small angle about an axis perpendicular to the grating grooves. In this paper we experimentally measured diffraction efficiency of rolled and pitched dielectric grating, and simulated it with two methods: numerical Fourier Modal Method in LightTrans Virtual Lab and semi-analytical Volume Integral Equation Method. Here we claim that roll is more preferable for dielectric diffraction gratings with high groove density. It is shown that the energy of laser pulse compressed by a Littrow-roll configured compressor is 2 to 5% higher than Littrow-pitch configured one.
B. Rus, P. Bakule, D. Kramer, J. Naylon, J. Thoma, J. Green, R. Antipenkov, M. Fibrich, J. Novák, F. Batysta, T. Mazanec, M. Drouin, K. Kasl, R. Baše, D. Peceli, L. Koubíková, P. Trojek, R. Boge, J. Lagron, Š. Vyhlídka, J. Weiss, J, Cupal, J. Hřebíček, P. Hříbek, M. Durák, J. Polan, M. Košelja, G. Korn, M. Horáček, J. Horáček, B. Himmel, T. Havlíček, A. Honsa, P. Korouš, M. Laub, C. Haefner, A. Bayramian, T. Spinka, C. Marshall, G. Johnson, S. Telford, J. Horner, B. Deri, T. Metzger, M. Schultze, P. Mason, K. Ertel, A. Lintern, J. Greenhalgh, C. Edwards, C. Hernandez-Gomez, J. Collier, T, Ditmire, E. Gaul, M. Martinez, C. Frederickson, D. Hammond, C. Malato, W. White, J. Houžvička
Overview of the laser systems being built for ELI-Beamlines is presented. The facility will make available high-brightness multi-TW ultrashort laser pulses at kHz repetition rate, PW 10 Hz repetition rate pulses, and kilojoule nanosecond pulses for generation of 10 PW peak power. The lasers will extensively employ the emerging technology of diode-pumped solid-state lasers (DPSSL) to pump OPCPA and Ti:sapphire broadband amplifiers. These systems will provide the user community with cutting-edge laser resources for programmatic research in generation and applications of high-intensity X-ray sources, in particle acceleration, and in dense-plasma and high-field physics.
A diffraction grating based on all-dielectric multi-layer structure is designed for compression of ultrafast pulses with spectrum centered at 900 nm. The grating at Littrow angle with an out-of-plane configuration shows more than 96% efficiency over the reflective band of 100 nm for the angle of incidence 41 degrees. We suggest grating grooves and the very first layer under the grooves to be made of fused silica. Reflective mirror under corrugated layer is designed as a stock of three types of dielectric nanolayers. Tolerances for groove depth and angle of incidence are estimated and, optimal duty-cycle parameter is found out. Electric field distribution inside of the grating is also numerically studied. The model is simulated by two methods: numerical Fourier Modal Method in LightTrans Virtual Lab and semi-analytical Volume Integral Equation Method. The results obtained by both methods show an excellent agreement.
All-dielectric grating with more than 98% efficiency over the reflective band of 40 nm with the central wavelength at 1053 nm is simulated for the angle of incidence 72 degrees. For the grating design we used the fact that chirped mirrors give wider reflective band than usual quarter-wavelength dielectric mirrors. Grating grooves and the very first layer under the grooves in our model is made of fused silica; underneath of the top layer we placed a chirped stack of 13 HfO2/SiO2 layers. Tolerances for groove depth and angle of incidence are estimated, optimal duty-cycle parameter is found out. Electric field distribution inside of the grating is also numerically studied. The model is simulated by two methods: numerical Fourier Modal Method in LightTrans Virtual Lab and semi-analytical Volume Integral Equation Method. The results obtained by both methods show excellent agreement.
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