Energy spectra of ions and fast electrons accelerated by a channeling laser pulse in near-critical plasma are studied using three-dimensional (3D) Particle-In-Cell simulations. The realistic 3D geometry of the simulations allows us to obtain not only the shape of the spectra, but also the absolute numbers of accelerated particles. It is shown that ions are accelerated by a collisionless radial expansion of the channel and have nonthermal energy spectra. The electron energy spectra instead are Boltzmann-like. The effective temperature $T_{\mathrm{eff}}$ scales as $I^{\text{1/2}}.$ The form of electron spectra and $T_{\mathrm{eff}}$ depends also on the length of the plasma channel. The major mechanism of electron acceleration in relativistic channels is identified. Electrons make transverse betatron oscillations in the self-generated static electric and magnetic fields. When the betatron frequency coincides with the laser frequency as witnessed by the relativistic electron, a resonance occurs, leading to an effective energy exchange between the laser and electron. This is the inverse free-electron laser mechanism. Electrons are accelerated at the betatron resonance when the laser power overcomes significantly the critical power for self-focusing.

We demonstrate that a beam of x-ray radiation can be generated by simply focusing a single high-intensity laser pulse into a gas jet. A millimeter-scale laser-produced plasma creates, accelerates, and wiggles an ultrashort and relativistic electron bunch. As they propagate in the ion channel produced in the wake of the laser pulse, the accelerated electrons undergo betatron oscillations, generating a femtosecond pulse of synchrotron radiation, which has keV energy and lies within a narrow (50 mrad) cone angle.

The generation of harmonics by interaction of an ultrashort laser pulse with a step boundary of a plane overdense plasma layer is studied at intensities Iλ²=10¹⁷–10¹⁹ W cm⁻² μm² for normal and oblique incidence and different polarizations. Fully relativistic one‐dimensional particle‐in‐cell (PIC) simulations are performed with high spectral resolution. Harmonic emission increases with intensity and also when lowering the plasma density. The simulations reveal strong oscillations of the critical surface driven by the normal component of the laser field and by the ponderomotive force. It is shown that the generation of harmonics can be understood as reflection from the oscillating surface, taking full account of retardation. Describing the oscillations by one or more Fourier components with adjustable amplitudes, model spectra are obtained that well reproduce the PIC spectra. The model is based on relativistic cold plasma equations for oblique incidence. General selection rules concerning polarization of odd and even harmonics depending on incident polarization are derived.

We present 3D particle-in-cell simulations for laser pulses with relativistic intensity propagating in slightly underdense plasma. We observe strong flows of relativistic electrons, axially comoving with the pulse. They generate magnetic fields up to 100 MG and strongly influence light propagation. After a phase of filamentation, a single light channel with a width of 1–2 wavelengths is formed by magnetic interaction. A representative case is discussed in which the intensity on axis increases from initially $1.2\times {10}^{19}$ to $2.0\times {10}^{20}$ W/c${\mathrm{m}}^2$. Ion acceleration and plasma cavitation are also discussed.