Stimulated Raman backward scattering of an X-mode laser beam propagating in a homogenous plasma is studied in the presence of a transverse magnetic field. As the laser propagates in its X-mode in plasma, it decays into an upper hybrid wave and a down-shifted sideband wave. Beating the incident laser with the sideband exerts a nonlinear ponderomotive force acting on plasma electrons driving the excited upper hybrid wave. The incident wave then parametrically couples with the upper hybrid wave to drive the sideband. Using the fluid model and nonlinear current density, the nonlinear ponderomotive force is obtained to find the dispersion relation of the scattered sideband wave and the growth rate of the instability in the weakly relativistic regime. It is shown that the growth rate decreases and the cut-off points in the normalized wave number of the upper-hybrid wave become smaller by increasing the static magnetic field.

We developed a comprehensive model for simulating laser/target interaction in the presence of external axial magnetic fields. The model was integrated into the framework of the HEIGHTS-LPP computer simulation package and benchmarked with recent experimental results. The package was then used to study the angular distribution of extreme ultraviolet (EUV) photon output in plasmas produced in tin planar targets by a Nd:YAG laser. A moderate (0.5 T) permanent magnetic field does not affect EUV source evolution and output. More effective control of plasma plume expansion should be based on magnetic field gradients, that is, on the temporary varying magnetic fields as a magnetic pinch. Analysis of angular EUV output showed strong anisotropy of photon emissions. We found that the correct monitoring angle (i.e., at which the measured EUV flux corresponds to the averaged value after the correctly integrated angular distribution) does not depend on laser irradiance in the studied range and is equivalent to ~60°. We recommend arranging the EUV sensors accordingly in experiments with planar tin targets.

The wavelength scalings of soft X-ray and hot electron generation efficiencies were studied using 1·05, 0·53, 0·35 and 0·26 μm lasers. A coupling efficiency from absorbed laser energy to compressed fuel core of 4.5% was obtained by using the GEKKO XII green laser.

A detailed study of direct laser-driven electron acceleration in paraxial Laguerre–Gaussian modes corresponding to helical beams LG0m with azimuthal modes m = {1,2,3,4,5} is presented. Due to the difference between the ponderomotive force of the fundamental Gaussian beam LG00 and helical beams LG0m, we found that the optimal beam waist leading to the most energetic electrons at full width at half maximum is more than twice smaller for the latter and corresponds to a few wavelengths Δw0 = {6,11,19}λ0 for laser powers of P0 = {0.1, 1,10} PW. We also found that, for azimuthal modes m ≥ 3, the optimal waist should be smaller than Δw0 < 19λ0. Using these optimal values, we have observed that the average kinetic energy gain of electrons is about an order of magnitude larger in helical beams compared to the fundamental Gaussian beam. This average energy gain increases with the azimuthal index m leading to collimated electrons of a few 100 MeV energy in the direction of the laser propagation.

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.

The longitudinal dynamics of drift compression and pulse shaping for a space-charge-dominated heavy ion fusion beam is studied. A nonperiodic quadrupole lattice is designed for a beam undergoing drift compression, and an adiabatically matched solution is found for the transverse envelope equations in the nonperiodic lattice.

Inertial confinement fusion (ICF) requires high compression of fusion fuel to densities approaching 1000 times liquid density of deuterium-tritium (D–T) at central temperatures in excess of 5 keV. The goal of ICF is to achieve high gain (of the order of 100 or greater) in the laboratory. To meet this objective with minimum driver energy, a number of central issues must be addressed. Research in ICF with laser drivers has shown the importance of using short wavelength (λ < 0.5 µm). To achieve conditions for high gain at driver energies of a few megajoules or less, high intensities (>1014W/cm2) are required. The directdrive approach to ICF is more energy efficient than indirect drive if the stringent drive symmetry and hydrodynamic stability requirements can be met by a suitable laser irradiation scheme and target design. Experiments carried out at 351 nm on the 2-kJ, 24-beam OMEGA laser system at the Laboratory for Laser Energetics (LLE) at the University of Rochester, and future experiments to be performed on a 30-kJ upgrade of this laser, can resolve the remaining physics issues for direct drive: (1) energy coupling and transport scaling; (2) irradiation-uniformity requirements for high gain; (3) hydrodynamic stability constraints; and (4) hot-spot and main-fuel-layer physics. We review progress made on achieving uniform drive conditions with the OMEGA system and present results for direct-drive cryogenic-fuel-capsule and CD-shell, “surrogate” cryogenic-capsule implosion experiments that illustrate the constraints imposed by hydrodynamic instabilities and drive uniformity on the design of high-performance direct-drive targets. Target designs have been identified that will explore the ignition-scaling regime using the OMEGA Upgrade. Experiments on the OMEGA Upgrade will signal whether or not there is a high probability of achieving modest to high gain using direct drive on an upgrade of the NOVA facility.

