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.

The interaction of medium sized Argon clusters (30 Å) with high-intensity femtosecond laser pulses (806 nm, 8 × 1016 W/cm2) of durations ranging from 10 fs to 120 fs have been studied using a three-dimensional relativistic time dependent molecular dynamic approach. The dynamics of cluster expansion is explained in terms of temporal evolution of electron population in the cluster and snapshots of particle positions at various times. The effects of inter-cluster distance on ionization dynamics are presented. It is observed that the collisional ionization increases with decreasing inter-cluster distance. The effect of pulse duration on laser energy absorption is also studied. For a laser pulse of gaussian time profile, there exists an optimum pulse duration for maximum absorption. No such optimum exists for a nearly flat top (super-gaussian) laser pulse. Results indicate the existence of resonance absorption inside the cluster. It is also observed that the high energy component of ion emission from cluster is anisotropic, showing a preferential direction of emission along laser polarization while the low energy ions emerge almost isotropically.

Enhancement of the line X-ray emission from iron plasma is investigated by simulating laser irradiation of both porous and solid targets. Spectral line intensities are calculated for selected lines of the iron plasma within the extreme ultra-violet lithography wavelength range 13.3–13.7 nm. The calculations show that X-ray yield in porous targets can be enhanced significantly in comparison with solid density targets. The results also show that for specified conditions of the driving laser, there are optimums conditions of the porous target in which maximum yield can be obtained.

Dependence of quasistatic Wakefield and high harmonic generation on the pulse-shape of an ultrashort, intense, few-cycle laser in the reflected radiation from a thin dense plasma layer is investigated. The pulse envelopes considered are Gaussian, Lorentzian, and hyperbolic secant having identical full width at half maximum of intensity. The reflected radiation from the strongly driven surface plasma layer embodies a quasistatic Wakefield, which exists after the main pulse is passed over. A phase modulation is also experienced by the laser light upon reflection from plasma surface motion. As a result harmonics of center carrier frequency of the laser-pulse are generated in the reflected signal. Intensity of the laser harmonics and magnitude of the Wakefield in the reflected radiation are found to depend on the pulse-shape, number of cycles, carrier envelope phase difference, plasma density, angle of incidence, and intensity of the incident pulse.

Tesla transformers are widely used in short pulse, repetition pulsed power generators. In this paper, a high repetitive rate intense electron beam accelerator (IEBA) based on high coupling (~1) Tesla transformer, which consists of a primary charging system, coaxial pulse forming line (PFL) charged by Tesla transformer and gas spark switch is described, especially stressed on the high coupling Tesla transformer. By introducing magnetic core to enhance the coupling factor between the primary and secondary windings, the transformer is capable of producing high voltage pulse up to 1.4 MV in approximately 45 µs. A coaxial pulse forming line is closely attached to the transformer that the outer and inner magnetic cores are parts of the PFL's outer and inner conductors respectively. In addition, the parameters of the Tesla transformer and PFL are calculated, including the dimension of the PFL and Tesla transformer. Some experiment results showed that the IEBA is capable of producing electron beams of 300–700 kV/7–13 kA at repetitive rate 100 Hz, with the pulse width 35 ns. The maximal energy efficiency of the Tesla transformer is 83%.

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.

A new multiphase buoyancy-drag model is developed for the study of Rayleigh-Taylor and Richtmyer-Meshkov instabilities in dusty gases, extending on a counterpart single-phase model developed in the past by Srebro et al. (2003). This model is applied to single- and multi-mode perturbations in dusty gases and both Rayleigh-Taylor and Richtmyer-Meshkov instabilities are investigated. The amplitude for Rayleigh-Taylor growth is observed to be contained within a band, which lies within limits identified by a multiphase Atwood number that is a function of the fluid densities, particle size, and a Stokes number. The amplitude growth is subdued with (1) an increase in particle size for a fixed particle number density and with (2) an increase in particle number density for a fixed particle size. The power law index for Richtmyer-Meshkov growth under multi-mode conditions also shows dependence to the multiphase Atwood number, with the index for the bubble front linearly decreasing and then remaining constant, and increasing non-linearly for the spike front. Four new classes of problems are identified and are investigated for Rayleigh-Taylor growth under multi-mode conditions for a hybrid version of the model: (1) bubbles in a pure gas rising into a region of particles; (2) spikes in a pure gas falling into a region of particles; (3) bubbles in a region of particles rising into a pure gas; and (4) spikes in a region of particles falling into a pure gas. Whereas the bubbles accelerate for class (1) and the spikes for class (4), for classes (2) and (3), the spikes and bubbles, respectively, oscillate in a gravity wave-like phenomenon due to the buoyancy term changing sign alternatively. The spike or bubble front, as the case may be, penetrates different amounts into the dusty or pure gas for every subsequent penetration, due to drag effects. Finally, some extensions to the presently developed multiphase buoyancy-drag model are proposed for future research.

