## 7 The sub-PW FLAME Installation  
Establishment of the FLAME installation started in March 2007 with the excavation of the underground area and the placement of the 1 m thick concrete basement of the laser room. Construction works were completed in May 2008 followed by installation of the clean room. Laser installation was completed in June 2009 and first laser shots on gas target took place early during 2010. An overview of the FLAME installation is shown in Figure 7. The key components of the FLAME installation are the laser system, which is hosted in the clean room and a radiation shielded, underground target area. The custom made FLAME Amplitude laser is based upon Ti:Sa, chirped pulse amplification (CPA) system that will deliver 20 fs, 800 nm, up to 220 TW, laser pulses with a 10 Hz repetition rate at a fundamental wavelength of 800 nm. The system features a high, sub-ns contrast ratio (\(>10^{10}\)) and has a fully remotely controlled operation mode. The system includes a front-end with pulse contrast enhancement (booster), bandwidth control and regenerative amplifier and yields pulses with 0.7 mJ in 80 nm bandwidth. These pulses are then further amplified by the first amplifier to the 25 mJ level while the second amplifier brings the energy to the 600 mJ level. The third cryogenic amplifier is based upon a 50 mm Ti:Sa crystal pumped by ten frequency doubled Nd:YAG laser pulses for a total energy of up to 20 J of energy at 532 nm. The extraction energy is as high as 35%, leading to a final energy in the stretched pulses in excess of 7 J. Pulses are then transported in air to the vacuum compressor placed in the underground target area. Once compressed, the pulse is transported under vacuum to the target chamber via remotely controlled beam steering mirrors. The plot of Figure 8 shows the cross-correlation curve of the FLAME laser system showing the level of ASE just above \(10^{-9}\) of the peak intensity. This high laser pulse contrast ensures that in our experimental conditions, no premature plasma formation occurs and the CPA pulse can be focused directly in the gas. In fact, at a maximum focused intensity of \(10^{20}\) W/cm\({}^{2}\), with our ASE contrast, we can expect a precursor laser intensity on target below \(10^{11}\) W/cm\({}^{2}\), which is below the plasma formation threshold in gases for laser pulses of sub-nanosecond duration [40], the typical duration of ASE. Therefore we can reasonably assume that, in our experimental conditions. However, according to the cross-correlation curve of Figure 8 (right) some precursor radiation may be present on the ps time scale before the pulse reaches the peak intensity. This radiation may give rise to premature ionisation of the gas. This is a key aspect to be evaluated to ensure a control of the interaction regime. In the experiment, the main laser pulse is focused onto a gas-jet target using an F/10 off-axis parabola at a maximum intensity above \(10^{19}\) W/cm\({}^{2}\). The main diagnostics used in the experiment are similar to those discussed above for the ILIL experimental campaigns and for previous experiments carried out elsewhere [21, 41, 42, 43]. Thomson scattering and Nomarski interferometry are set up perpendicularly to the main laser pulse propagation axis to study and characterise ionisation and basic laser-plasma interaction issues. A second group of diagnostics, including scintillators coupled to photomultipliers, a LANEX screen and custom [31] dose sensitive, radiochromic film stacks, enables indirect and direct detection and characterisation of the accelerated electron bunches. Our objective is to produce acceleration in a 4-10 mm long plasma. Since this distance has to be matched with the dephasing length, namely the distance that the relativistic electrons have to travel before they overtake the accelerating region of the plasma wake, the required electron density is less than \(3\times 10^{18}\) cm\({}^{-3}\).