[en] We experimentally demonstrate a new regime of high-order harmonic generation by relativistic-irradiance lasers in gas jet targets. Bright harmonics with both odd and even orders, generated by linearly as well as circularly polarized pulses, are emitted in the forward direction, while the base harmonic frequency is downshifted. A 9 TW laser generates harmonics up to 360 eV, within the 'water window' spectral region. With a 120 TW laser producing 40 uJ/sr per harmonic at 120 eV, we demonstrate the photon number scalability. The observed harmonics cannot be explained by previously suggested scenarios. A novel high-order harmonics generation mechanism [T. Zh. Esirkepov et al., AIP Proceedings, this volume], which explains our experimental findings, is based on the phenomena inherent in the relativistic laser - underdense plasma interactions (self-focusing, cavity evacuation, and bow wave generation), mathematical catastrophe theory which explains formation of electron density singularities (cusps), and collective radiation due to nonlinear oscillations of a compact charge.

No abstract available

[en] We study the process of resonant scattering of radiation by the laser excited plasma wake in the wavebreaking regime. The resonant interaction arises between the cusp electrons and the slow wave of ponderomotive potential which is formed by the incident and scattered electromagnetic waves. Scattered wave is amplified when the density cusp is moving slightly faster than ponderomotive wave, and is absorbed in the opposite case. This FEL-like interaction leads to nonlinearity in reflection, the intensity-dependent reflection coefficient is calculated

[en] The acceleration of charged particles by plasma waves excited with a strong laser pulse is discussed. PIC simulations demonstrate that, by a proper choice of the laser pulse shape and the plasma density profile it is possible to increase the efficiency of the particle acceleration. (author)

[en] We present an analytical model and three dimensional particle simulations of intense laser interaction with a cluster of overdense plasma. When laser intensity is above a critical value, it blows off all of electrons from the cluster and forms a non-neutral ion cloud. During the Coulomb explosion of the ion cloud, ions acquire their energy. Ion energy spectra are discussed in detail for different densities and sizes of clusters with various laser intensities. It is shown that ultra-fast ions are produced for relatively large clusters, and that the ion energy becomes three times greater than the maximum electrostatic potential energy of the ion cloud. The laser driven Coulomb explosion of a cluster may provide a new high energy ion source

[en] The paper presents the results of a numeric modelling of the propagation of ultra short relativistically strong laser pulses in a rarefied plasma by the 'particle in cell'. Primary attention is paid to the process of the formation of electromagnetic solitons which can not be described in the approximation of envelopes. It is found that under certain conditions a significant portion of pulse energy can transform is solitons. The soliton excitation mechanism is related to a decrease of local frequency of electromagnetic radiation due to the generation of wave plasma waves. From one soliton to a stub of solitons can be generated in the wake of a relatively long pulse depending on the parameters of laser pulse in plasma. Particles are effectively accelerated forwards radiation propagation in the electric field of wake plasma waves. 22 refs., 7 figs

[en] We propose a new mechanism of high-order harmonic generation during an interaction of a high-intensity laser pulse with underdense plasma. A tightly focused laser pulse creates a cavity in plasma pushing electrons aside and exciting the wake wave and the bow wave. At the joint of the cavity wall and the bow wave boundary, an annular spike of electron density is formed. This spike surrounds the cavity and moves together with the laser pulse. Collective motion of electrons in the spike driven by the laser field generates high-order harmonics. A strong localization of the electron spike, its robustness to oscillations imposed by the laser field and, consequently, its ability to produce high-order harmonics is explained by catastrophe theory. The proposed mechanism explains the experimental observations of high-order harmonics with the 9 TW J-KAREN laser (JAEA, Japan) and the 120 TW Astra Gemini laser (CLF RAL, UK) [A. S. Pirozhkov, et al., arXiv:1004.4514 (2010); A. S. Pirozhkov et al, AIP Proceedings, this volume]. The theory is corroborated by high-resolution two-and three-dimensional particle-in-cell simulations.

