Vacuum UV fluorescences from ionic alkali-halide and rare-gas alkali excimers were observed by e-beam excitation. The two diffuse emissions centered at 185 and 154 nm from the CsF vapor were attributed to the bound-free B2Σ+ → X2Σ+ and D2Π → X2Σ+ bands of (CsF)+, respectively. And, the 164- and 133-nm diffuse bands from gas the mixture of Xe or Kr with hot vapor of Rb were assigned to the emissions of (XeRb)+ and (KrRb)+ ionic excimers. In addition, amplified spontaneous emissions of the Na2 violet, K2 yellow, and Rb2 orange diffuse bands were observed from e-beam-excited alkali vapor with argon buffer gas. The continuum spectra centered at 436, 574, and 604 nm were attributed to triplet-triplet Na2, K2, and Rb2 23Πg → 13Σu+ bound-free transitions, respectively. The dissociative recombination of Na3+, K3+, or Rb3+ is discussed as an efficient formation process of the upper states in e-beam pumping.

Bandwidth effects on laser-plasma interaction were investigated with a ¼-μm laser. Planar targets were irradiated with a 1- to 40-cm–1 bandwidth laser at I = 1 × 1013–4 × 1015 W/cm2. Above 3 × 1013 W/cm2, stimulated Brillouin scattering (SBS) was observed with 1 cm–1 light. This process was strongly reduced with a 40-cm–1 light. Evolution of the convective SBS was studied with 1-D fluid simulation code. Bandwidth effect on the SBS growth was discussed to compare the theoretical prediction and experimental results. The scalelength dependence exists for the SBS reduction with a broadband laser.

An atomic model of the laser-produced Al plasma has been developed and used to analyze excitation processes of recombination pumping soft X-ray lasers. A soft X-ray gain for H-like Balmer-α line and He-like 3d-2p transition in short-pulse intense KrF laser (IL = 1014–1015 W/cm2, T = 10–100 ps)-produced Al plasmas are calculated for various laser temporal pulse shapes to find the condition for efficient production of population inversion. Results from different models are compared and requirements for the atomic model for X-ray laser design are discussed.

A couple of advanced techniques of high-efficiency pulse compression for KrF lasers have been developed in our laboratory. As is well known, electron beam-excited KrF lasers have a capability of large output energy of 100- to 1,000-ns pulse duration. However, high-efficiency operation under a short-pulse input is difficult to achieve because of the reaction kinetics and short lifetime of KrF excimers. For developing ultrahigh peak-power lasers, the efficient pulse compression of ns to ps and ps to fs pulses is essential. We demonstrated advanced techniques, Raman pulse compression in a transient Raman regime, and pulse compression in saturated amplifiers with saturable absorbers.

A γ-ray laser would stimulate the emission of radiation of wavelengths below 1 Å from excited states of nuclei. However, the anticipation of a need for high pump powers tended to discourage early research and the difficulties in demonstrating a device were first assumed to be insurmountable. Over the past decade, advances in pulsed-power technology have changed these perceptions and studies have built a strong momentum. A nuclear analog of the ruby laser has been proposed and many of the component steps for pumping the nuclei have been demonstrated experimentally. A quantitative model based upon the new data and concepts has shown the γ-ray laser to be feasible if some real isotope has its properties sufficiently close to the ideals modeled. The greatest positive impact upon feasibility has come from the discovery of giant resonances for pumping nuclei that greatly reduce the levels of pump power needed.

Measurements on nonlinear processes caused by multiphoton absorption and electron avalanche in optical thin films have been carried out using KrF lasers of 20-ns and 1.7-ps pulse duration. Multiphoton absorption of the order of 10-7 J was detected by a photoacoustic signal, and the nonlinear growth of photo-induced current due to the electron avalanche was analyzed dynamically. The correlation between damage threshold and carrier lifetime was investigated for oxide and fluoride coatings.

Diamond film technology has advanced to the point where electronic devices are now becoming feasible. In addition, diamond has outstanding mechanical properties. The energy given off in fusion reactions may be converted to a narrow-band light spectrum that can be absorbed by wide-bandgap photovoltaic cells to directly produce electricity. The properties of possible wide-bandgap photovoltaic cells are examined for the purpose of fusion energy conversion.

We have developed a new opacity modeling of partially ionized high-Z plasma to solve radiation transport in fluid codes. The average atom model is used to describe the electronic state of the plasma. The electronic state of the plasma is determined by solving the collisional radiative equilibrium model. We have taken into account the electron energy level splitting owing to the difference in the azimuthal quantum number. To model the line groups made of the same electronic transitions from ions indifferent charge states, we used a statistical method and calculated the distribution of the charge states from the averaged electron population in each bound state. By using the new opacity model, we can well reproduce the X-ray spectra from the plasmas. It is found that the Δn = 0 transition can explain the peaked spectra near hv = 300 eV and l–splitted emission of the n = 5–4 transition can explain the flat spectra in the region of hv = 400–800 eV seen in the experiments.

