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Laser particle acceleration and applications


The fast-growing field of particle acceleration using lasers is now in a new trend due to the enormous worldwide efforts in increasing the peak power of the existing systems, driving the laser technology to a new plateau, accompanied by the increase of the average power that is required for practical applications. One of the most symbolic examples is the new research infrastructure in Europe, the Extreme Light Infrastructure (ELI) initiative, with the establishment of three (and a forth coming soon) research Laboratories devoted to host the most intense lasers world-wide (more than PW peak power). Besides the basic physics that is being brought to light due to these new regimes, several applications are very promising with these systems, and one of those is the goal of this program, the acceleration of charged particles. When high intensity laser pulses impinge on a target, its atoms are ionized, and the electrons interact with the laser electromagnetic field, being accelerated directly by the laser field (Lorentz force or ponderomotive force). The electrons and the parent ions are separated, and a plasma is formed. For the sake of simplicity, if the target is a solid thin material, the pulse passes the target followed by an electron cloud (sheath) that travels at a relativistic speed. The remaining cations in the target are both attracted by the electrons and repelled by themselves, producing both a proton and a hadron current. This technique is used in PW systems to produce 100 MeV proton beams. Another powerful electron acceleration method is the Laser Wakefield Plasma Acceleration (LWPA), that is based on the idea of Tajima and Dawson to produce an electric Wakefield on the plasma to drive the electrons. The implementation of this idea has produced 4.2 GeV electrons in a length of 10 cm. This was accomplished after the understanding that it is possible to produce a nonlinear plasma wave that is resonant with the laser pulse duration and energy. There is a synchrony in the longitudinal e transverse directions. This condition demands a very high peak power of the laser to have the necessary intensity at the focus with a diameter of tens of microns. The targets are gas systems with densities in the range of 1018 - 1021 atoms/cm3 that correspond to gas pressures from a fraction to tens of bar. The further requirement is a low backing pressure to avoid plasma formation before the target, with a minimal transition layer. This requirement imposes especial vacuum conditions and pulsed gas sources. Recently, a high density plasma created by a kHz repetition rate laser int eh TW regime has shown proton acceleration to few MeV in high density liquid targets. It is the aim of this project to collaborate in understanding and establishing these new regimes for medical purposes. (AU)

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