The many years' of difficulty with laser fusion owing to the complexity of the interaction with the plasma has been clarified experimentally from the 10-ps pulsation of the interaction. The theoretical understanding of this pulsation is presented here from a hydrodynamic simulation of the interaction in which the generation of partial standing waves and the subsequent nonlinear-force-produced density ripple produces ideal Bragg reflection in the outermost part of the plasma. This all decays within about 10 ps hydrodynamically. The theory explains immediately why the 2-ps coherence at induced spatial incoherence avoids the pulsation and arrives at a smooth interaction similar to the random-phase plate or the use of broad-band laser spectrum irradiation.

Long-scale preformed plasmas are generated inside the cone by the pre-pulse of the heating laser in the cone-guided fast ignition scheme and it is found that coupling efficiency from the heating laser to fast electrons especially suitable for core heating is drastically reduced by the preformed plasmas. To mitigate this serious problem, an extremely thin film is suggested to cover the entrance of the cone. This method, however, introduces long rarefied plasmas around the entrance of the cone and the main pulse must propagate through these plasmas. Therefore, fast electron characteristics produced by the main pulse could be affected, and effects of long rarefied plasmas on fast electron generation are investigated. It is found that the electron beam intensity becomes larger than that without the rarefied plasma, but the energy coupling rate from the heating laser to the core decreases due to lack of appropriate electrons for core heating. To achieve less than 10% degradation of the core electron temperature, the thin film must be expanded by irradiation of the pre-pulse so that the length and the density of rarefied plasmas become less than 500 µm and one-tenth of the critical density. A thickness of the thin film can be determined by these criteria and the intensity of the pre-pulse.

A new method to focus a relativistic charged particle beam is suggested and studied. This idea is based on the use of the ponderomotive force which arises when a periodic electromagnetic field is created, as in the case of two crossed laser beams.

In this paper, the robustness of the dynamic instability mitigation mechanism is first examined, and then the instability mitigation phenomenon is demonstrated in a deuterium–tritium (DT) fuel target implosion by wobbling heavy-ion beams (HIBs). The results presented here show that the mechanism of the dynamic instability mitigation is rather robust against changes in the phase, the amplitude and the wavelength of the wobbling perturbation applied. In general instability would emerge from the perturbation of the physical quantity. Normally the perturbation phase is unknown, so that the instability growth rate is discussed. However, if the perturbation phase is known, the instability growth can be controlled by a superposition of perturbations imposed actively: if the perturbation is induced by, for example, a driving beam axis oscillation or wobbling, the perturbation phase could be controlled and the instability growth is mitigated by the superposition of the growing perturbations. In this paper, we realize the superposition of the perturbation by the wobbling HIBs’ illumination onto a DT fuel target in heavy-ion inertial fusion (HIF). Our numerical fluid implosion simulations present that the implosion non-uniformity is mitigated successfully by the wobbling HIBs illumination in HIF.

We have performed an initial assessment of the feasibility of producing heavy negative ion beams as drivers for an inertial confinement fusion reactor. Negative ion beams offer the potentially important advantages relative to positive ions that they will not draw electrons from surfaces and the target chamber plasma during acceleration, compression, and focusing, and they will not have a low energy tail. Intense negative ion beams could also be efficiently converted to atomically neutral beams by photodetachment prior to entering the target chamber. Depending on the target chamber pressure, this atomic beam will undergo ionization as it crosses the chamber, but at chamber pressures at least as high as 1.3 × 10−4 torr, there may still be significant improvements in the beam spot size on the target, due to the reduction in path-averaged self-field perveance. The halogens, with their large electron affinities, are the best negative ion candidates. Fluorine and chlorine are the easiest halogens to use for near-term source experiments, whereas bromine and iodine best meet present expectations of driver mass. With regard to ion sources and photodetachment neutralizers, this approach should be feasible with existing technology. Except for the target chamber, the vacuum requirements for accelerating and transporting high energy negative ions are essentially the same as for positive ions.

