Non-paraxial approximation based study of transient self-focusing of an intense short-pulse laser in plasma has been investigated by considering the effect of a transverse magnetic field. The laser with non-uniform distribution of intensity exerts a ponderomotive force on electrons and sets in an ambi-polar diffusion of the plasma. The ambient magnetic field, however, strongly inhibits the process, when the electron Larmor radius is comparable to or shorter than the laser spot size. As the plasma density is depleted, the laser beam becomes more self-focused. This study addresses a significant enhancement in the laser self-focusing rate by including the correction terms due the off-axis approximation.

This paper is devoted to the question of the conversion of nuclear energy into the energy of coherent optical radiation. The two possible ways to convert nuclear energy into laser radiation are discussed: direct and combined nuclear pumping. The concept of using laser gas active media capable of working with both direct and combined nuclear pumping is considered. The results of an investigation by Kazakh scientists on nuclear pumped lasers active media on bound–bound atomic transitions are presented.

The laser-driven acceleration of proton beams from a double-layer cone target, comprised of a cone shaped high-Z material target with a low density proton layer, is investigated via two-dimensional fully relativistic electro-magnetic particle-in-cell simulations. The dependence of the inside diameter (ID) of the tip size of a double-layer cone target on proton beam characteristics is demonstrated. Our results show that the peak energy of proton beams significantly increases and the divergence angle decreases with decreasing ID size. This can be explained by the combined effects of a stronger laser field that is focused inside the cone target and a larger laser interaction area by reducing the ID size.

This article presents the results of a study on transportation of a pulse ion beam of gigawatt power. This beam is formed by a self-magnetically insulated diode with an explosive-emission cathode. The experiments have been performed using the TEMP-4M pulsed ion accelerator configured in double-pulse formation mode with the first negative pulse (300–500 ns, 100–150 kV), followed by the second positive pulse (150 ns, 250–300 kV). To increase the effectiveness of ion beam focusing, a metal shield is installed on a grounded electrode. Investigations are performed using a strip focusing diode, a cone diode, and a spiral diode with metal shields of different constructions. We observed that the beam diameter at the focus decreases from 60 mm (without shield) to 40–42 mm (with a shield), which leads to an increase in the energy density by a factor of 1.5–2 being 4–5 J/cm2 at the focus. We analyzed different mechanisms for ion trajectories deviation from an ideal one: Coulomb repulsion due to incomplete space charge neutralization, influence of electromagnetic fields, etc. It is found that for a strip focusing diode the concentration of low-energy electrons accompanying the ion beam exceeds the concentration of ions by 1.3–1.5 times. The use of a metal shield improves the transportation properties of the ion beam by keeping neutralizing electrons within the beam volume which ensures its space charge neutralization during the transport.

This paper presents an investigation for excitation of ion acoustic wave and resulting stimulated Brillouin scattering in a collisionless plasma due to presence of a laser beam carrying null intensity at center (hollow Gaussian beam). In presence of ponderomotive nonlinearity, the pump beam get focused and affects the back stimulated Brillouin scattering process. To understand the nature of laser plasma coupling, a paraxial-ray approximation has been invoked for the propagation of the hollow Gaussian beam, ion acoustic wave, and stimulated Brillouin scattering. It is observed from the result that self-focusing and back reflectivity reduces for higher order of hollow Gaussian beam.

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.

The nonlinear Compton scattering by a linearly polarized laser pulse of finite duration is analyzed, with a focus on the spin effects of target electrons. We show that, although the Compton scattering accompanied by the electron no-spin flip is dominant, for some energy regions of Compton photons their emission is dominated by the process leading to the electron spin flip. This feature is observed for different pulse durations, and can be treated as a signature of quantum behavior. Similar conclusions are reached when analyzing the scattered electron energy spectra. This time, the sensitivity of spin effects to the carrier-envelope phase of the driving pulse is demonstrated. The possibility of electron acceleration by means of Compton scattering is also discussed.

Brillouin backward scattering is investigated in the interaction of linearly polarized short laser pulse with a homogenous underdense transversely magnetized plasma by taking into account the relativistic and nonlinearity effects up to third order. The plasma is embedded in a uniform magnetic field perpendicular to both of propagation direction and electric vector of the radiation field. Temporal growth rate of instability is calculated by using of the nonlinear wave equation. Results are significantly different in comparison with lower order computations. The growth rate of Brillouin backward instability shows a decrease due to the presence of external magnetic field, while relativistic and higher order nonlinearities due to the external magnetic field, give rise the Brillouin backward scattering instability.