In the last few years, systematic studies on radiation hydrodynamics in the X-ray confining cavity and a fuel capsule have attained remarkable progress. This makes it possible to analyze quantitatively the energy transfer processes from laser to the fusion capsule and find uniform irradiation conditions of the fusion capsule driven by thermal X rays. As a result, reproducible and stable implosions were achieved. Throughout implosion experiments with the Gekko XII blue laser system (351 nm, kJ, 0.8 ns), good agreement of implosion has been obtained between the experiment and numerical simulations, assuming perfectly spherical symmetry, up to a radial convergence ratio of 15. Described are particularly the issues of (1) energy transfer processes from laser to a fuel capsule and conditions for uniform irradiation, (2) properties of the X-ray propagation through aluminum heated by X-ray radiation, and (3) dependence of the convergence ratio of Ri/Rf (where Ri and Rf are the initial and final radii) of the capsule on the initial fill pressure of D–T gas and its influence on the core parameters and fusion products to evaluate implosion sphericity.

Theoretical and numerical analyses are given for direct- and indirect-driven reactor targets, from which 3-GJ fusion output energy is released. For a reactor target, which has a large radius and long implosion time, supersonic flow of imploding D–T fuel in the converging nozzle (in sphere) is important for adiabatic compression of fuel. For a direct-driven target, pellet gain depends much upon the region where the beam deposits its energy. Phase mixing is also important to increase the absorption rate of irradiating laser light. When a strong light with a phase is concentrated on a small region of target surface, collective electron motion on the target surface radiates electromagnetic waves, which reduce the electron motion. When beam irradiation on the target is not uniform, an indirect-driven target must be used because a direct-driven target is weak for nonuniform irradiation. For an indirect-driven reactor target, which requires a long implosion time, expansion of radiator and absorber layers causes the decrease in radiation temperature. There is the optimum target structure with respect to the aspect ratio (radiation gap distance).

We describe the properties of the exploding foil neon-like germanium soft X-ray lasers having wavelengths of 19–28 nm and gain length product of more than 10. The measured X-ray intensity of lasing lines from an exploding foil target has been explained with the results of the plasma hydrodynamic code from the viewpoint of suitable condition of electron density and temperature for creating population inversion.

Atomic modeling of high-Z partially ionized plasma is essential for simulating radiation hydrodynamics of laser-produced plasma. A collisional-radiative model based upon an average atom model is used to calculate plasma opacity and emissivity. Because line radiations are most dominant in such plasma, the detail configuration accounting (DCA) for electronic state is required. We propose a statistical method to carry out the DCA with the use of the average population of bound electrons. Further modeling of line group made of the same transition from ions in different change states is discussed by considering the detail structure (hierarchy) of the line group.

The present review is of the experimental investigations on laser-plasma interaction being carried out in past years at IAE. Experiments were conducted on the “Mishen” facility. The laser system of Mishen consists of two channels with output beam parameters as follows: the main beam—output energy 100–200 J (λ = 1.054 μm) in 3-ns pulse, divergence ∼2 × 10-4 rad, contrast ratio ∼106, power density at the target surface ∼1013–1014 W/cm2; the diagnostic beam–output energy 10–20 J (λ = 1.054 μm) and 5–10 J (λ = 0.53 μm) in 0.3-ns pulse, divergence ∼10-4 rad, power density 1013 - 1014 W/cm2. Our aim in this experiment is to study the different aspects of the ICF processes in flat geometry. The main issues of our studies are hydrodynamic aspects, including acceleration efficiency, high-velocity impact in cascade targets, hydrostability, and X-ray physicsconversion efficiency, heat transfer, and X-ray-driven targets.

A parameter study of implosion, burn, and gain of D–T ignitor/D3He fuel pellets is presented for a D3He inertial confinement fusion reactor. It is found from burn simulation that attaining a quasi-isobaric state with a temperature of 4 keV and pR value of 2.5 g/cm2 for the D–T ignitor and 0.8 keV and 9.5 g/cm2 for the D3He main fuel would suffice to obtain a pellet gain of ∼40–50 required for the D3He reactor. With 30-MJ laser irradiation and the coupling efficiency of 10%, the density of the target is assumed to be imploded to 5,000 times the liquid density. However, in the implosion simulation to realize the above configuration it is found that after void closure the central hot D–T ignitor region is ignited, while the bulk of the D3He main fuel is still imploding with high velocities. This preignition of the D–T ignitor leads to a low compression of the main fuel and prevents the D–T/D3He pellet from obtaining the required pellet gain. The pellet gain obtained is only ∼3.

