## 3 The Experiment at ILIL  
A detailed description of the experiment and all the diagnostics is described elsewhere [23]. Here we give a summary of those results relevant for the foreseen applications. In the experiment, the main pulse was focused onto a gas-jet target using an F/6 off-axis parabola. The gas-jet target was irradiated at full laser energy varying the gas backing pressure, _i.e._, the pressure in the pipe before the fast valve controlling the nozzle to change the value of the maximum density of the neutral gas.  
Two gases, namely He and N\({}_{2}\), were used explore the role of different physical properties mainly related to the atomic number, and, in particular, to the ionisation properties under irradiation of ultrashort, intense laser pulses. The gas-jet nozzle was characterised by a 4 mm long, 1.2 mm wide slit and was mounted on a micrometric motorised support in order to move the interaction point along the three Cartesian axes (position scan). During the pulsed operation, the gas flows out of the slit at supersonic speed in order to produce steep interfaces between gas and vacuum. The vacuum in the chamber before the shot is maintained at a pressure below \(\sim 10^{-4}\) Torr by a turbo-molecular pump (Varian Turbo-V 550) connected to the chamber by a gate-valve.  
Thomson scattering imaging and plasma interferometry were set up perpendicularly to the main laser pulse propagation axis to measure plasma density distribution. A second group of diagnostics, including scintillators coupled to photomultipliers, a phosphor screen (LANEX), an electron spectrometer based upon permanent magnets and dose sensitive, radiochromic film stacks SHEEBA [24], enabled indirect and direct detection and characterisation of the electron bunches accelerated during the laser-gas interaction.  
We use Thomson imaging to derive information on the laser intensity as discussed in detail in [25]. Indeed, a careful analysis of the Thomson scattering images obtained in our experiment suggests that in our ILIL configuration, laser pulse channelling takes place that significantly extends the length of the interaction region. This observation is confirmed by the interferometry measurements obtained by using a Nomarski-type interferometer (see [26] and references therein) arranged with the line of sight set to be perpendicular to the main laser pulse propagation axis. It is well known that optical probing techniques are based upon the dependence of the plasma refractive index on the electron density according to the following expression:  
\[\eta=\sqrt{1-\frac{n_{e}}{n_{c}}}\simeq 1-\frac{n_{e}}{2n_{c}} \tag{1}\]  
In the interferometer, a probe beam is set to propagate through the region of interest. A lens is used to image the region of interest with the required magnification and resolution. A Wollaston prism is then used to split the probe beam into two partially overlapping beams. In the overlapping region an interference pattern is produced with a fringe separation set to be perpendicular to the laser propagation direction.  
Figure 1(left) shows a typical interferogram taken after propagation throughout the gas-jet. This image clearly shows a collimated propagation of the laser pulse over a distance of approximately 200 \(\mu\)m. Beyond this point, the beam rapidly expands with a clear evidence of beam break-up and filamentation.  
The extraction of the electron density from the interferogram was performed using the code IACRE [27,28]. The density map thus obtained shows a maximum electron density of \(7\times 10^{19}\) cm\({}^{-3}\) in the 200 \(\mu\)m long plasma channel.