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.
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.
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 HfO<sub>2</sub>/SiO<sub>2 </sub>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.
Experiments at the HERCULES laser facility, originally reported by C. Zulick, <i>et al</i> in Applied Physics Letters (2013), have produced neutron beams with energies up to 16:8(±0:3) MeV using <sup>7</sup> <sub>3</sub>Li(d,n)<sup>8</sup> <sub>4</sub>Be reactions. These efficient deuteron reactions required the selective acceleration of deuterons through the introduction of a deuterated plastic or cryogenically frozen D<sub>2</sub>O layer on the surface of a thin film target. It was shown that a optimized frozen D<sub>2</sub>O layer, formed <i>in situ</i>, yielded the highest efficiency deuteron acceleration with deuterons constituting over 99% of the accelerated light ions. The deuteron signal was optimized with respect to the delay between the heavy water deposition and laser pulse arrival, as well as the temperature of the target. A total conversion efficiency of laser energy to neutron energy of 1(±0:5) × 10<sup>−5</sup> was obtained. The simulated neutron signal was found to be in reasonable agreement with the experimental spectra. The scattering of neutrons through shielding and target materials was investigated with MCNPX and determined to have a small effect on the observed neutron energies.
A semi-analytical method to simulate the local and integral pumping rates of spectral lines in multi-component hollow cathode discharge plasma has been proposed. The contribution of different mechanisms into pumping rate of upper level of laser line 441.6 nm CdII in He-Cd mixture is discovered for CW regime. The excitation of discharge by current pulses has been analyzed. The results of theoretical calculations are in good agreement with experimental measurements.
The properties of cadmium ion level excitation by means of second-kind collisions with buffer gas ions or metastable atoms in a negative glow of steady-state hollow cathode discharge are discussed. The electron energy distribution function, electric field distribution, and relevant level excitation rates were calculated and measured. The laser output power was measured as a function of discharge parameters and laser tube design for blue (441.6 nm), green (533.7 and 537.8 nm), and red (636 nm) He-Cd<SUP>+</SUP> laser lines in continuous wave hollow cathode discharge both in lasing at individual lines and in three-color output regime that gives white- light emission. Output powers of 115 mW, 25 mW, and 13 mW for blue, green, and red lines, respectively, and a noise level less than 1% rms were obtained.