We study theoretically and numerically the acceleration of protons by a combination of laser radiation pressure acceleration and Coulomb repulsion of carbon ions in a multi-ion thin foil made of carbon and hydrogen. The carbon layer helps to delay the proton layer from disruption due to the Rayleigh-Taylor instability, to maintain the quasi-monoenergetic proton layer and to accelerate it by the electron-shielded Coulomb repulsion for much longer duration than the acceleration time using single-ion hydrogen foils. Particle-in-cell simulations with a normalized peak laser amplitude of a 0 = 5 show a resulting quasimonoenergetic proton energy of about 70 MeV with the foil made of 90% carbon and 10% hydrogen, in contrast to 10 MeV using a single-ion hydrogen foil.An analytical model is presented to explain quantitatively the proton energy evolution; this model is in agreement with the simulation results. The energy dependence of the quasi-monoenergetic proton beam on the concentration of carbon and hydrogen is also studied. Contents

Structure formation of high-beta plasma flow in a perpendicular magnetic field is investigated in the ion kinetic regime by a fully kinetic particle-in-cell simulation. We demonstrate that directional plasma flow is spontaneously focused to form a sharp density structure. The primary focusing process comes from field-aligned electron inflow associated with the whistler mode and plasma confinement due to a self-generated magnetic field. The resulting concave magnetic field lines modulate ion gyration to cause a secondary focusing process with significant plasma concentration. Required conditions for these processes are determined by a dimensionless parameter α ≡ βi0(ΔW/ρi0), where βi0, ΔW, and ρi0 denote the plasma kinetic beta, window size, and ion gyration radius, respectively. The focusing process is apparent for small α, whereas diamagnetic expansion is dominant for large α. This condition describes a transition between diamagnetic cavity formation and the focusing process.

Linear and nonlinear behaviors of gyrotron backward wave oscillators (gyro-BWO) were investigated by both analytical theories and direct numerical calculations. Employing two-scale-length expansion, an analytical linear dispersion relation corresponding to absolute instabilities in a finite-length system has been derived. Detuning from the beam-wave resonance condition due to the finite amplitude radiation fields, meanwhile, was found to play the crucial roles in the nonlinear physics. Near the start oscillation of the gyro-BWO, the radiation field amplitude saturates when the resonance broadening is comparable to the linear growth rate. Far beyond the start oscillation threshold, the beam-wave resonance detuning effectively shortens the interaction length toward that corresponding to the critical oscillation length for the given beam current. The theoretically predicted scaling laws for the linear stability properties and nonlinear stationary states of the gyro-BWO are in good agreement with numerical results.

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