[en] Complete text of publication follows. Laser ion acceleration has caught significant attentions due to its various applications such as medical applications, ion beam fast ignition, and proton radiography. For medical applications, laser ion acceleration is expected to realize a compact and reliable ions source for the cancer therapy. One of the critical issues on laser-driven ion source is the enhancement of laser-accelerated ion energy, where protons with maximum energy of 200 MeV are requested from the medical point of view. The mechanism known as a target normal sheath acceleration (TNSa) has been widely investigated theoretically and experimentally, and it is shown that a relatively high power laser such as PW-class laser is needed for generating 200 MeV protons. Recently, high energy ions are observed from gas-cluster targets with relatively small laser energy, where 20 MeV/u ions are generated by using 4 TW laser pulse. We consider that these high energy ions, the energy of which is roughly one order higher than the TNSA energy scaling, are generated via formation and evolutions of magnetic dipole vortex. In this paper, we investigate a detail of the ion acceleration via magnetic dipole vortex and derive an ion energy scaling. By the propagation of an intense laser pulse thorough underdense plasma, a dipole vortex is induced when the laser energy is almost depleted. In Fig. 1(a), an ion distribution is plotted when a magnetic dipole vortex is formed. Electrons and resultant ions are pushed outward from the vortex, forming an ion shell around the vortex and wall along the vortex axis. Electron, ion and electric field distributions along the axis are plotted in Fig. 1.(b). The ion distribution has a sharp spike and electrons are located in front of it, which results in generation of strong electro-static field. This structure moves to the right together with expanding dipole vortex. This leads to an ion acceleration by moving electric field. We modeled the magnetic dipole acceleration and obtained energy scaling, which predicts that a 100 TW laser can generate 200 MeV protons by magnetic dipole acceleration.

[en] Study results within the framework of the Hasegawa-Mima equation of a single and double layer are presented. Analytical solution of the problem on instability of singular chains of discrete vortices and vortex path is obtained. It is shown that anti-symmetrical configuration, similar to the von Carman path, is stable by the corresponding parameters ratio. The instability nonlinear stage is studied through numerical modeling

[en] Complete test of publication follows. The generation of coherent high-frequency radiation is the topic of great interest since the invention of lasers. Among the proposed schemes are the x-ray laser, free-electron laser, high-order harmonic generation in gases, relativistic harmonics from the solid targets, and so on. Recently, the relativistic frequency upshifting accompanied by the light intensification and pulse shortening was proposed using the Flying Mirror technique. According to the relativity theory, the frequency of light pulse reflected at the relativistic mirror moving toward it, is upshifted by the factor ∼ 4γ². Here we describe new method of super-high frequency generation, using simultaneously the relativistic upshifting and harmonic generation, resulting in the net factor of 4Nγ², where is N is the harmonic number. When a relativistic-irradiance laser pulse ('driver', subscript '0') propagates in the underdense plasma, it creates a wake wave with the phase velocity equal to the group velocity of the driver pulse, which is close to the velocity of light c for a small plasma density n₀. The gamma-factor is γ ∼ ω₀/ω_pe >> 1, where ω₀ is the driver pulse frequency, ω_pe = (4πn₀e²/m)^1/2 is the Langmuir frequency, and e and m are the electron charge and mass. At at near the wave breaking condition, the electron density profile in the wake wave has cusps with the peak density much higher than the unperturbed plasma density. The electrons within the cusps move with the velocity close to the phase velocity of the wake wave. Counter-propagating source pulse (subscript 's') is partially reflected from the cusps. If the source pulse is substantially strong, the reflected pulse contains not only the upshifted fundamental frequency 4γ²ω_s, but also harmonics. This nonlinear reflection leads to the spectrum containing components with the frequencies equal to 4Nγ²ω_s. Even for moderate values of γ and N, the source pulse frequency can be upshifted hundreds or thousands times. Furthermore, the gamma-factor is relatively easy to control via the plasma density, which gives rise to the tunable source of high-frequency pulses. In the time domain, the reflected pulse duration shrinks approximately by the factor 4γ² compared to the source pulse. Selecting several harmonics by a spectral filter, even shorter pulses are possible to produce. We developed an analytical model of the super-high frequency upshifting based on the model developed for the laser - thin-foil interaction. We also addressed the range of laser and plasma parameters for which the model is applicable.