A spherical scheme is introduced for indirect drive, having as reference the application to inertial fusion energy. The concept uses the transmission of a radiation wave through a high-Z, low-density spherical radiator surrounding the capsule to be imploded. The low conversion efficiency to in-cavity radiation is somewhat compensated by using modest radiator-to-capsule gaps, the smoothing effects on the longest dominant nonuniformity wavelengths due to this separation being of the order of 10. Another factor of the order of 100–500 is related to the consequent use of a large number of beams. From the application of the loop condition relative to power plants, it is found that this approacrTbecomes practicable if drivers with efficiencies greater than 10% are assumed and if the fraction of circulating power is allowed to be 0.5, this implying a unitary cost of energy about a factor 1.5 greater than that associated to the more conventional fraction of 0.25.

The effect of surface finish on the evolution of ICF targets has been studied. As driver for the implosion has been used a temporally profiled thermal radiation pulse (25 ns in duration) as that expected for the case of a high-gain (cm sized) capsule enclosed in a cavity. 2-D numerical simulations have been performed on a planar analog to investigate the stability to perturbations having wavelengths in the interval 2.5–40 μm and initial amplitudes of 0.03 and 1 μm. The simulations show perturbation growing due to hydrodynamic instabilities. The distortion of the ablation front reaches such a level to strongly enhance the ablation rate because of the increase in the exposed area and in the power absorbed per unit area. Two different phenomena have been identified: one, preferentially occurring for small wavelengths, in which the motion behind the first shock wave becomes turbulent and the symmetry is lost; the other, appearing for long wavelengths, in which a thermal radiation wave penetrates into the fuel. This preheating phenomenon is related to the formation of holes in the thermal shield due to the enhanced ablation and to matter transverse piling up. All these effects may render useless the 1-D-based design of a high-gain capsule exposed to a given thermal bath. However, it has been also observed that, for the longer wavelength (40 μm) and a smaller initial perturbation amplitude (e.g. 0.01 μm), during the temperature pulse the preheating does not occur and most of the fuel can be accelerated as a whole to speeds of the order of 2 × 107 cm/s or greater without a substantial onset of transverse velocities, even in the presence of the ablator piling up.

The ASTERIX iodine laser delivers after frequency tripling to λ = 0.44-μm laser pulses with energies up to 500 J at a pulse duration of 300 ps for target experiments. Experimental investigations of radiative transfer in low- and high-Z materials are reported.

Smoothing of laser-plasma interaction by ISI, RPP, SSD, etc. was mainly directed to overcome lateral nonuniformity of irradiation. While these problems are in no way less important, we derived numerically the model of the Laue rippling and hydrorelaxation model for explanation of the measured temporal pulsation in the 10- to 40-ps range and how the smoothing schemes suppress these pulsations. The partial standing wave fields of the normally coherent laser-irradiated plasma corona is then suppressed by smoothing and conclusion for tests for this model, e.g., by the “question mark experiment” is given. The result provides a physics solution of the laser interaction problem for direct-drive inertial fusion energy

The production of laser-induced plasmas, which ordinarily occurs only at high laser powers, occurs at far lower laser powers using resonant and collisional phenomena in alkali metal vapors. For example, 10 torr of Na vapor can be significantly ionized to produce a bright white ball with as little as 1 mW of focused cw dye laser power (e.g., at the 3p → 4d transitions at 568.3 and 568.8 nm). We first discuss the alkali metal vapors in general (1) and their important ionization processes (2), especially those occurring at photon energies below the atomic ionization energy. We then discuss the striking wavelength dependences of ionization. For example, cw laser irradiation of these vapors gives completely different behavior at slightly different wavelengths (Stwalley 1990): 10 torr of sodium vapor at 568.2 nm, where the vapor shows only molecular fluorescence and is sufficiently quiescent that highly stable, narrow-band, optically pumped lasers based upon the fluorescent transitions can easily be constructed (Bahns 1983; Bahns et al. 1983; Verma et al. 1983); and, at 568.3 nm, where a “quasiresonant” laser-induced plasma forms and strong emission (including violet excimer bands, a potential laser) is produced (Koch & Collins 1979; Bahns et al. 1989a; Bahns et al. 1989b). We will analyze and reinterpret the classic subthreshold ionization studies (3) and survey recent laser-induced plasma studies (4). We also discuss dissociative recombination and its selectivity for producing strong excimer emission (5) the predicted electronic spectra of the alkali metal molecular ions (6).