Generation of giga-electron-volt proton beams by micronozzle acceleration

Summary

Two-dimensional PIC simulationsWe perform two-dimensional (2D) PIC simulations using the open-source fully relativistic code EPOCH.53 In the 2D (x, y) PIC simulations, the system is assumed to be uniformly long-stretched along the z-axis. The curves of the inner surfaces of the nozzle head and nozzle skirt are designed using parts of a circle and an ellipse, respectively. The nozzle head and nozzle skirt are then smoothly joined on the nozzle-neck boundary. The boundary corresponds to the vertical dashed line in Fig. 1 passing through the H-rod center. In practice, the resultant MNA target structure can be specified by the following scale parameters: the vertical lengths of the nozzle (\(H_1\), \(H_2\), and \(H_3\)), the horizontal lengths (\(L_1\) and \(L_2\)), the H-rod diameter (D), and the nozzle wall thickness (d). The default settings in the following simulations are \(H_1=5.3\upmu\)m, \(H_2=2.8\upmu\)m, \(H_3=12.0\upmu\)m, \(L_1=3.1\upmu\)m, \(L_2=9.9\upmu\)m, \(D=2.0\upmu\)m, and \(d=0.6\upmu\)m. This paper focuses on clarifying the underlying physics of MNA in terms of the default parameters. Therefore it is not our main purpose in this paper to optimize the set of parameters for the target structure and the laser pulse. The simulation box size placed on the x-y plane is 100 – \(200\upmu\)m (along the x-axis, depending on the applied laser intensity) \(\times 40\upmu\)m (along the y-axis) at a rate of 100 cells/\(\upmu\)m or equivalently 10 nm/cell. The simulation box size, (100 – \(200)\upmu\)m\(\times 40\upmu\)m, is set such that significant amount of charged particles are not lost out of the simulation box to affect accelerated proton dynamics with energies of \(\sim\) GeV.From the left boundary of the simulation box, a p-polarized laser pulse (the laser electric field oscillates along the y-axis) with the laser wavelength \(\lambda _{\textrm{L}}=0.8\, \upmu\)m is irradiated along the x-axis. The other three sides of the simulation box are treated as ope...