High-Flux 100 kHz Attosecond Pulse Source Driven by a High-Average Power Annular Laser Beam

Abstract

High-repetition rate attosecond pulse sources are indispensable tools for time-resolved studies of electron dynamics, such as coincidence spectroscopy and experiments with high demands on statistics or signal-to-noise ratio, especially in the case of solid and big molecule samples in chemistry and biology. Although with the high-repetition rate lasers, such attosecond pulses in a pump-probe configuration are possible to achieve, until now, only a few such light sources have been demonstrated. Here, by shaping the driving laser to an annular beam, a 100 kHz attosecond pulse train (APT) is reported with the highest energy so far (51 pJ/shot) on target (269 pJ at generation) among the high-repetition rate systems (>10 kHz) in which the attosecond pulses were temporally characterized. The on-target pulse energy is maximized by reducing the losses from the reflections and filtering of the high harmonics, and an unprecedented 19% transmission rate from the generation point to the target position is achieved. At the same time, the probe beam is also annular and low loss of this beam is reached by using another holey mirror to combine with the APT. The advantages of using an annular beam to generate attosecond pulses with a high-average power laser are demonstrated experimentally and theoretically. The effect of nonlinear propagation in the generation medium on the annular-beam generation concept is also analyzed in detail.

Liquid-cooled modular gas cell system for high-order harmonic generation using high average power laser systems

“…The vacuum chamber around the gas cell (system) was modeled with a cube geometry of 300 mm edges, which corresponds to the experimental conditions in the beamlines where the gas cell system was tested. [55][56][57] The flange opening of the turbomolecular pump, mimicking real conditions, was a circular hole with a 250 mm diameter positioned at the center of one of the side walls. Single cells were used in the gas flow simulations, while thermal load calculations were performed on whole cell assemblies for simulating the complete cooling system.…”

“…In this way, high repetition rate XUV sources can be implemented with extreme high flux. 55 The operation of gas cells at such high average laser powers poses challenges: balance must be found between minimizing apertures for the reduced gas load and maximizing apertures for the reduced clipping and heat load. 55 The thermal load from illumination can degrade the cell, cause overheating or damage the aperture edges, and consequently influence the gaseous medium, or even accidentally destroy the whole gas cell target.…”

“…55 The operation of gas cells at such high average laser powers poses challenges: balance must be found between minimizing apertures for the reduced gas load and maximizing apertures for the reduced clipping and heat load. 55 The thermal load from illumination can degrade the cell, cause overheating or damage the aperture edges, and consequently influence the gaseous medium, or even accidentally destroy the whole gas cell target. Therefore, an active cooling technology and special aperture design are needed to provide reliable protection from such degradations and overheating even in cases of extreme thermal loads.…”

“…the boundaries of the nonlinear medium. The cell system is designed to be used as a target of the HHG process in the high repetition rate, gas-based attosecond beamlines (HR GHHG GAS 55,56 and HR GHHG COND) of the ELI ALPS facility. 57 The HHG process is driven by a postcompressed fibre-based, chirped-pulse amplification laser system (Active Fiber Systems GmbH) delivering 6.5 fs ultrashort laser pulses with 1030 nm central wavelength and 1 mJ pulse energy at 100 kHz repetition rate, [58][59][60] up to 100 W average power (reaching 500 W in the second implementation phase of the laser providing 5 mJ pulse energy with otherwise identical parameters [61][62][63] ).…”

“…The beamlines generate -optionally monochromatizedhigh-harmonic radiation of the fundamental infrared (IR) field in the XUV spectral range of 17-90 eV photon energies, resulting in an attosecond light source operating at 100 kHz repetition rate with unprecedented XUV fluxes. 55 This article is structured as follows: Sections II and III present the design and modeling of the thermal and flow characteristics of the gas cell. Section IV focuses on the experimental assessment of the gas cell.…”