# Dosimetric Optimization of a Laser-Driven Irradiation Facility Using the G4-ELIMED Application
## 1 Introduction  
High power laser-plasma interaction is a new and innovative approach to produce and accelerate particle beams [1]. The interaction of ultrahigh laser intensities (\(>\)10\({}^{19}\) W/cm\({}^{2}\)) with a thin (\(\sim\)um) solid target results in the generation of extremely high magnetic and electric fields that produce a plasma and relativistic electrons (known as \"hot electrons\") propagating into the vacuum and creating a quasi-static sheath electric field at the target-vacuum interface. Such a field ionizes the target rear side and accelerates the ions outwards. The characteristics of the laser-accelerated ion beam will depend on the used laser and target parameters. A laser-plasma ion accelerator can be considered as a \"multi-color\" source where different kinds of ionizing radiations (protons/ions, gamma/X-rays, electrons, and neutrons) can be produced simultaneously. Additionally, such accelerators are expected to generate ultra-high dose rate beams, which are orders of magnitude higher than those currently being proposed for the \"FLASH\" radiotherapy approach [2, 3]. Moreover, a laser-based approach could potentially reduce the overall size and cost of an accelerator installation. In this framework, the ELIMAIA (ELI Multidisciplinary Applications of laser-Ion Acceleration) beamline [4] at the ELI Beamlines (Extreme Light Infrastructure) Centre aims to provide ion beams accelerated by high repetition-rate petawatt-class lasers suitable for multidisciplinary user applications. The two major subsystems of ELIMAIA are the Ion Accelerator and ELIMED (ELI Beamlines MEDical and multidisciplinary applications) sections [5,6]. ELIMED, in turn, consists of three main sub-sections: (i) the ion collection and focusing part, (ii) the ion energy selection, and (iii) the in-air transport section. The collection and focusing section aims to collimate the laser-accelerated ion beam and reduce its peculiar large divergence. This part is made of a set of five Permanent-Magnet Quadrupoles (PMQs). As described in [7], the PMQs have different lengths (one is 160 mm long, two are 120 mm, and the other two 80 mm) and a field gradient of around 100 T/m over a 36 mm magnetic bore. The PMQs are used to properly inject the accelerated particles downstream into the Energy Selection System (ESS). Hence, the full section from the target to the first collimator of the chicane is arranged in a way that the matching condition between collection and selection sections are respected. This means that the drift between quadrupoles is chosen to keep the emittance and Twiss Parameters within the required values of the chicane (Table 1). Also, the transport matrix conditions to have a waist on the horizontal axis and a parallel beam on the vertical axis are respected (it means M12 = 0 and M44 = 0). The ESS is a chicane made of four laminated resistive dipoles [8]. The technology used for the laminated yokes of the magnets (98% of packing factor) allows fast changes (1 Hz) in the magnetic field intensity based on the required ion species, ion energy, and energy bandwidth. This allows to change the selected ion energy between different shots, i.e., the ESS can be used as an active energy modulator. This is a unique feature of the ELIMED beamline not available at other laser-based accelerator facilities. Downstream of the ESS, a set of electromagnets (two electromagnetic quadrupoles and two steerers) are available to allow a final shaping of the particle beam and to correct for systematic misalignments prior to its final delivery onto the user sample in the in-air dosimetry end-station, which is separated from the in-vacuum section by a thin kapton window. A detailed description of the ELIMAIA beamline, along with the ELIMED transport magnetic elements can be found here [4,6,9]. The ion and proton beams transported along ELIMED are characterized and monitored online in terms of energy, fluence, and spatial profile through a set of in-line detectors [10].