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.

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.

The high power microwave (HPM) synthesis method is presented in this paper for gigawatt level. The gigawatt level HPM could be synthesized from two separate input wave-guides according to the coupled-wave and orthogonal polarization theory. The synthesizer is used by two back to back circular wave-guides. The main channel is the circular wave-guide connected to the output port, which transmits horizontal polarization TE011 mode. The operating bandwidth is only limited by the barrier wave-length λc of circular wave-guide. The sub-channel transmits vertical polarization TE011 mode and the operating bandwidth is up to several hundred MHz. The energy of sub-channel could be coupled into main channel through continuous long-slit coupling structure. The synthesizer can be analyzed using numerical simulation method, which focuses on the power capability. The simulation results indicate that the transmission efficiency of the main channel is above 99%, the coupling efficiency of the sub-channel is above 96%, which also validates the reasonability of synthesizer design. At the same time, the prototype of synthesizer is designed and the HPM experiment system is established. The transmitting and coupling efficiency are both greater than 95% in cold test condition and they are also greater than 90% in gigawatt class test condition, the power capability of the synthesizer reaches about 1.2GW. The test results validate the feasibility of synthesizer for gigawatt class HPM.

Charge transfer of 4.3 MeV/u chlorine ions passing through a discharge plasma target is used as a probe to determine the plasma density and the ratio of impurities inside the plasma column. Charge-state distributions of 2 MeV/u chlorine ions passing through the plasma are then measured and compared to corresponding measurements in the cold gas. Stopping power measurements are also performed in both cases.

A feasibility of a new approach of laser fusion in plasma without implosion has been proposed and discussed using an intense laser. The cross-section of nuclear reaction is increased by the enhanced penetrability of nuclei through the Coulomb barrier. In this approach, an intense laser field of more than 10 EW was required to distort the Coulomb barrier to obtain enough penetrability. In the new improved model, a nuclear potential with meson attractive force is considered. Enhancement is observed for penetrability around EW or less power laser due to a nuclear potential. Energy gain even with Deuterium-Deuterium reaction can be obtained on this scheme in Deuterium plasma with energetic nucleon theoretically.

This paper presents a model to study the two prominent coexisting instabilities, stimulated Raman (SRS), and stimulated Brillouin scattering (SBS) in the presence of background axial magnetic field. In the context of laser-produced plasmas, this model is very useful in the situations where a self-generated axial magnetic field is present as well as where an external axial magnetic field is applied. Due to the interplay between both the scattering processes, the behavior of one scattering process is greatly modified in the presence of another coexisting scattering process. The impact of this coexisting phenomenon and axial magnetic field on the back reflectivity of scattered beams has been explored. It has been demonstrated that the back reflectivity gets modified significantly due to the coexistence of both the scattering processes (SRS and SBS) as well as due to the axial magnetic field. Results are also compared with the three-wave interaction case (isolated SRS or SBS case).

Experimental results on the determination of the color temperature in shock waves produced with lasers are presented. The method is based on imaging the target rear side in two different spectral windows and on using phased zone plates to produce high-quality shocks. The shock velocity is also measured, allowing, with the use of the equation of state, the real shock temperature to be deduced and compared with the measured color temperature.

With the aim to provide more economic systems for acceleration of electrons, the possibilities of laser accelerators are discussed. The driving of electrons in laser wave fields in a longitudinal direction is based on nonlinear forces, which in simplified form cause the ponderomotion. Since the formulation of the Sessler theorem it is evident that optical signals moving with the speed of light in vacuum may not transfer energy to charged particles. A slowing down—even for a very minor difference only—does result in acceleration. Since the beat-wave accelerator using plasma led to numerous problems, a scheme is proposed that works without such dielectric effects by the superposition of laser wave fields in vacuum. Then the reduction of the signal velocity is performed by a controlled time dependence of the frequency with a phase or direction of at least one of the laser beams in interference.