## 4 Electron Beam Characterisation  
Characterisation of the laser accelerated electrons was carried out using four different diagnostics. A set of 3 scintillators were arranged to detect the \(\gamma\)-rays produced by the electrons via Bremsstrahlung on the target chamber. A phosphor screen (Kodak LANEX Regular Screen) was placed on the laser axis to image out the accelerated electron beam. In a preliminary stage, a dosimetric film stack SHEEBA [24] based upon radiochromic films was used to obtain simultaneous spectro-angular distribution of electrons. This detector provides information on the entire electron bunch since angular selection (collimation) is not needed prior to detection as in the case of magnetic spectrometers. This makes SHEEBA very useful when the properties of the accelerated bunch are not known or when significant angular/energy spread is expected. After this preliminary stage, highly collimated electron bunches were analyzed using magnetic spectrometers as discussed in [29]. In the case of scintillators, the scattering of energetic electrons within the walls of the chamber generates \(\gamma\)-ray that can be detected and measured by an oscilloscope to show basic evidence of high energy electrons. The LANEX phosphor screen was filtered with a 25 \(\mu\)m Al foil and was coupled with an optical system imaging its rear surface. When an electron hits the sensitive material, as shown in Figure 2, the phosphors emit optical (green) radiation from the rear surface of the screen. This radiation was then imaged out of the vacuum chamber using a photographic objective, on to a commercial digital SLR camera (Pentax Digital Camera K 100D Super). The camera was equipped with a custom built trigger unit to enable synchronised shooting with the shortest possible shutter operation of 250 ms. This set up enabled direct observation of the electron bunch divergence and allowed us to obtain information on the shot by shot fluctuations of the bunch charge. Figure 1(right) shows a raw image of the entire exposed LANEX area, with the electron beam pattern obtained in two shots taken in two different experimental conditions. Figure 1(right) presents the typical electron signal from N\({}_{2}\) gas (@50 bar backing pressure) that shows a large bunch divergence  
Figure 1: Left: Interferogram of the gas-jet interaction region taken 2 ps after propagation of the laser pulse. The shot was taken in N\({}_{2}\) at a backing pressure of 35 bar, with the laser focal plane located 50 \(\mu\)m into the gas-jet and 400 \(\mu\)m from the nozzle output plane. Right: typical outputs of LANEX showing moderately collimated laser-accelerated electrons in case of N\({}_{2}\) gas @ 50 bar. of approximately 20 deg. A much narrower angular distribution was found in the case of He gas (@ 50 bar), which is of the order of 2 degrees (FWHM). While the narrow beam condition was not highly reproducible, the beam pattern of Figure 1(right) was very stable and reproducible, in a range of pressures from 30 to 40 bars. These experimental conditions are indeed interesting for applications of high-charge electron beams [21]. A spectral analysis of the accelerated electrons was carried out using a magnetic spectrometer coupled with the LANEX screen. The spectrometer, based upon permanent NeFeB magnets generating a quasi-uniform magnetic field (BMax \(\sim\) 0.45 T), was placed at a distance of 44 mm in front of the LANEX screen. The magnetic field amplitude was mapped in the region of interest using a millimetre-sized Hall magnetic probe. A 2 mm thick Pb foil with a \(\sim\) 0.5 mm slit width was placed in front of the magnet, with the slit direction parallel to the magnetic field, in order to limit the transverse momentum of electrons accepted by the electron spectrometer and consequently to increase the resolution of the spectrometer.