A combination of electrical and optical diagnostics has been used to investigate the time evolution of the two-dimensional expansion velocity distributions of the cathode plasma in pulsed high-power diodes. The perveance model based on the Child-Langmuir law was used to calculate the expansion velocity of the diode plasmas from voltage and current profiles. Additionally, a four-channel high speed framing camera was used to observe the formation and subsequent movement of the cathode plasma. More accurate and valuable information about the dynamics of the cathode plasma was also acquired by utilizing the digital image processing methods. Results from the experiments and theoretical analysis were compared. In this paper, the experiments have been performed using a high-voltage pulse generator with 200 kV output voltage and 110 ns pulse duration. Current densities up to 440 A/cm2 were produced. The observation of the cathode plasma expansion in transverse direction indicated that the diode current was cathode-limited in the current rising stage (the first 60 ns of the current pulse). The perveance model should be modified taking in account the time dependent expanding plasma surface (i.e., not the whole cathode surface) for this period. The velocity in the direction parallel to the cathode surface (transverse velocity) was much larger than that in the direction perpendicular to the cathode surface (longitudinal velocity), and further, it dropped from 90 cm/μs to nearly 20 cm/μs rapidly. It was shown that, during the current flattop stage, the plasma filled out all the surface of cathode and the diode current was space-charge-limited. The values of the transverse velocity and longitudinal velocity were nearly the same and decreased relatively slowly. The satisfactory coincidence of experimental and calculated (both were in the range of 6–8 cm/μs) values of the cathode plasma expansion velocities was obtained.

Two of the key issues of a krypton fluoride (KrF) laser driver for inertial fusion energy are the development of long life, high transparency pressure foils (to isolate vacuum in the electron beam diode from a working gas in the laser chamber), and the development of durable, stable, optical windows. Both of these problems have been studied on the single-pulse e-beam-pumped KrF laser installation GARPUN. We have measured the transport of electron beams (300 keV, 50 kA, 100 ns, 10 × 100 cm) through aluminum-beryllium and titanium foils and compared them with Monte Carlo numerical calculations. It was shown that 50-μm thickness Al-Be and 20-μm Ti foils had equal transmittance. However, in contrast to Ti foil, whose surface was strongly etched by fluorine, no surface modification nor fatal damages were observed for Al-Be foils after ∼1000 laser shots and protracted fluorine exposure. We also measured the 8% reduction in the transmission of CaF2 windows under irradiation by scattered electrons when they were set at 8.5 cm apart from the e-beam-pumped region. However an applied magnetic field of ∼0.1 T significantly reduced electron scattering both across and along the laser cell at typical pumping conditions with 1.5 atm pressure working gas. Thus the e-beam-induced absorption of laser radiation in optical windows might be fully eliminated in an e-beam-pumping scheme with magnetic field guiding.

The effect of long-wavelength irradiation nonuniformities on the performance of singleshell, reactor targets is investigated by means of 2-D numerical simulations. The stages of target collapse, hot-spot formation, ignition, and burn are illustrated. Evidence is shown for the occurrence of the Rayleigh–Taylor instability during target stagnation. It is then shown that the sensitivity of a given family of targets to the nonuniformities of the driving pressure critically depend on the 1–D ignition margin and on the spark convergence ratio of the target. The tolerable levels of nonuniformity as a function of the perturbation mode number are determined, for selected targets, by means of a parametric numerical study.

Generation of hot electrons at grazing incidence of a subpicosecond relativistic-intense laser pulse onto a plane solid target is analyzed for the parameters of petawatt class laser systems. We study preplasma formation on the surface of solid aluminum targets produced by laser prepulses with a different time structure. For modeling of the preplasma dynamics, we use a wide-range two-temperature hydrodynamic model. As a result of simulations, the preplasma expansion under the action of the laser prepulse and the plasma density profiles for different contrast ratios of the nanosecond pedestal are found. These density profiles are used as the initial density distributions in three-dimensional particle-in-cell simulations of electron acceleration by the main P-polarized laser pulse. Results of modeling demonstrate a substantial increase of the characteristic energy and number of accelerated electrons for the grazing incidence of a subpicosecond intense laser pulse in comparison with the ponderomotive scaling of laser–target interaction.

