Search

We developed a longitudinally excited N

We developed a longitudinally excited CO2 laser with a tandem discharge tube. The tandem scheme was constituted of two 30-cm long discharge tubes connected with an intermediate electrode. Two parts, each consisting of a charged capacitance and a 30-cm long discharge tube, were electrically connected in parallel and switched by a spark gap. The tandem scheme produced a short laser pulse like that of a TEA-CO2 laser with a charging voltage of -24.8 kV, which was smaller than the -40.0 kV charging voltage of our previous CO2 laser. At a gas pressure of 3.8 kPa, the spike pulse width was 145 ns, the pulse tail length was 58.8 μs, the output energy was 52.0 mJ, and the spike pulse energy was 2.4 mJ. We also investigated the dependence of the laser pulse and the discharge voltage on gas pressure.

We developed a longitudinally excited N2 laser (337 nm) with low beam divergence without collimator lenses. The N2 laser consisted of a 30 cm long Pyrex glass tube with an inner diameter of 2.5 mm, a normal stable resonator formed by flat mirrors, and a simple, novel driver circuit. At a N2 gas pressure of 0.4 kPa and a repetition rate of 40 Hz, the N2 laser produced a circular beam with an output energy of 2.6 μJ and a low full-angle beam divergence of 0.29 mrad due to the uniform discharge formed by the longitudinal excitation scheme, the long cavity with the small aperture, and the low-input energy oscillation.

X-ray laser research performed at the Institute of Laser Engineering, Osaka University, are reported. The performance of X-ray lasing pumped by a train of 100-ps pulses at the wavelength range between 14 and 4 nm was investigated. The irradiance on the target was up to /spl sim/3/spl times/10/sup 14/ W/spl middot/cm. The double target amplification was successfully demonstrated with two beam irradiation for Ag, Nd, Yb, Hf, and Ta lasing. Moreover double-pass amplification at 7.9 nm has been demonstrated. The higher atomic number X-ray lasing strongly depends upon the transverse distance of the two targets.

We have carried out the design studies of KOYO-Fast laser fusion power plant, using fast ignition cone targets, DPSSL lasers, and LiPb liquid wall chambers. Using fast ignition targets, we could design a middle sized 300 MWe reactor module, with 200 MJ fusion pulse energy and 4 Hz rep-rates, and 1200MWe modular power plants with 4 reactor modules and a 16 Hz laser driver. The liquid wall chambers with free surface cascade flows are proposed for cooling surface quickly enough to a 4 Hz pulse operation. We examined the potential ofYb-YAG ceramic lasers operated at 150 ∼ 225 K for both implosion and heating laser systems required for a 16-Hz repetition and 8 % total efficiency.

The progress of laser fusion research at the Institute of Laser Engineering, Osaka University is reviewed. They are physics investigations on target implosion, development of laser technologies, and R&D for laser fusion power plants. The long term strategy of Inertial Fusion Energy (IFE) development has been reexamined taking into account the recent progress of implosion physics and reactor related technologies. 15 refs., 12 figs.

An oxidizing agent is needed for silicone oil to be photo-oxidized with Xe2 excimer-lamp. However, the lamp light did not reach the silicone oil on the surface of the substrate satisfactorily, and the photo-oxidation reaction of the silicone oil layer was hard to take place properly. In order to find the appropriate conditions for supplying the proper amount of an oxidizing agent to silicone oil, the vacuum ultraviolet light that passed the silicone oil layer was made fluoresce in the phosphor to monitor the progress of the photo-oxidation reaction. As the vitrification by photo-oxidation reaction of the silicone oil layer improved, the fluorescence intensity of the phosphor increased. While monitoring the change of the fluorescence intensity, the supply of the oxidizing agent and the irradiation time of the vacuum ultraviolet light were controlled; as a result, the new method to efficiently form a transparent, photo-oxidized thin film has been established.