We present a numerical study of the acceleration of ions in the interaction of a high intensity circularly polarized laser beam normally incident on an overdense plasma target, and the subsequent formation of neutral plasma ejected toward the rear side of the target. We compare the results obtained from two different numerical codes. We use an Eulerian Vlasov code for the numerical solution of the one-dimensional relativistic Vlasov-Maxwell set of equations, for both electrons and ions, and a particle-in-cell code applied to the same problem. We consider the case when the laser free space wavelength λ0 is greater than the scale length of the jump in the plasma density at the target plasma edge Ledge (λ0 ≫ Ledge), and the ratio of the plasma density to the critical density is such that n/ncr ≫ 1. The ponderomotive pressure due to the incident high-intensity laser radiation pushes the electrons at the target plasma surface, producing a sharp density gradient at the plasma surface, which gives rise to a charge separation. The resulting electric field accelerates the ions that reach a free streaming expansion phase, where they are neutralized by the electrons. A neutral plasma jet is thus ejected toward the rear side of the target. Two cases are studied: In the first case, the laser intensity rises to a maximum and then remains constant, and in the second case, the laser intensity is a Gaussian-shaped pulse. The results show substantial differences in the phase-space structure of the ions and the electrons between these two cases. There is good agreement between the quantitative macroscopic results obtained by the two codes, and good qualitative agreement between the results showing the kinetic details of the phase-space structures. The low noise level of the Eulerian Vlasov code allows a more detailed representation of the phase-space structures associated with this system, especially in the low density regions of the phase-space where ions are accelerated.

A model for plasma channel formation by a laser pre-pulse in a low Z gas (Hydrogen) embedded with high Z atoms (Ar) is developed. The laser of intensity I ≅ 1014 W/cm2 ionizes hydrogen atoms fully whereas Ar atoms are ionized only singly. After the first pulse is gone, plasma expands on the time scale of a nanosecond to produce a hydrogen plasma channel with minimum density on the axis. A second intense short pulse laser of intensity I ≥ 1016 W/cm2 gets focused. It tunnel ionizes the remaining Ar. The Ar acquires Ar8+ charge state after loosing 8 ions and acquires Ne like configuration and could emit X-rays.

Detailed spectroscopic identification and analysis of lines emitted by Ni-like ions may infer on plasma parameters, such as electron density, temperature, and ionization state. Spatially, resolved X-ray spectra of samarium laser produced plasma were recorded in the 7 to10 Å wavelength range. Measured line intensity ratios of Ni-like 3d-5f, 3p-4d, 3p-4s, and 3s-4p transitions were used for electron density diagnostic as a function of the distance from the target. Calculations using Hebrew University Lawrence Livermore Atomic Code show that these ratios are not very sensitive to the electron temperature in the range from 500 to 1000 eV. Self-absorption of some lines is found to be important at electron densities higher than 1021cm−3. The inferred ranges of electron density and temperature are found to be consistent with results of hydrodynamic simulations and models of ionization in plasma.

The anomalous absorption of laser, incident at an arbitrary angle of incidence on a metal surface embedded with nanoparticles, is studied. The electrons inside a nanoparticle resonantly absorb laser energy when the laser frequency equals the frequency of surface charge oscillations of the nanoparticle. A monolayer of nanoparticles of radius rnp0 ≈ 50 A with inter-particle separation d ~ 10rnp0 can cause up to 40% reduction of the reflection of p-polarized laser light. The absorption coefficient increases with the angle of incidence and has a sharp peak at a resonant frequency width of about 1%. At high laser power, even if the nanoparticles are initially off resonant with the laser, the particle heating and subsequent expansion reduces the resonance frequency, and the resonance absorption is realized after a time delay. The delay is found to be directly proportional to the cluster size and inversely proportional to the 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.

It is shown that scattering of the laser-accelerated carbon ions in the material of the protective membrane of a fast ignition, indirect compression target without cone can result in a significant increase in the hot spot radius and will complicate significantly or even prevent the effective shaping of the bombarded region of the compressed fuel.