In order to measure the phase conjugation-fidelity of stimulated Brillouin scattering with high pump energy more accurately and conveniently, a new measurement is proposed. According to the definition of the fidelity with optical field correlated integral, the method of recording the far-field by array camera is analyzed theoretically. By the method, an experiment for high energy stimulated Brillouin scattering is arranged, and the maximum energy of 3.42 J is achieved in a single-cell stimulated Brillouin scattering generator. For small pump pulse, the fidelity maintains a high value of more than 0.9. However, the reflectivity fluctuation is significant when the pump energy is larger than 1.5 J. As a result, the fidelity decreases gradually to 0.5 at the pump energy of 3.42 J. The reflectivity shows the same character of great fluctuation as the fidelity at high pump energy.

We describe here the design, main parameters, and characteristics of a forevacuum-pressure plasma-cathode electron source based on a hollow-cathode discharge. The source generates a continuous focused electron beam with energy up to 30 keV and current up to 300 mA at a pressure of 10–50 Pa. The focused electron beam reaches a maximum power density of 106 W/cm2. The source utility has been demonstrated by its application for processing and cutting of ceramic.

Within the next few years X-ray free-electron lasers (XFELs) now under construction are expected to generate highly collimated XUV pulses with 1013 photons and a duration of 100 fs. Focusing this radiation to a spot some 10 μm in diameter generates intensities of up to 1016 W/cm2. Such pump intensities make feasible the investigation of photopumped XUV lasers using this radiation. We present simulations taking into account two different mechanisms generating the gain: (1) photoionization with subsequent three-body recombination, which takes advantage of the monochromaticity of the pump radiation to generate very cold electrons; (2) inner-shell ionization in which transient inversion is obtained by generating a hole in an otherwise completely filled shell. The simulations show that under appropriate conditions both mechanisms generate very high gain. However, a number of further issues must be considered, such as the propagation of the pump pulse in the medium to be pumped.

This paper reports on properties of a plasma formed by sequential action of two laser beams on a flat target, simulating the conditions of shock-ignited inertial confinement fusion target exposure. The experiments were performed using planar targets consisting of a massive copper (Cu) plate coated with a thin plastic (CH) layer, which was irradiated by the 1ω PALS laser beam (λ = 1.315 μm) at the energy of 250 J. The intensity of the fixed-energy laser beam was scaled by varying the focal spot radius. To imitate shock ignition conditions, the lower-intensity auxiliary 1ω beam created CH-pre-plasma which was irradiated by the main beam with a delay of 1.2 ns, thus generating a shock wave in the massive part of the target. To study the parameters of the plasma treated by the two-beam irradiation of the targets, a set of various diagnostics was applied, namely: (i) Two-channel polaro-interferometric system irradiated by the femtosecond laser (~40 fs), (ii) spectroscopic measurements in the X-ray range, (iii) two-dimensional (2D)-resolved imaging of the Kα line emission from Cu, (iv) measurements of the ion emission by means of ion collectors, and (v) measurements of the volume of craters produced in a massive target providing information on the efficiency of the laser energy transfer to the shock wave. The 2D numerical simulations have been used to support the interpretation of experimental data. The general conclusion is that the fraction of the main laser beam energy deposited into the massive copper at two-beam irradiation decreases in comparison with the case of pre-plasma. The reason is that the pre-formed and expanding plasma deteriorates the efficiency of the energy transfer from the main laser pulse to a solid part of the targets by means of the fast electrons and the wave of an electron thermal conductivity.

