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Relativistic mirrors can be realized with strongly nonlinear Langmuir waves excited by intense laser pulses in underdense plasma. On reflection from the relativistic mirror, the incident light affects the mirror motion. The corresponding recoil effects are investigated analytically and using particle-in-cell simulations. It is found that if the fluence of the incident electromagnetic wave exceeds a certain threshold, the relativistic mirror undergoes a significant back reaction and splits into multiple electron layers. The reflection coefficient of the relativistic mirror and the factors of electric field amplification and frequency upshifty of the electromagnetic wave are obtained.

The critical electric field of quantum electrodynamics is so strong that it produces electron-positron pairs from vacuum, converting the energy of light into matter. This field has become feasible through the construction of extremely high power lasers or/and with the sophisticated use of nonlinear processes in relativistic plasmas. A feasibility of the experiments on the collision of laser light and high intensity electromagnetic pulses, generated by relativistic flying mirrors, with relativistic electrons for the studying of extreme field limits in the nonlinear interaction of electromagnetic waves is discussed.

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.

A study is made of the ultra-short laser pulse irradiation of Ar cluster targets. Experiments have been performed with large cluster sizes and using very high laser contrasts, which have allowed clear and unambiguous observation of exotic inner-shell transitions in near-neutral Ar ions. The interaction of the main laser pulse with the unperturbed target is a necessary requirement for observing these lines. Our measurements are supported by kinetics calculations in which a very detailed atomic model is used. The calculations predict all of the spectral features found experimentally, and support the notion that the X-ray emission arises from many ion stages of the Ar plasma, from near-neutral through He-like ions, and from a range of plasma temperatures and densities. Differences between X-ray argon clusters excited at the laser-cluster and laser-droplet interactions have been analyzed. X-ray spectral methods have been proposed to determine the parameters of the plasma formed at the early stages of its evolution. It has been shown that the spectra of hollow ions are the most informative in the first moments of the heating of a cluster, whereas the diagnostics of the late stages can be performed using the conventional lines of multicharged ions.

We report on novel methods to generate ultra-short, coherent, X-rays using a laserplasma interaction. Nonlinear interaction of intense laser pulses with plasma creates stable, specific structures such as electron cusps. For example, wake waves excited in an underdense plasma by an intense, short-pulse laser become dense and propagate along with the laser pulse. This is called a relativistic flying mirror. The flying mirror can reflect a counter-propagating laser pulse and directly convert it into high-frequency radiation, with a frequency multiplication factor of ∼ 4γ2 and pulse shortening with the same factor. After the proof-of-principle experiments, we observed that the photon number generated in the flying mirror is close to the theoretical estimate. We present the details of the experiment in which a 9 TW laser pulse focused into a He gas jet generated the Flying Mirror, which partly reflected a 1 TW pulse, giving up to ∼ 1010 photons, 60 nJ (1.4×1012 photons/sr) in the XUV spectral region (12.8-22 nm).

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.

Application of polychromatic (1.5–15 nm) soft X‐ray emission of a spatially large (>0.1 mm) bright femtosecond laser driven plasma source for propagation based phase contrast imaging of nanometer thick foils and biological samples is considered. Diffraction and phase contrast effects increased quality and contrast of the experimental images, registered by LiF crystal X‐ray detector with submicron resolution.

A novel ultra‐bright high‐intensity source of X‐ray and gamma radiation is suggested. It is based on the double Doppler effect, where a relativistic flying mirror reflects counter‐propagating electromagnetic radiation causing its frequency multiplication and intensification, and on the inverse double Doppler effect, where the mirror acquires energy from an ultra‐intense co‐propagating electromagnetic wave. The role of the flying mirror is played by a high‐density thin plasma slab accelerating in the radiation pressure dominant regime. Frequencies of high harmonics generated at the flying mirror by relativistically strong counter‐propagating radiation undergo multiplication with the same factor as the fundamental frequency of the reflected radiation, approximately equal to the quadruple of the square of the mirror Lorentz factor.

A strongly nonlinear wake wave driven by an intense laser pulse can act as a partially reflecting relativistic mirror (the flying mirror) [S. V. Bulanov, et al., Bulletin of the Lebedev Physics Institute, No. 6, 9 (1991); S. V. Bulanov, et al., Phys. Rev. Lett. 91, 085001 (2003)]. Upon reflection from such mirror, a counter‐propagating optical‐frequency laser pulse is directly converted into high‐frequency radiation, with a frequency multiplication factor ∼4γ2 (the double Doppler effect). We present the results of recent experiment in which the photon number in the reflected radiation was at least several thousand times larger than in our proof‐of‐principle experiment [M. Kando, et al., Phys. Rev. Lett. 99, 135001 (2007); A. S. Pirozhkov, et al., Phys. Plasmas 14, 123106 (2007)]. The flying mirror holds promise of generating intense coherent ultrashort XUV and x‐ray pulses that inherit their temporal shape and polarization from the original optical‐frequency (laser) pulses. Furthermore, the reflected radi...

When an intense short laser pulse interacts with plasmas, the nonlinear interaction causes various phenomena such as high energy particle generation, strong radiation in the x‐ray to THz frequency range. Formation of solitons is one of these interesting topics and has been studied mainly from theoretical aspects. Solitons are formed in an underdense plasma in the process of the laser pulse frequency downshift. They store a portion of the EM wave energy with the polarization inherited from the laser pulse‐driver and emit it at the plasma‐vacuum interface in the form of low frequency EM bursts. Solitons were observed experimentally by proton‐beam imaging and in the visible‐near‐infrared emissions from the plasma. A low frequency EM radiation emission has also been observed in our experiments. The interpretation invokes the relativistic electromagnetic solitons. We present the results of the polarization‐resolved measurements of low frequency radiation from the plasma region irradiated by the intense laser pulse. A dominant component of the observed THz radiation has the same polarization as the driver laser pulse.

Generation of a parametric X‐rays using a Silicon crystal plane and a 150 MeV electron beam generated by a Microtron accelerator were performed. 10 keV class X‐rays have been detected at the generation condition of the X‐rays. We consider the X‐rays flux is ∼1 photons/(electrons sr). Also high‐energy X‐rays generated by harmonics of parametric process were observed.

Recent activities related to laser‐plasma acceleration at Advanced Photon Research Center, Japan Atomic Energy Research Institute are presented. Gas density measurements of a pulsed gas valve and wakefield measurements have been performed with two types of interferometers. A wakefield of 20 GV/m excited in the gas‐jet plasma has been obtained at a plasma density of 7×1017 cm−3. We have shown by 1D‐3V particle‐in‐cell simulation that a high quality electron beam with a transverse emittance of 0.3 π mm‐mrad and with an energy of 7.0±0.5 MeV can be produced using a colliding optical injection. Our proposed experiments using a 100 TW laser beam and a 150 MeV electron beam are also presented.

We investigate a laser wakefield excited by intense laser pulses, and the possibility of generating an intense bright electron source by an intense laser pulse. The coherent wakefield excited by 2 TW, 50 fs laser pulses in a gas‐jet plasma around 1018 cm−3 is measured with a time‐resolved frequency domain interferometer (FDI). The results show an accelerating wakefield excitation of 20 GeV/m with good coherency. This is the first time‐resolved measurement of laser wakefield excitation in a gas‐jet plasma. The experimental results agree with the simulation results and linear theory. The pump‐probe interferometer system of FDI will be modified to the optical injection system as a relativistic electron beam injector. In 1D particle in cell simulation we obtain results of high quality intense electron beam generation.

The beam monitor system at ICR 7 MeV proton linac is described. Because of the pulsed beam (