The HR GHHG Gas attosecond beamline of ELI-ALPS is driven by the 100 kHz high average-power HR-1 laser system (in long pulse mode) and produces attosecond pulse trains (APTs) for XUV – IR pump-probe measurements on gas-phase targets. Under the current operating conditions, the average pulse duration in the generated APT is ~400 as measured using the RABBITT technique. 2 pJ attosecond pulse trains with a bandwidth from 25 eV to 50 eV and a flux of 2.8 × 1010 photons per second were recorded at the target position during the commissioning of the beamline in 2019 . Different spectral ranges and fluxes can be provided utilizing different rare gas targets. The beamline is under continuous upgrade and optimization, currently providing up to 50 pJ XUV APTs on target in the 20-65 eV photon energy range at 100 kHz repetition rate. The schematic optical layout is given in Figure 1. The beamline is equipped with an electron time-of-flight (TOF) spectrometer, which serves as the primary tool for temporally characterizing the XUV pulses via RABBITT, positioned in the “Target area 1”. In the “Target area 2” an experimental end-station can be positioned according to user needs. For flux and spectral characterization of the XUV, a photodiode and a flat-field spectrometer are continuously available.
Figure 1. Schematic optical layout of the HR GHHG Gas beamline.
Table 1 shows the measured specifications of the HR-1 laser system and the XUV radiation generated using HHG. After the planned upgrade of the HR-1 laser system and arrival of the HR-2 laser in the first half of 2021, the specifications of the beamline are expected to substantially improve for experiments planned during the second half of 2021.
Table 1. Measured parameters of the HR-1 laser system (in long pulse mode) and the XUV radiation generated thorugh HHG.
Representative data of the achievable XUV spectrum, XUV flux and XUV APT temporal characteristics are given in Figure 2 and Figure 3.
Figure 2. The measured XUV flux and spectrum.
Figure 3. (a) Measured RABBITT trace, (b) corresponding XUV spectrum and reconstructed phase, (c) reconstructed average pulse shape in the XUV APT. 
 Peng Ye et al., J. Phys. B: At. Mol. Opt. Phys. 53 154004 (2020).