In this paper we report on an experimental study of high harmonic radiation generated in nanometer-scale foil targets irradiated under normal incidence. The experiments constitute the first unambiguous observation of odd-numbered relativistic harmonics generated by the v × B component of the Lorentz force verifying a long predicted property of solid target harmonics. Simultaneously the observed harmonic spectra allow in-situ extraction of the target density in an experimental scenario which is of utmost interest for applications such as ion acceleration by the radiation pressure of an ultraintense laser.

The inelastic interaction between heavy ions and an electron plasma in the presence of an intense radiation field (RF) is investigated. The stopping power of the test ion averaged with a period of the RF has been calculated assuming that ω0 > ωp, where ω0 is the frequency of the RF and ωp is the plasma frequency. In order to highlight the effect of the radiation field we present a comparison of our analytical and numerical results obtained for nonzero RF with those for vanishing RF. It has been shown that the RF may strongly reduce the mean energy loss for slow ions while increasing it at high–velocities. Moreover, it has been shown, that acceleration of the projectile ion due to the RF is expected at high–velocities and in the high–intensity limit of the RF, when the quiver velocity of the plasma electrons exceeds the ion velocity.

In the present analysis we study the time dependent, self-consistent propagation of nonlinear electromagnetic pulses in plasmas. Interactions of the electromagnetic pulses and wakefields are fully taken into account, from which one obtains accurate information on pulse time dependent dynamics and stability. While wide pulses may or may not retain the localized shape depending on their power, narrower pulses always tend to delocalize as time evolves.

This paper presents an investigation of self-focusing of Gaussian laser beam in collisional plasma and its effect on second harmonic generation. Due to non-uniform heating, collisional non-linearity arises, which leads to redistribution of carriers and hence affects the plasma wave, which in turn affects the second harmonic generation. Effect of the intensity of the laser beam/plasma density on the harmonic yield is studied in detail. We have set up the non-linear differential equations for the beam width parameters of the main beam, plasma wave, second harmonic generation and second harmonic yield by taking full non-linear part of the dielectric constant of collisional plasma with the help of moment theory approach. It is predicted from the analysis that harmonic yield increases/decreases due to increase in the plasma density/intensity of the laser beam respectively.

300 keV electron beams with energy peaked in the range 280–390 keV were generated by focusing a high contrast ratio but temporally double pulsed 800 nm ultrafast laser onto a flowing water jet under both helium atmosphere at ambient pressure and water aspirator vacuum conditions, using laser intensities in the range 1015–1018 Wcm−2. Their characteristics have been investigated as functions of inter-pulse delay, incidence geometry and laser pulse chirp. Shot-to-shot variation of the beams' equatorial and azimuthal distributions was also recorded in real time. Measurements of the emitted charge and energy have been performed. Secondary X-ray emission arising from impingement of the electron beams on the target chamber walls and other parts of the apparatus have been identified. Preliminary results after transition to a high repetition rate laser system have shown similar behavior. Approaches for improvements and applications are suggested.

Experiments with an air electrode gap have been performed where the current/charge of a picosecond beam of runaway electrons was measured over a wide range (up to four orders of magnitude) downstream of the absorbing foil filters. Measurements and calculations have made it possible to refer the beam current to the rise time of the accelerating voltage pulse to within picoseconds. It has been shown that, in contrast to a widespread belief, the runaway electron energies achieved are no greater than those corresponding to the mode of free acceleration of electrons in a nonstationary, highly nonuniform electric field induced by the cathode voltage. The experimental data agree with predictions of a numerical model that describes free acceleration of particles. It has been confirmed that the magnitude of the critical electric field that is necessary for electrons to go into the mode of continuous acceleration of electrons in atmospheric air corresponds to classical notions.

