_Applied_  
_sciences_  
**HELL: High-Energy Electrons by Laser Light, a User-Oriented Experimental Platform at ELI Beamlines**  
**Tadzio Levato 1,*, Stefano Bonora 2, Gabriele Maria Grittani 1,3, Carlo Maria Lazzarini 1, Muhammad Fahad Nawaz 1, Michal Nevrka 1, Leonardo Villanova 1, Roberto Ziano 1,+, Silvano Bassanese 4, Nadezhda Bobrova 5, Katia Casarin 4, Edwin Chacon-Golcher 1, Yanjun Gu 1, Danila Khikhlukha 1, Daniel Kramer 1, Marco Lonza 4, Daniele Margarone 1, Veronika Olsovcova 1, Marcin Rosinski 6, Bedrich Rus 1, Pavel Sasorov 1, Roberto Versaci 1, Agnieska Zaras-Szydlowska 6, Sergei V. Bulanov 1 and Georg Korn 1**  
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beams also at small and medium size research infrastructures [2; 3; 4; 5]. Nowadays, laser-accelerated electron beams are produced with table-top laser systems from TW to PW peak power [6; 7; 8; 9; 10], thus accessing a broad energy range up to the multi-GeV level [11]. The use of an ultrashort laser pulse driver enables at the same time the possibility to pump and probe a sample with secondary laser or particle beams, thus offering ultrafast temporal resolution in the 10-fs range. These features are extremely interesting for several potential multidisciplinary applications [12]. On the other hand, the non-optimal electron beam quality in terms of pointing stability, average energy and energy spread impose severe limitations to the usability of such beams. At the same time, such challenge requirements have increased the number of test experiments aiming to improve the beam reliability for applications. As recently shown, innovative injection approaches can stabilize the average electron energy and limit the energy spread down to 1% [13]. These achievements are promising and encouraging for further improvements of the accelerated electron beam features, but at the same time, also allow for reaching a sufficient level for their use for specific applications [12]. The ELI Beamlines project [2] main mission is to deliver laser-produced secondary beams to the scientific community, both for applied and fundamental research in different fields. For this reason, different user-oriented beamlines are being implemented with the goal of delivering particle and radiation with unique features, especially when compared to conventional facilities. Since the capability to produce secondary beams using ultra-high-power laser systems is still in a fast development phase, some of these beamlines are geometrically organized to be flexible and modular to offer the possibility to arrange advanced optical setups for future development of the source or just the preparation of a complex user experiment. This is, for example, the case of the High-energy Electrons by Laser Light (HELL) experimental platform, a project that is mainly based on the laser wake field acceleration (LWFA) concept [1; 5; 11; 14; 15]. Figure 1 shows a typical three-dimensional (3D) Particle In Cell (PIC) simulated electronic density of the LWFA process using the code EPOCH [16] on the ELI Beamlines clusters ECLIPSE. In this case, a 0.2 PW, 30 fs laser pulse is focused to a waist of 19.8 um onto a supersonic gas-jet generating an electron plasma density of \(1.5\times 10^{18}\)\(\mathrm{e}^{-}/\mathrm{cm}^{3}\). This condition is shown to produce strong electron injection, already at 1 mm of propagation in the plasma, as its evident at the center of the dark region also called wake-field. Typically, such an acceleration regime [14] requires long focal lengths; furthermore, a laser beam counter-propagating with the accelerated electrons is requested for fundamental studies [17; 15]. Such requirements in terms of electron energy and setup flexibility, in the case of PW-class lasers suggest to use two separate vacuum chambers: one for the laser driver, which includes the main focusing optics, and one for the acceleration areas.