The propagation of a high-current finite-length ion charge bunch through a background plasma is of interest for many applications, including heavy ion fusion, plasma lenses, cosmic ray propagation, and so forth. Charge neutralization has been studied both analytically and numerically during ion beam entry, propagation, and exit from the plasma. A suite of codes has been developed for calculating the degree of charge and current neutralization of the ion beam pulse by the background plasma. The code suite consists of two different codes: a fully electromagnetic, relativistic particle-in-cell code, and a relativistic Darwin model for long beams. As a result of a number of simplifications, the second code is hundreds of times faster than the first one and can be used for most cases of practical interest, while the first code provides important benchmarking for the second. An analytical theory has been developed using the assumption of long charge bunches and conservation of generalized vorticity. The model predicts nearly complete charge neutralization during quasi-steady-state propagation provided the beam pulse duration τb is much longer than the inverse electron plasma frequency ωp−1, where ωp = (4πnp e2/me)1/2 and np is the background plasma density. In the opposite limit, the beam head excites large-amplitude plasma waves. Similarly, the beam current is well neutralized provided ωpτb >> 1 and the beam radius is much larger than plasma skin depth δp = c/ωp. Equivalently, the condition for current neutralization can be expressed in terms of the beam current as Ib >> 4.25Zb βb(nb /np)kA, where nb is the beam density, Zb is the ion charge, and Vb = βb c is the beam velocity; and the condition for charge neutralization can be expressed as Ib >> 4.25βb3(nb /np)(rb /lb)2kA, where lb and rb are the beam length and radius, respectively. For long charge bunches, the analytical results agree well with the results of numerical simulations. The visualization of the data obtained in the numerical simulations shows complex collective phenomena during beam entry into and exit from the plasma.

Collective absorption of intense laser pulses by atomic clusters is studied by PIC simulations. The cluster is modeled in two-dimensional calculations as a cylindrical plasma column with a diameter of D = 6.4 nm and an initial electron density of ne0 = 1023 cm−3. The frequency and intensity dependence of absorption is discussed. It is found that nonresonant absorption by electron emission increases as a power law with the laser intensity. The absorbed energy per electron reaches a maximum of about Wmax = mωp2D2 (ωp: plasma frequency, m: electron mass) at the intensity where ionization saturates.

This paper summarizes the results of several laser–plasma-interaction experiments using multikilojoule lasers, and considers their implications for laser fusion. The experiments used 1·06-, 0·53-, 0·35-, and 0·26-μm light to produce relatively large, warm, planar plasmas and to study the effect of laser wavelength and density-gradient scale length on the Stimulated Raman Scattering and on the scattering of light at frequencies near the incident laser frequencey by Stimulated Brillouin Scattering or other processes. The results of these experiments suggest that some laser wavelength between 0·2 and 0·6 μm will be required for high-gain laser fusion.

As the pulse power and high power microwave source technologies gradually matured, technologies for enhancing the output capacities of high power microwaves are becoming more and more attractive. In this paper, two different methods for the increasing of X band microwave powers are discussed: diplexers based on microwave filter and photonic crystal. For the case of diplexer based on microwave filter, the dual channel X band microwaves transmit through the filters with high efficiencies, the polarization and radiation directions for the microwaves are the same. With the application of metal photonic crystal, the reflection and transmission of TE/TM polarized X band microwaves are realized simultaneously; thus, the dual channel microwaves have the same radiation direction. A pulse of 25 ns, 3.9 GW has been successfully obtained. However, according to the experimental results, the internal breakdown in these devices limits their power handling capacities when the peak power and pulse duration of the microwaves increase. In the end, several methods for enhancing the power handling capacities the diplexer have been proposed.

A new approach to measure stimulated Brillouin scattering threshold based on the output energy characteristic of stimulated Brillouin scattering optical limiting is proposed. The stimulated Brillouin scattering threshold or its exponential gain, Gth, can be accurately and conveniently determined by the intersection point of linear-fitting lines of the output energy below and above the threshold. The values of Gth in CS2 and FC-72 for different wavelengths and interaction lengths are measured in Continuum's Nd:YAG Q-switched laser and its frequency-doubled system. We show that Gth for transient regime is larger than that for steady state, and increases with the pump wavelength and the interaction length.

An attosecond electron beam generation is studied by an intense short-pulse TEM (1,0) + TEM (0,1)-mode laser with a plasma separator in vacuum. The TEM (1,0) + TEM (0,1)-mode laser has a ring-shaped intensity peak in the radial direction. Electrons are accelerated and compressed near the focus point of the TEM (1,0) + TEM (0,1)-mode laser. However, after the focus point, some electrons move to its deceleration phase of the laser pulse and are decelerated. As a result, a longitudinal velocity deference of electrons generated causes a density lowering. In order to suppress the deceleration and the density lowering, we set an overdense plasma-foil separator before the electrons move to the deceleration phase of the laser pulse. Since only the laser is reflected by the plasma separator, the electrons do not experience the deceleration phase and the density of the electron bunch is kept high after passing through the plasma separator. Consequently, a high-density electron beam is generated and at the same time, the pulse length of the electron bunch becomes sub-femto second.

