## 2 Laser Layout and Features
### Front End  
The FLAME CPA chain begins with a femtosecond oscillator, specifically a commercial Femtosource Sinergy from Femtolasers (Spectra-Physics, Vienna, Austria, which delivers 10 fs, 4 nJ pulses at an 80 MHz repetition rate. A Pockels cell acts as a pulse picker, reducing the oscillator output repetition rate to 10 Hz.  
To enhance the contrast ratio (\(<\)10\({}^{-9}\)), the 10 fs pulse is amplified using a booster module. This module consists of a compact 14-pass amplifier able to increase the energy to tens of \(\upmu\)J and a saturable absorber to clean the pulse by removing the residual Amplified  
Figure 1: **(Top)** FLAME facility sketch comprehensive of the clean room, where the entire laser system is set, and of the two different interaction areas. **(Bottom)** FLAME facility sketch comprehensive of the laser pulse properties at each stage. White: Laser stages; Green: Laser pumping system; Red: Interaction chambers; Red arrows: Laser pulse; Green arrows: Pumping systems pulses  
Spontaneous Emission (ASE) background from the amplifier. The booster amplifier is pumped with a 7 mJ, 532 nm pump pulse from a CFR Ultra Nd:YAG laser (Quantel Laser, Lannion, France), which also pumps the following amplification stage, namely the regenerative amplifier.  
At this point, for safe amplification in solid-state materials, the booster pulses are temporally stretched using an optical stretcher based on an all-reflective triplet combination of Offner-type, comprising two spherical concentric mirrors: the first concave and the second convex. This symmetrical configuration eliminates aberrations such as on-axis coma and chromatic aberration but does not cancel out the symmetrical ones such as spherical aberration and astigmatism.  
After the stretcher, with the pulse temporally stretched to about 600 ps, an acousto-optic programmable dispersive filter (Fastlite Dazzler, Orsay, France) is introduced. The Dazzler acts both as a phase and intensity modulator. The former operational mode is used to pre-compensate for phase distortions that occur during laser transport, while the latter is used to enhance the laser output spectrum, enabling the generation of shorter pulses.  
The regenerative amplifier (REGEN) or second amplification stage delivers around 0.6 mJ pulses when pumped with 10 mJ, 532 nm pulses emitted by the CFR Ultra Nd:YAG pumping laser. This setup incorporates an intracavity acousto-optic programmable gain filter (Fastlite Mazzler, Orsay, France) to mitigate gain narrowing throughout the amplification stages.  
The laser pulse then moves to the first multi-pass amplifier (MP1), a 5-pass amplifier, which is pumped by a 230 mJ, 532 nm pulse from a CFR 200 laser (Quantel Laser, Lannion, France), resulting in a final pulse energy of 20 mJ. The amplified pulse is subsequently expanded to a 10 mm diameter before entering the second multi-pass amplification stage (MP2), a 4-pass amplifier designed to deliver approximately 600 mJ of energy. This amplifier features a water-cooled Ti:Sapphs crystal, 15 mm in diameter and 20 mm in length, with anti-reflection coating on both faces, and it is pumped with 2 J, 532 nm pulses from a ProPulse+ Nd:YAG laser (Amplitude Technologies). The amplified 800 nm laser pulse is then expanded to approximately 36 mm in diameter before reaching the final cryo-cooled amplification stage.  
At this point in the amplification chain, the laser beam is split into two: one beam continues along the main transport line, and the other follows the auxiliary/probe transport line.