Advancement of ion acceleration by intense laser pulses is studied with ultra-thin nanometer-thick diamond like carbon and micrometer-thick Titanium target foils. Both investigations aim at optimizing the electron density distribution which is the key for efficient laser driven ion acceleration. While recently found maximum ion energies achieved with ultra-thin foils mark record values micrometer thick foils are flexible in terms of atomic constituents. Electron recirculation is one prerequisite for the validity of a very simple model that can approximate the dependence of ion energies of nanometer-thick targets when all electrons of the irradiated target area interact coherently with the laser pulse and Coherent Acceleration of Ions by Laser pulses (CAIL) becomes dominant. Complementary experiments, an analytical model and particle in cell computer simulations show, that with regard to ultra-short laser pulses (duration ~45 fs at intensities up to 5 × 1019 W/cm2) and a micrometer-thick target foil with higher atomic number a close to linear increase of ion energies manifests in a certain range of laser intensities.

Based upon the Ammosov-Delone-Krainov ionization model, it is shown that two-color laser interaction with neutral gas generates strong ionization currents, which lead to electromagnetic emission at terahertz frequency when the gas density is at proper values. The emission efficiency depends on the difference of the phases between the fundamental and its second harmonic. The intensity ratio between the two pulses also affects the emission strength. An optimum intensity ratio has been found within our parameter region. The above ionization current theory is in agreement with one-dimensional particle-in-cell simulations with field ionization included.

This work is devoted to improving relativistic self-focusing of intense laser beam in underdense unmagnetized plasma. New density profiles are introduced to achieve beam width parameter up to the wavelength of the propagating laser. By investigating variations of the beam width parameter in presence of different density profiles it is found that the beam width parameter is considerably decreased for the introduced density ramp comparing with uniform density and earlier introduced density ramp profiles. By using this new density profile high intensity laser pulses are guided over several Rayleigh lengths with extremely small beam width parameter.

In the tape helix slow-wave system, discontinuous dielectrics have great effects on the dispersion characteristics. In this paper, the tape helix slow-wave system, including an inner and an outer metal shield, tape helix, nylon support and de-ionized water as filling dielectric, was analyzed. Effects of dielectric discontinuity caused by the support dielectric and filling dielectric on the dispersion characteristics were studied in detail. The dispersion relations, phase velocities, slow-wave coefficients and electric lengths of the spatial harmonics in the system were calculated. Results showed that, if the permittivity of support dielectric was smaller than that of the filling dielectric, frequencies of the spatial harmonics in the system rose, phase velocities and slow-wave coefficients increased, the slow-wave effect of the system was weakened so that the previous electric length was shortened. The reverse condition corresponded to the reverse results, and the electromagnetic simulation also proved it. By use of the helical pulse forming line of accelerator based on the studied tape helix slow-wave system, the electric lengths of the system were tested as 188.5 ns and 200 ns in experiment when the thicknesses of nylon support were 6 mm and 3 mm, respectively. The theoretical calculation results 198 ns and 211 ns basically corresponded to experimental results, which only had relative errors as 5 and 5.5%, respectively.

A theoretical formalism is developed to obtain the mode structure of right circularly polarized nonlinear laser Eigen mode in a self created plasma channel in the presence of an axial magnetic field. The nonlinearity in electron response arises due to relativistic mass effect and ponderomotive force induced density redistribution. The Eigen mode is seen to be unstable to stimulated Raman backscattering involving an electrostatic quasi-mode and a scattered electromagnetic wave. The growth rate increases with ambient magnetic field.

A new direction for high-power microwave (HPM) development is to investigate devices capable of producing HPMs with a complicated spectrum. In recent years, some HPM sources with two stable and separate frequencies have been investigated theoretically and experimentally. However, many short-comings still exist in these devices. Especially, the beam-wave interaction efficiency and the output microwave power are low in such devices. This paper proposes a novel dual-frequency HPM generator based on transition radiation. In the device, the electromagnetic fields are localized near the resonator cavities in the form of standing waves, and thus the interference between the different HPM components with different frequencies is weak. Compared with the existing dual-frequency devices, the new structure allows high beam-wave interaction efficiency and high output microwave power. As indicated in particle-in-cell simulation, with an electron beam of 500 kV voltage and 15.0 kA current guided by a magnetic field of 0.8 Tesla, an average power of 1.60 GW with a total power conversion efficiency of 21.3% is obtained, and the frequencies are 1.53 GHz and 3.29 GHz, respectively. Power level between two HPMs is comparable. The simulation results verify the feasibility of the dual-frequency HPM generator.