Recent progresses have shown the possibility of the ignition at the inertial confinement fusion experiment in the direct drive scheme. The imploded core of burning temperature, 5-10 keV, and the imploded core of 600 times liquid fuel density have been realized separately at Osaka University by using less than 10 kJ energy with 3% energy balance. While the high temperature core could only generate the neutrons up to a level of one tenth to a hundredth predicted by the one dimensional calculation at a compression ratio over ten. The required energy and its uniformity on the spherical pellet are trade-off for given the resultant of the implosion. The key issue in the inertial fusion energy development is now the realization of the ignition which is one of milestones to the energy production by the ICF. In this report, the basic design of the Gekko XII upgrade system named {open_quotes}Kongoh{close_quotes} at Osaka University is discussed with system components being developed now to improve the uniformity, controllability, and the energy delivering in the existing building of Gekko XII.

Thermonuclear ignition and subsequent burn are key physics for achieving laser fusion. In fast ignition, a highly compressed fusion fuel generated with multiple ns‐laser beams is rapidly heated with a large energy, ps‐laser pulse in prior to core disassembly. This scheme has a high potential to achieve ignition and burn since driver energy required for high fusion gain is predicted to be about one tenth of that needed for the central ignition scheme. In Japan, Fast Ignition Realization Experiment (FIREX) project has been started to clarify the physics of energy transport and deposition in the core plasma and to demonstrate fuel temperature of above 5 keV. After the success, FIREX‐I will be followed by the second phase of the project (FIREX‐II) to demonstrate ignition and burn. LFEX laser, designed to deliver a laser pulse of 10 kJ in 10 ps, are operational and the first phase of FIREX experiments has been stated. A new target is proposed to attain dense compression of fuel and improve laser‐core coupling efficiency by adopting double‐cone structure, a low‐density inner liner, low‐Z outer coating, and Br‐doped fuel shell. In this paper, present status and near term prospects of the FIREX‐I project will be reported together with activities on target designing, laser development, and plasma diagnostics.

The functional damage threshold (FDT) and conventional damage threshold (1-on-1) were examined for silica plate and two mirrors. The FDTs were also compared with R-on-1 thresholds, and both were in good agreement.

Rapid heating of a compressed fusion fuel by a short-duration laser pulse is a promising route to generating energy by nuclear fusion, and has been demonstrated on an experimental scale using a novel fast-ignitor geometry. Here we describe a refinement of this system in which a much more powerful, pulsed petawatt (10(15) watts) laser creates a fast-heated core plasma that is scalable to full-scale ignition, significantly increasing the number of fusion events while still maintaining high heating efficiency at these substantially higher laser energies. Our findings bring us a step closer to realizing the production of relatively inexpensive, full-scale fast-ignition laser facilities.

Lasing in various Ni-like ions at the wavelengths below 10 nm has been demonstrated [1,2]. Achieving saturated amplification with the Ni-like ions is considered to be difficult compared with the Ne-like ions at longer wavelengths, since the plasma parameter space for amplification is narrower with the Ni-like ions [3]. In this paper we report generation of very intense x-ray laser radiation at 8 nm in Ni-like Nd ions by amplification using double curved targets. This result shows that it will be possible to achieve saturated amplification in Ni-like ions by properly optimizing the irradiation conditions and target configurations.

Laser fusion is one of the most feasible approach in the fusion energy development. The goal of inertial fusion energy (IFE) development is to prove that fusion energy can be available to society as electric power source. Recent progress of laser fusion research and development enable us to examine technical and economical feasibility, and to plan the realistic strategy and program to the commercial power plant. The most important key issue for IFE is driver technologies. The development of the laser fusion driver may establish new industrial technologies based on the photon processes and is attracting attentions in wide industrial fields.Laser fusion is one of the most feasible approach in the fusion energy development. The goal of inertial fusion energy (IFE) development is to prove that fusion energy can be available to society as electric power source. Recent progress of laser fusion research and development enable us to examine technical and economical feasibility, and to plan the realistic strategy and program to the commercial power plant. The most important key issue for IFE is driver technologies. The development of the laser fusion driver may establish new industrial technologies based on the photon processes and is attracting attentions in wide industrial fields.

The two‐dimensional multi‐lens array of 350 mm in diameter with spherical element lenses to improve the irradiation uniformity is tested. Circular and hexagonal shape of element lens aperture have been examined. The circular aperture is chosen for eliminating azimuthally asymmetric intensity distribution. An approximate flat‐top intensity distribution is obtained with a lens array of 37 lenses. Beam patterns of coherent laser and partially coherent light (PCL) with the random phase plate (RPP) and the multi‐lens array (MLA) are measured at GEKKO XII glass laser system. Spherical mode structure of the irradiation nonuniformity of MLA on a spherical target is calculated from the measured beam patterns.

