## EOS Detector
### Scintillator Screen Diagnostics
#### 3.2.2 Laser-Gas Jet Interaction Experimental Campaign  
LWFA experiments have been performed with the FLAME laser at SPARC_LAB since 2016. Non-linear excitation of electron plasma waves in the wake of an ultra-intense laser pulse allows for the generation of extremely large local electric fields. These fields can be used for the acceleration of electron beams to ultra-relativistic energies. Electron beams  
Figure 16: Electron beam profile with 30 mrad FWHM divergence. Figure 17: Electron beam spectral content retrieved by coupling the lanex screen to a magnetic spectrometer. at FLAME were accelerated in the laser-driven plasma wakefields via the self-injection scheme. In this scheme, the background plasma acts as the reservoir of the electrons that form the final accelerated beam. Plasma electrons can be injected from the rear-front of the plasma wave. In 2016, at FLAME we reached an acceleration of electrons to several hundred MeV in a few mm, corresponding to an acceleration gradient of the order of GV/cm. In Figure 18, a typical spectrum of electron kinetic energy is reported, showing a mean energy of 320 MeV and a relative energy spread of 20% for a bunch of electrons carrying \(\sim\)100 pC charge. Acceleration to 320 MeV was obtained by the interaction between the laser pulse and the helium gas ejected by a supersonic gas nozzle, over an acceleration length of \(\sim\)1 mm and \(n_{ep}\sim 10^{19}\) cm\({}^{-3}\). The length of the plasma channel has been measured by means of Thomson scattered light. The electron plasma density was characterized through of a Mach-Zehnder interferometer. Electron acceleration in laser-plasma wakefields is accompanied by the emission of so-called betatron radiation. Acceleration is due to an electric field polarized along the propagation direction of the laser driver pulse. However, plasma wakefields also provide transverse focusing fields putting the injected electrons into oscillation. Such oscillations are called betatron oscillations. Focusing fields provide transverse oscillations, with the consequence of an efficient emission of radiation. Betatron radiation is synchrotron-like, where the radiation spectra can extend from soft X-rays to \(\gamma\)-rays [45; 46; 47; 48; 49]. Figure 19 shows the betatron radiation spectrum emitted by the electron beam with the energy spectral content reported in Figure 18. In analogy with synchrotron radiation, the critical energy of the spectral distribution lays around 4 keV. The emitted photon number is about \(10^{8}\) per pulse, corresponding to a electron-to-photon conversion efficiency of 10%. The critical energy is connected with the electron beam emittance, offering the possibility of diagnosing the latter [39; 50; 51]. Furthermore, betatron X-rays show interesting features of directionality. Figure 20 shows the betatron radiation angular distribution corresponding to the spectrum depicted in Figure 19.  
Figure 18: Energy spectrum of LWFA electron beams generated by the FLAME laser. Mean electron kinetic energy \(\sim\)320 MeV, relative spread 20%, bunch charge \(\sim\)100 pC. The X-ray beam divergence is smaller than 10 mrad, which is useful for intense irradiation of small samples [52]. The divergence of betatron radiation is approximately equal to the divergence of the electron beam accelerated in the plasma wakefields. In 2023, within the EuAPS framework, we restarted the experimental activity on LWFA and the production of betatron radiation, with the aim of realizing an X-ray source for users. The FLAME laser was enhanced in performance, allowing the increment of critical energy of the betatron spectrum (working at lower electron plasma density to increase the acceleration length but also the betatron oscillation amplitude). A typical spectrum from the most recent results is reported in Figure 21.  
Figure 19: Betatron radiation spectrum from LWFA electron beams generated by the FLAME laser. Critical energy \(\lesssim\)4 keV, emitted photon number \(\sim\)10\({}^{8}\) in a single shot.