We theoretically and experimentally investigate the conditions of obtaining a narrowband flat-top Brillouin gain spectrum (BGS) based on single-frequency and multi-frequency phase modulations. Using the unequal-amplitude spectral lines, the flat-top BGS can be realized by controlling the intensity ratio and the frequency separation between them. In experiment, we obtain the flat-top gain spectra with the bandwidths of 40 MHz and 125 MHz, and with the top fluctuation of less than 0.21 dB. Based on this, we also achieve low distortion Brillouin amplification of a probe signal pulse and spectrum.

Coaxial relativistic backward wave oscillator with the rippled inner conductor not only increases the output efficiency but also results in the serious phenomenon of pulse shortening in experiments. Our research indicates that the two main mechanisms leading to the pulse shortening are the electron beam interruption and combining effects of the explosive field electron emission and the secondary electron multipactor on the surface of the slow-wave structure. In order to enhance its power capacity the electrodynamic structure is modified by detailed analysis of the field distribution in the coaxial slow-wave structure. The appropriate resonant reflector and the electron collector are developed for the application of the coaxial relativistic backward wave oscillator. A series of surface treatment is applied to enhance the power capacity of the coaxial RBWO. In the experiment, the microwave pulse duration is increased from less than 10 ns to 20 ns, and the output efficiency is enhanced from less than 20% to 34% employing the electron beam pulse of the full width at half maximum 28 ns. The peak power of 1.01 GW at the frequency of 7.4 GHz is achieved. It is found that the output efficiency of the coaxial RBWO is likely to be advanced if its power capacity can be boosted further.

Two-dimensional particle-in-cell simulations are performed to investigate the propagation of low energy continuous ion beams through background plasmas. It is shown that the continuous ion beam can be modulated into periodic short beam pulses by the induced wakefield, which can be adopted as a method to produce ultrashort ion beam pulses. Furthermore, the transport of the continuous ion beam in plasma with density gradient in the beam propagation direction is proposed and an enhanced longitudinal compression by density gradient is found due to the phase lock of ion pulses in the focusing regions of wakefield and reduced heating of plasma electrons.

The results of the experimental studies of the high-power e-beam accelerator producing six radially convergent electron beams are presented. The studies are aimed to increase the energy of the electron beam transported through the foil into the gas-filled chamber by using the rod-shaped current returns in the diode at small inter-electrode gaps. Installation of these rod current returns shields the periphery regions of the diode from the current field in the central part thus reducing the field at the diode edge. The inter-electrode distance, the shape, and the sizes of the cathodes are chosen by taking into account the magnetic field reduction in the diode. It is shown that in such type of the diode the electrons impact the foil almost normally to its surface, and the electron beams enter the output windows completely. Such type of the diode allows increasing the efficiency of the electron beam energy transfer into the gas by 30%.

Direct-drive target designs below self-ignition threshold are proposed for the laser megajoule in the context of shock-ignition. Two distinct initial aspect ratios are considered and laser pulses are shaped following a classical Kidder's law in order to achieve an implosion velocity of 300 km/s, an in-fight adiabat close to unity and to maximize the peak areal density. The pulse shapes are adjusted to arrange shock timing at the inner side of the DT fuel. The robustness of the laser pulse is addressed by the means of random variations around the initial Kidder's laws. Correlation matrices show no significant correlations between laser parameters. An admissible envelope of laser pulse is given for both designs in order to warrant more than 80% of the best peak areal density. Variations of laser drive power produce variations of implosion velocities in the range 250–370 km/s. Self-ignition threshold is achieved and thermonuclear energy are produced in the range 3 kJ–27 MJ. Finally, the random procedure shows that it is possible to improve the first deterministic optimization and the laser pulses are given.

This paper examines the reliability of a widely used method for temperature determination by multi-wavelength pyrometry. In recent warm dense matter experiments with ion-beam heated metal foils, we found that the statistical quality of the fit to the measured data is not necessarily a measure of the accuracy of the inferred temperature. We found a specific example where a second-best fit leads to a more realistic temperature value. The physics issue is the wavelength-dependent emissivity of the hot surface. We discuss improvements of the multi-frequency pyrometry technique, which will give a more reliable determination of the temperature from emission data.

A theoretical model for electrons in the conduction band intend to investigate the optical breakdown threshold in femtosecond laser pulse-fused silica interaction is presented. The model is derived from a rate equation that includes the avalanche and multi-photon ionization processes of Thornber and Keldysh, respectively, and also the three-body and exciton recombination mechanisms. In addition, the time evolution of electron mean energy is also considered through the energy balance equation. The mean energy acts as a trigger for the avalanche mechanism. The evolution of electron density profiles is calculated and discussed with respect to the ionization and recombination mechanisms. The results for the fluence threshold as a function of the pulse duration fall in good agreement with the experimental data reported in the literature.