New GEKKO XII glass laser system has been improved by the precision project of the power balance with the partially coherent light (PCL) source to attain a few percent nonuniformity of the spherical irradiation of the pellet in a directly driven implosion scheme. The technical issues for balancing the twelve beam intensity and the theoretical treatment of the PCL optics are summarized.

The suppression of the speckles produced by a random phase plate is described. The contrast of the speckles is reduced at 30% by a polarization rotator array (PRA). The PRA is made from the liquid crystal (LCPRA). The aperture of the LCPRA is 80 mmφ and element size is about 2 mm square. The number of the element is about 1200. The damage threshold is about 11 J/cm2 (532 nm, 0.9 ns).

Physics investigation of implosion and relevant technology development at ILE, Osaka University are reviewed. Since the completion of GEKKO XII and its first shot in December 1983, high temperature implosion demonstrated 10 keV and 1013 neutron/shot, and high density compressions more than 600 times over initial solid density were achieve. Fundamental physic relevant to the implosion process such as thermal smoothing, R‐T instability, pre‐heat effect and fuel pusher mixing are investigated. Precision GEKKO XII project and fuel capsule development for cryogenic target with preliminary implosion test are going on.Conceptual design of commercial reactor KOYO has been proceeded. LD pump solid state laser is designed showing the feasibility for reactor driver. Fuel pellet design and gain scaling reveals the key physics issues to be investigated in the next step of program. Applications of ICF technologies performed at ILE with outer collaborators are also shown.

A mount system for segmented meter-sized gratings has been developed, which has a high precision grating support mechanism and drive mechanism to minimize both deformation of the optical surfaces and misalignments in setting a segmented grating for obtaining sufficient performance of the pulse compressor. From analytical calculations, deformation of the grating surface is less than 1/20 lambda RMS and the estimated drive resolution for piston and tilt drive of the segmented grating is 1/20 lambda, which are both compliant with the requirements for the rear-end subsystem of FIREX-1.

The effects of the discharge parameters on laser performance have been studied experimentally and theoretically in a TEA N 2 laser driven by a Blumlein circuit. Unmeasurable circuit constants such as the resistance and the inductance of discharge plasma are derived by circuit analysis, and the dependence of the laser output energy on the discharge parameters is discussed in terms of the theoretical quantity which denotes the figure of merit of the discharge circuit.

The pumping mechanism of the N2+ ion laser excited by a rapid discharge in a He-N2 mixture has been examined experimentally. The dependence of the laser output on temperature and pressure of gas mixture and the time history of the gain of active medium are measured, leading to the conclusion that the electron impact excitation of the Na2+ (B2 Sigma u+) state is the dominant excitation process in the case of high speed excitation.

A new design of TEA N2 laser for producing high-density excitation by combined use of a modified Marx generator and a pair of high-speed capacitors is described. Laser output of 1.5 MW (1.2 mJ, 0.8 ns) is obtained from a 12 cm long uniform discharge at a charging voltage of 25 kV.

The recent rapid extension of inertial confinement fusion requires more sophisticated beam control, such as uniform irradiation and dynamic wavefront control for strong coupling during the implosion process. Phase randomizing has been attained in a twelve-beam, 35-cm aperture Gekko XII glass laser system using a random phase plate (RPP) with 2-mm square, λ/2 optical thickness coated segments located randomly on the glass substrate. The RPP generates many beamlets focused on the spherical target in the far field region where the sharp speckle pattern is smoothed out by the lateral and longitudinal thermal energy transfer effects in plasma. The RPP has a sufficiently higher damage threshold of 5 J/cm2 at 0.53 μm of 1-ns pulse duration.

Observation of gain in Balmer-α (H α) transitions in Na XI, Mg XII and Al XIII is reported. Planer stripe targets were irradiated with intense, line-focused 351 nm laser light. Time evolution of the gain in Na XI Hα is also presented.