K. Okada

Soft x-ray emission from ω0, 2ω0, and 4ω0 laser-produced plasmas

The soft x‐ray spectra from Au plasmas in 0.1–1.6 keV range and x‐ray conversion efficiencies of Au (high Z) and Al (low Z) targets with three laser wavelengths of 0.26, 0.53, and 1.05 μm are measured. In addition, the intensity dependence of the x‐ray conversion efficiency at 0.26 μm is obtained for Au targets. For 0.26‐μm laser light, the conversion efficiency of Au targets can be as high as 80% at an intensity of 2.4×1013 W/cm2.

Development of a two-dimensional space-resolved high speed sampling camera

We developed a two-dimensional spatial resolved high-speed UV sampling camera (HISAC) to study energy transport in laser-produced plasmas. The HISAC is composed of an optical bundle of fibers coupled to a streak camera to obtain a two-dimensional spatial resolution with a temporal resolution of less than a few 10 ps. This HISAC system was demonstrated in the experiments on inferred uniformity measurements of laser-ablation pressures as well as on energy transport in ultraintense laser-plasma interactions. Two-dimensional shock heating was temporally resolved in a hemispherical target, resulting in the distribution of the ablation pressure as a function of laser incidence angle.

Studies of intense laser-plasma interactions for the fast ignitor concept at ILE, Osaka University

We experimentally studied laser-hole boring with 100 ps/1 TW laser light and ultra-intense laser-plasma interactions using a 0.5 ps/100 TW laser system. Investigation into laser-hole boring was made by measurements of backscattered light spectra, x-ray image and electron density profiles in under- and overdense regions. For the first time x-ray laser light was used to directly observe the channel in the overdense plasma. We further examined short pulse laser interactions as well as the generation of high-energy particles with 100 TW laser light. Neutrons by D-D fusion reaction in the interaction plasma were measured to diagnose ion acceleration. High-energy electrons (MeV) were measured by a x-ray method.

Energy confinement effect on soft x‐ray generation in 0.53‐μm laser‐heated cavity target

The first experimental investigation has been made of the time‐resolved soft x‐ray spectra from an empty high Z cavity target irradiated by 0.53‐μm wavelength laser. The soft x‐ray emission spectra are compared with those from planar targets with respect to temporal and spectral profiles and brightness temperature. Evidence on the radiation confinement and the plasma confinement in the cavity geometry is discussed in conjunction with the energy dissipation flow.

X‐ray and radioactive measurements in ICF research at ILE Osaka (invited)

The experimental basis and instrumentation for the Gekko XII laser fusion program developed at the Institute of Laser Engineering (ILE), Osaka University are presented. Many diagnostics are in use to evaluate the key parameters of target implosion in an extremely dense, short period region. In this paper, we describe the present status and future planning of these diagnostics, such as x‐ray, radioactive, and reaction particle measurement, because of their importance for fuel ρR determination.

Diode‐array coupled time‐resolved transmission grating spectrometer

A new type of sub‐keV x‐ray (0.1–1.2 keV) spectrometer diode array time‐resolved transmission grating spectrometer (DATTS) with a fast time response (≤270 ps) has been developed to measure transient soft x‐ray spectra from laser‐produced plasmas. DATTS, the combination of the transmission grating (TG) and fast biplanar diodes, offers many advantages over the conventional x‐ray diagnostics. Among the advantages are flexibility of the energy windows, fast time response, large dynamic range, and the capability of measuring absolute x‐ray energy. All of these measurements have not been possible with a single diagnostic. The performance of the DATTS was compared with the data taken with the x‐ray diode system, showing excellent agreement in the sub‐keV x‐ray energy range.

The analysis of the high‐resolution X‐ray spectra emitted from laser‐irradiated gold plasma

The fifteen lines in the x‐ray spectra between 3.1 and 4.5 keV emitted from a laser‐irradiated Au plasma were observed and identified with the relativistic Hartree‐Fock‐Slater (RHFS) calculation. It is shown that these lines are emitted from Au ions with the charge state of 26+∼31+ containing multiple inner‐shell vacancies: one in the 3p1/2 or 3p3/2 shell and one or two more in the M or N shell. It is seen that the energy separation of the corresponding lines between the two groups in the spectra observed, which is about 400 eV, comes from the difference of the binding energy between the 3P1/2 and 3p3/2 shell in Au ions due to the spin‐orbit interaction. A possible mechanism of creating multiple vacancies in inner‐shells of the ions in the plasma is also discussed.

Measurements of nonuniformly heated plasmas with a 2D space-resolved high-speed sampling camera

We developed a two-dimensional space resolved high speed UV sampling camera (2D-HISAC) to study energy transport in laser produced plasmas. The HISAC is composed of an optical bundle fiber coupled to a streak camera to get a two dimensional spatial resolution. The temporal resolution of a few 10 ps was obtained with the 2D-HISAC using multi mode fibers. This 2D- HISAC was applied to the experiments to investigate laser- produced plasmas. Nonlocal heating by high-density relativistic electrons in ultra-intense laser interactions was temporally and 2-D spatially resolved. Observed were filament of the relativistic electron heating in solid targets. The 2D- HISAC was also used in the experiment to estimate uniformity of laser-ablation pressures at an oblique incidence of laser light with a large spot area. Two-dimensional shock heating was temporally resolved, resulting in the distribution of the ablation pressure.

Experimental and simulation studies on soft‐x‐ray emission from 0.53 μm‐laser irradiated solid targets

The spectrum‐resolved radiant energy in 0.1∼1.6 keV range from various plane targets irradiated by a 0.53μm laser at 0.1∼1.0 nsec pulse through a f/1.6 lens with a nominal incidence angle of 54° were obtained. Atomic number dependences of the spectrum and x‐ray conversion efficiency are described. The conversion efficiency increased as laser pulse duration.These x‐ray emission spectra are investigated by the 1‐D hydrodynamic Lagrangian code HIMICO and 2‐D particle in‐cell code IZANAMI, both of which are omission thermodynamic equilibrium (non‐LTE) average ion model and multi‐group radiation transport. The physical meaning of the experimentally obtained spectrum is clarified and the total emitted density proves to have maximum at certain combination of density and temperature.