Multi-coaxial relativistic backward wave oscillator that generates the beating wave of high power microwave pulse driven by a single accelerator and a single guiding magnet system is presented. Making use of the coaxial annular cathodes that can synchronously produce three annular beams at one shot, the average power of 5.88 GW consisting of two frequency components 9.0 and 9.7 GHz is obtained under the diode voltage and current 724 and 19.57 kA, corresponding to the conversion efficiency 41.5%. The conversion efficiency and the beating frequency are considerably stable with the diode voltage. The coaxial transmission supporter developed from our previous experimental research is employed to conductively connect the coaxial structure and to incoherently combine the microwave pulse of two frequencies with little reflection. It is found that the equipotential connection of the coaxial structure modifies the field distribution in the diode structure to facilitate the operation of the coaxial annular cathodes. The coaxial cathodes of different lengths are proved to be efficient at depressing the space charge effect in order to prevent the explosive emission of the inner cathode from being shielded by the outer annular beams.

We study the generation of resonant third harmonic laser radiation in a density non-uniform rippled plasma channel. An introduction of plasma channel non-uniformity strongly enhances the self-focusing and compression of main laser pulse at lower powers. In a deeper plasma channel, self-focusing is less sensitive to laser amplitude variation but increases compression. Plasma density ripple ‘nq’ leading to resonant third harmonic generation when kq = 4ω2p/3meω0cγ0, where ‘ω’p is electron plasma frequency, ‘ω0’ is laser frequency, and ‘γ0’ is the electron Lorentz factor. Third harmonic is produced through the beating of ponderomotive force induced second harmonic density oscillations and the oscillatory velocity of electrons at main laser frequency. The self-focusing and compression of the fundamental pulse periodically enhances the intensity of the third-harmonic pulse at lower powers of main laser. In a deeper plasma channel, the third harmonic power is less effective by self-focusing and the compression of main laser, and increase with main laser pulse power.

A scheme of terahertz radiation generation is proposed by beating of two extra-ordinary lasers having frequencies and wave numbers $\lpar {\rm \omega}_1\comma \; \vec k_1 \rpar $ and $\lpar {\rm \omega}_2\comma \; \vec k_2 \rpar $, respectively in a magnetized plasma. Terahertz wave is resonantly excited at frequency $\lpar {\rm \omega}_1 - {\rm \omega}_2 \rpar $ and wave number (k1 − k2 + q) with a wave number mismatch factor q which is introduced by the periodicity of plasma density ripples. In this process, the lasers exert a beat ponderomotive force on plasma electrons and impart them an oscillatory velocity with both transverse and longitudinal components in the presence of transverse static magnetic field. The oscillatory velocity couples with density ripples and produces a nonlinear current that resonantly excites the terahertz radiation. Effects of periodicity of density ripples and applied magnetic field are analyzed for strong THz radiation generation. The terahertz radiation generation efficiency is found to be directly proportional to the square of density ripple amplitude and rises with the magnetic field strength. With the optimization of these parameters, the efficiency ~10−3 is achieved in the present scheme. The frequency and power of generated THz radiation can be better tuned with the help of parameters like density ripple amplitude, periodicity and applied magnetic field strength in the present scheme.

Strong electromagnetic fields are radiated during the operation of the intense electron-beam accelerator (IEBA), and it is investigated in this paper. It is obtained that the triggered spark-gap switch and the main switch of the IEBA are the radiation resources. Here, the radiation of the triggered spark-gap switch is researched carefully. Firstly, circuit simulation is used to simulate the current going through the switch. The electric fields generated by the current and its frequency spectra are both calculated. The electric fields of the IEBA are measured by using a bi-cone antenna. When the primary capacitor of the IEBA is charged to 20 kV, the radiation field with amplitude of 753.4 V/m, dominant frequency of 20.8 MHz and high frequency 100 MHz is obtained. The experiment results show reasonable agreements with the analysis. It is believed that the gas-gap switch is a new method to achieve wide-band radiation.

Nuclear fusion in ultra-dense deuterium D(-1) was reported previously to be induced by 0.2 J pulses with 5 ns pulse length, ejecting particles with energies in the MeV range. The ns-resolved signal from D(-1) to two in-line collectors at up to 1 m distance can be observed directly on an oscilloscope, showing particles with energies in the range 1–20 MeV u−1. They are probably mainly protons and deuterons in the form of neutral ultra-dense hydrogen H(-1) fragments. Electrons and photons give only small contributions to the fast signal. The observed signal at several mA peak current corresponds to 1 × 1013 particles released per laser shot and to an energy release >4 J assuming isotropic formation and average particle energy of 3 MeV. This corresponds to an energy gain of 30 in the process. A movable slit close to the laser target gives lateral resolution of the signal generation, showing almost only fast particles from the point of laser impact and penetrating photons from the plasma outside the laser impact point. The observation of multi-MeV particles indicates nuclear fusion, either as a source or as a result.