Ow leading to sheath formation and expansion at the

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Unformatted text preview: associated return current. start to accelerate ions. As a rough estimate, a test ion crossing the sheath would acquire the energy E i $ ZeEs Ls ¼ ZTh , resulting in MeV energies and a scaling as I 1=2 if Th ’ E p given by Eq. (6) holds. Protons from a thin layer of hydrogencontaining impurities on the surface will be in a very favorable condition for acceleration because of both their initial position, located at the maximum of the field, and their highest charge-to-mass ratio so that they will be more rapid than heavier ions in following electrons and screening the sheath field. This is the qualitative scenario for TNSA of protons as introduced by Wilks et al. (2001) to explain their experimental results on proton acceleration (Hatchett et al., 2000; Snavely et al., 2000). The essential features of the TNSA mechanism have been supported by several experiments and TNSA has become the reference framework to interpret observations of multi-MeV protons from the target rear side. Various schemes for beam optimization and control have been designed on the basis of TNSA. Detailed discussions of main experimental findings are reported in Secs. III.A, III.B, and III.E. From a theoretical viewpoint, there are two main categories of models which describe TNSA, namely, ‘‘static’’ and ‘‘dynamic’’ models which, depending on the starting assumptions, allow one to provide simplified analytical descriptions useful for interpreting experimental data. These models and related numerical investigations are presented in Sec. III.C. 2. Front surface acceleration Already in the first measurements of proton acceleration in the forward direction, the possibility of a contribution originating at the front surface of the target was conceived (Clark et al., 2000a; Maksimchuk et al., 2000). As a consequence, mechanisms leading to ion acceleration in such a region have also been extensively investigated. At the front surface, the intense radiation pressure of the laser pulse pushes an overdense target inward, steepening the density profile and bending the surface; this process is commonly named hole boring. The recession velocity vHB of the plasma surface Andrea Macchi, Marco Borghesi, and Matteo Passoni: Ion acceleration by superintense laser-plasma . . . may be estimated by balancing the electromagnetic and mass momentum flows I=c $ ni ðmi vHB ÞvHB . This corresponds to an energy per nucleon E i ¼ mp v2 =2 $ I=Ani c. The scaling with HB the laser intensity I is more favorable than the I 1=2 scaling for TNSA and suggests that RPA effects should become more important for higher intensities. More accurate, relativistic, and dynamic modeling is presented in Sec. IV.A.1 along with related experimental indications. Radiation pressure action and hot electron temperature may also lead to the generation of collisionless shock waves (Tidman and Krall, 1971) with high Mach number M. Such waves are associated with the reflection of ions from the shock fron...
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