This paper introduces recent activities on Marx-based compact repetitive pulsed power generators at the Institute of Applied Electronics (IAE), China Academy of Engineering Physics (CAEP), over the period 2010–2018. A characteristic feature of the generators described is the use of a simplified bipolar charged Marx circuit, in which the normal isolation resistors or inductors to ground are removed to make the circuit simpler. Several pulse-forming modules developed to generate a 100 ns square wave output are introduced, including thin-film dielectric lines of different structures, a pulse-forming line based on a Printed Circuit Board, and non-uniform pulse-forming networks. A compact repetitive three-electrode spark gap switch with low-jitter, high-voltage, and high-current was developed and is used in the generators. A positive and negative series resonant constant current power supply with high precision and high power is introduced. As an important part of the repetitive pulse power generator, a lower jitter pulse trigger source is introduced. Several typical high-power repetitive pulsed power generators developed at IAE are introduced including a 30 GW low-impedance Marx generator, a compact square-wave pulse generator based on Kapton-film dielectric Blumlein line, a 20 GW high pulse-energy repetitive PFN-Marx generator, and a coaxial Marx generator based on ceramic capacitors. The research of key technologies and their development status are discussed, which can provide a reference for the future development and application of miniaturization of compact and repetitive Marx generators.

For biological hard tissues as well as for artificial restoration materials, ultra-short laser pulses (USLP) have proven their suitability for ablation with negligible collateral damage providing many advantages. For this paper, it was the aim to investigate the suitability of scanned USLP for the ablation of dental restorative materials compared to the microsecond pulses of conventional Erbium lasers. Scanning electron microscopy allowed analyzing the quality of the cavity surfaces with respect to structural properties as well as temperature exposition. Quantitative results about the ablation efficiency are given for different laser parameters on dental hard tissue and a broad variety of dental restoration materials, the latter being reported for the first time.

Various problems involved in the simulation of current X-ray laser experiments are discussed. The methods are based on laser-plasma simulation techniques with simultaneous calculation of the ionization dynamics, and are particularly appropriate for collisional and recombination pumped systems.

Experimental data are reported on second harmonic generation (SHG) during the interaction between a Nd laser beam (20 ns pulse-duration and peak intensity 1013 W/cm2) and a plasma of density 1019 cm−3 produced in helium. The behavior of self-focusing and the onset of steep plasma gradients under the same experimental conditions have been previously characterized by independent techniques as already reported. In the light of these former findings, the observed SH magnitude, emission pattern and time dependence are briefly discussed and compared with expectations.

We investigate intermediate unstable modes between two stream and filamentation instabilities. We detail the problem of the angle between the wave vector and its electric field and use an electromagnetic formalism allowing for any value for this angle. We display analytical results for 3 different models: cold beam-cold plasma, cold beam-hot plasma and cold relativistic beam-hot plasma. We demonstrate that plasma temperature prompts a critical angle for which waves are unstable at any k and show that for a relativistic beam, the most unstable waves are obtained for wave vectors which are neither normal nor perpendicular to the beam.

Relativistic laser–plasma interaction, subsequent superhot electron transport, superhot electron energy deposition, and the overall implosion process are key subjects for fast ignition. All these phenomena couple with each other, and more studies by simulations are essential. We have a plan to simulate the whole of fast ignition self-consistently with four individual codes. Four codes are integrated into one big system in the Fast Ignition Integrated Interconnecting code project. In a first stage of this project, we integrate the Arbitrary Lagrangian Eulerian (ALE) hydro code with the collective particle in cell (PIC) code. The PIC code obtains density profile at maximum compression from the ALE hydro code to introduce imploded plasma into a PIC system, and we can simulate interaction between ignition laser and realistic plasma. We have evaluated reflected laser spectrum and electron energy distribution, and found many differences between the realistic plasma profile and the conventional one in PIC simulations.