Theoretical and experimental studies of radiative properties of substances heated by pulsed current devises or lasers and used as X-ray sources have been carried out depending on plasma conditions, and specific spectra of X-ray absorption and radiation for different materials have been calculated. Important features of the theoretical model, known as the ion model of plasma, are discussed. This model can be applied for calculations of the radiative properties of complex materials over a wide range of plasma parameters. For purposes of indirect-driven inertial fusion based on the hohlraum concept, an optimization method is used for the selection of an effective complex hohlraum wall material, which provides high radiation efficiency at laser interaction with the wall. The radiation efficiency of the resulting material is compared with the efficiency of other composite materials that have previously been evaluated theoretically. A similar theoretical study is performed for the optically thin X-pinch plasma produced by exploding wires. Theoretical estimations of radiative efficiency are compared with experimental data that have been obtained from measurements of X-pinch radiation energy yield using two exploding wire materials, NiCr and Alloy 188. It is shown that the theoretical results agree well with the experimental data. A symmetric multilayer X-pinch, where W and Mo wires are used, is as well considered. The theoretical explanation of experimental phenomena is discussed based on the W and Mo radiative spectra. The ion model was as well applied for interpretation of experimental results on opacities of CHO-plasma obtained via indirect heating of low density polymer layers by means of soft X-rays. The new diagnostics method based on the deformation of the of the Carbon absorption K-edge when foam layer is heated to plasma is discussed. The spectral coefficients for X-ray absorption in CHO-plasma are calculated in the photon energy region around the Carbon K-edge for different plasma temperatures and mean foam density. In this case, the Carbon K-edge position on the energy scale can be used for plasma temperature diagnostic.

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.

This article presents the cross-focusing of two high power dark hollow Gaussian beams in plasma when relativistic nonlinearity is operative. A paraxial like approach has been used in the present analysis. In this study, the non-linear dielectric function has been expanded in terms of radial distance from the maximum of the irradiance, rather than from the axis, as is the case of Gaussian beams. The nature of propagation of a hollow Gaussian beam propagating in plasmas has been studied under the influence of relativistic non-linearity. The effect on the order (n) of hollow Gaussian beam on the cross-focusing of two beams has been explored in relativistic non-linearity in this publication.

The scaling of the flux and maximum energy of laser-driven sheath-accelerated protons has been investigated as a function of laser pulse energy in the range of 15–380 mJ at intensities of 1016–1018 W/cm2. The pulse duration and target thickness were fixed at 40 fs and 25 nm, respectively, while the laser focal spot size and drive energy were varied. Our results indicate that while the maximum proton energy is dependent on the laser energy and laser spot diameter, the proton flux is primarily related to the laser pulse energy under the conditions studied here. Our measurements show that increasing the laser energy by an order of magnitude results in a more than 500-fold increase in the observed proton flux. Whereas, an order of magnitude increase in the laser intensity generated by decreasing the laser focal spot size, at constant laser energy, gives rise to less than a tenfold increase in observed proton flux.

A new approach to spectroscopy of laser induced proton beams using radiochromic film (RCF) is presented. This approach allows primary standards of absorbed dose-to-water as used in radiotherapy to be transferred to the calibration of GafChromic HD-810 and EBT in a 29 MeV proton beam from the Birmingham cyclotron. These films were then irradiated in a common stack configuration using the TARANIS Nd:Glass multi-terawatt laser at Queens University Belfast, which can accelerate protons to 10–12 MeV, and a depth-dose curve was measured from a collimated beam. Previous work characterizing the relative effectiveness (RE) of GafChromic film as a function of energy was implemented into Monte Carlo depth-dose curves using FLUKA. A Bragg peak (BP) “library” for proton energies 0–15 MeV was generated, both with and without the RE function. These depth-response curves were iteratively summed in a FORTRAN routine to solve for the measured RCF depth-dose using a simple direct search algorithm. By comparing resultant spectra with both BP libraries, it was found that the effect of including the RE function accounted for an increase in the total number of protons by about 50%. To account for the energy loss due to a 20 µm aluminum filter in front of the film stack, FLUKA was used to create a matrix containing the energy loss transformations for each individual energy bin. Multiplication by the pseudo-inverse of this matrix resulted in “up-shifting” protons to higher energies. Applying this correction to two laser shots gave further increases in the total number of protons, N of 31% and 56%. Failure to consider the relative response of RCF to lower proton energies and neglecting energy losses in a stack filter foil can potentially lead to significant underestimates of the total number of protons in RCF spectroscopy of the low energy protons produced by laser ablation of thin targets.

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.

The purpose of this paper is to calculate proton energy loss straggling using a full conserving dielectric function (FCDF) for plasmas at any degeneracy. This dielectric function takes into account plasma electron-electron collision considering density, momentum, and energy conservation. When only momentum conservation law is accomplished, the FCDF reproduces the well known Mermin dielectric function, when none of the conservations laws are obeyed, the random phase approximation (RPA) is recovered. Then, the FCDF is applied for the first time to the determination of the energy loss straggling. Differences among diverse dielectric functions to determine straggling follow the same behavior for all kind of plasmas then, they do not depend on the plasma degeneracy but essentially do on the value of the collision frequency. These discrepancies can rise up to 5% between FCDF values and the Mermin ones, and 2% between the FCDF ones and RPA ones for plasma with high enough collision frequency. The similarity between FCDF and RPA results is not surprising, as all conservation laws are also considered in RPA dielectric function. The fact that FCDF and RPA give similar results and the fact that FCDF considers electron-electron collisions and RPA does not, means that latter collisions are not significant for energy loss straggling calculations.