## 2 The Experimental Platform HELL: High-Energy Electrons by Laser Light
### Diagnostics for Plasma Interaction and Electron Beams
(**right**) Simulated neutral density profile of gas target at different distances from the nozzle exit. A preliminary set of measurements at a multi-GeV energy range as been performed and will be described in a different manuscript. According to the state of the art, the highest energy experimentally demonstrated with a PW-class laser is 4.6 GeV [11], the choice for the HELL platform implementation is to design a spectrometer that is capable of resolving with good resolution energies up to at least 10 GeV. In order to meet these requirements a series of different magnetic dipoles has been designed [43; 44]. Each dipole is mounted on a linear stage, and by their combination, it is possible to achieve the highest spectral resolution at the energy of interest. The modular configuration, in which different modules can be additionally used for a specific energy range, is shown in Figure 21. A 0.5 to 10 GeV case is analyzed. The electron beam enters the series of dipoles from the right side of the image. First, the electron beam is cleaned from its low energy component up to 6 MeV by the first small dipoles, which deflect the low energy on a dedicated local shielding, and do not affect the electrons above 100 MeV. After that, a H-shaped-magnet is used to bend electrons in the energy range of interest into the 80 cm long, 1 T, C-shape dipole. This last is the strongest dipole, the one that allows for high resolution GeV-spectrometry, as shown in Figure 22. The simulation is done in two steps: first the magnetic field is simulated with the Radia Software, then the electrons are propagated in such field with the software Simion. Due to the overall length of the system, and to the typical LWFA beam parameters, each gap has been designed to have the highest acceptance. The minimum gap for each dipole has been designed to be 15 mm, which corresponds to a 12 mrad full angle acceptance of the system. Figure 22 shows the simulated deflection for a pencil beam in the range 1-10 GeV. After designing the electron beam diagnostics, the performance of the low energy range have been characterized with the preinjector electron beam at the synchrotron Elettra [45]. The parameters of the preinjector beam are 100 MeV, 100 pC at 2.6 Hz repetition rate. During the test by varying the beam energy between 80 and 110 MeV, the bending of the 7 cm, 0.8 T magnetic dipole has been tested. The measured values are in very good agreement with the ones that were predicted by the simulations, as shown in Figure 23 (right). The spectrometer resolution over the entire energy range will be described in future work. Figure 21: Modular configuration design of the 10 GeV electron spectrometer. Figure 22: Simulated deflection for a pencil beam in the range 1–10 GeV using the softwares Radia for the magnetic field and Simion for the particle tracking. Being the HELL platform partly devoted to multi-disciplinary experiments, the first end station of a laser plasma electron accelerator has been realized. It has been designed and built by Elettra, according to the specifications that are provided by ELI-Beamlines. A picture of the station is shown in Figure 23 (left). The overall size of the User Station fits in a 1 m side square. The scope of the User Station is twofold: (i) to position with an accuracy of at least 10 m a user samples, and (ii) to characterize shot-to-shot the beam which hits the sample. Thanks to this system, the user sample can be scanned in XYZ and rotated of 2\(\pi\) around the vertical axis. The maximum weight allowed for the sample is 5 kg. The electron beam is characterized on-line by two movable fluorescent screens; one placed before the target and one after it. Finally, the User Station has been designed to be compatible with the HELL beamline and with the ELI-Beamlines control system, in order to provide to the Users an integrated set of data compatible with the other facility diagnostics devices.