Toshiatsu Oda

Photoluminescence and decay kinetics of indirect free excitons in diamonds under the near-resonant laser excitation

Indirect edge emission spectra were observed in types-IIa, -Ib and -Ia diamond crystals, excited with a pulsed laser just above the indirect band gap. In the type-IIa the lifetime of indirect free excitons was also measured, whose temperature dependence was successfully analyzed using a rate-equation model including a bound exciton formation. Weak luminescence observed in other diamonds was qualitatively explained by a strong nonradiative process related to deep impurities.

The fundamental absorption edge and electronic structure in Sb2S3

Absorption spectra near the fundamental absorption edge are studied in Sb 2 S 3 . Step-like structures characteristics to the indirect allowed exciton transition are found. The energy of the indirect exciton is determined to be 1.749 eV at 27 K. The band structure is calculated by the pseudopotential method and compared with the experimental results. It is shown that the top of the valence band and the bottom of the conduction band lie around \(\varGamma\) and X points, respectively, and that the smallest direct gap is located at around the X point.

Neutral beam probe spectroscopy for edge plasma diagnostics

Abstract A new method of neutral beam probing combined with spectroscopic and laser-induced fluorescence techniques has been developed in order to measure electron density profiles of the boundary layer plasma. The feasibility of this method using two kinds of Li0-beams of thermal and several eV energies is demonstrated with an ECR test plasma. The obtained electron density profile agrees well in form with that measured by a Langmuir probe. The absolute values are about one third of those obtained by the Langmuir probe but agree with results from a microwave interferometer. The applicability of this method to the TEXTOR tokamak is discussed.

Measurement of Intensity and Polarization of HeI Forbidden Lines for Diagnostics of Electric Field in a Plasma

Intensity of the HeI 491.1 nm (41P→21P) forbidden line due to the Stark effect, with respect to the HeI 396.5 nm (41P→21S) allowed line, was measured to obtain radial distribution of the electric field in a hollow cathode discharge. The polarization of the lines was also observed to show a capability of detecting the field direction in plasmas. The measured electric field distribution satisfactorily yielded the cathode fall potential, and the polarization was well in agreement with the theoretical prediction. The experiment demonstrated that field strength as low as 250 V/cm could be accurately measured using the present line pair.

Measurement of electron density with combined technique of laser blowoff and laser‐induced fluorescence

A method combining the techniques of laser blowoff and laser‐induced fluorescence has been developed in order to measure the local electron density below 1013 cm−3. Characteristics of a Li0 beam produced by laser blowoff of a thin Li film are investigated using beam–plasma interactions and laser‐induced fluorescence. Such a beam has a near‐Maxwellian velocity distribution with a temperature around 4.5 eV and a density of the order of 1010 cm−3 at a distance of about 1 m from the film target. The feasibility of measuring electron density with this Li0 beam and a dye laser is demonstrated with an ECR plasma.

Measurement of Density Distribution of Metastable He Atom in a Plane-Parallel Hollow Cathode He Plasma

The density distribution of the singlet metastable He atom in a plane-parallel hollow cathode plasma was measured by the laser-induced fluorescence (LIF) method along with the absorption method, where the alignment of the atom excited by the linearly polarized laser light, the self-absorption of the fluorescence light by the 21S atom in the plasma and the effect of the atomic collisions as well as the electronic ones were considered. The alignment is demonstrated to play important roles in the determination of distribution of the metastable He atom density in a glow plasma with the electron density of (2.0±0.5)×1011 cm-3, temperature of 1.3 eV and gas pressure of 0.7 Torr. Applicability of a simplified model used for the analysis of the present experiment was also discussed.

Spectrum Identification of Heliotron E Plasma Impurities by a Time-Resolving Grazing Incidence Spectrometer with a Multichannel Detector

A flat field vacuum ultraviolet spectrometer with a multichannel detector has been constructed to identify impurity spectral lines over the wavelength from 100 A to 400 A in Heliotron E plasmas. The resonance emissions of iron, titanium, chromium and nickel ions were identified in heated plasma by ion cyclotron range of frequency (ICRF) pulse. The lines were enhanced as ICRF power increased. The possible sources of these impurities are discussed briefly.

Forbidden Transitions in Helium and Lithium Due to Fluctuating Electric Fields for Plasma Diagnostics

Suitable line pairs of the forbidden and the allowed lines in the helium and lithium lines are proposed for measurement of fluctuating electric fields with low frequency in plasmas. The forbidden line intensity with respect to the allowed one is calculated as a function of the electric field strength by using the quasi-static theory. For the electric field of more than several kV/cm, the forbidden line intensities are detectable. The fluctuating electric fields are observed in a small linear theta pinch plasma by using the proposed method. Two line pairs of HeI (HeI 491.1 nm/HeI 396.5 nm and HeI 663.2 nm/HeI 501.6 nm) are used for the observation. Both results obtained almost agree within the experimental uncertainty. The peak intensity of the observed fluctuating field is 5 kV/cm in the early phase of the implosion of the plasma.

POLARIZED LASER-INDUCED FLUORESCENCE SPECTROSCOPY FOR MEASURING ELECTRIC FIELD DISTRIBUTION IN PLASMAS

A sensitive and reliable spectroscopic system measuring simultaneously polarized components of time-resolved laser induced fluorescence (LIF) is being developed for determining electric field distribution in transient plasmas under strong magnetic field. In this LIF method which we have already proposed, only the polarization has to be observed of a LIF due to a laser-excited forbidden transition caused by both the Stark effect and the electric quadrupole (QDP) moment in the plasmas. Temporal evolution of both the polarized LIF components is simultaneously observed using a Glan–Thompson polarizing beamsplitter with extinction ratio of 10−5 and a pair of photomultipliers with fast rise time. The polarization components observed in a hollow cathode discharge He plasma under magnetic field of 2.5 kG were successfully analyzed and electric field distribution in the sheath region was accurately estimated using this procedure. It was also shown that electric field strength was determined from the intensity rati...

Spectroscopic method to directly measure electric field distribution in tokamak plasma edge

A supersonic helium beam source operated in pulse mode was constructed for direct measurement of electric field distribution in the tokamak plasma edge region with the aid of laser-induced fluorescence (LIF) technique. In this technique only the polarization has to be observed of a LIF resulting from a laser-excited forbidden transition due to the Stark effect and the electric quadrupole to determine the electric field strength. No calibration is needed of the absolute intensity of LIF and tunable laser used. The helium atom beam density was obtained (about 1020 He atoms cm−2 s−1) at a distance of 7 cm from the pulsed nozzle. A model-type experiment to make clear the influence of a magnetic field on the LIF is reported. Design study was also made to install the supersonic beam and spectroscopic measurement system on a medium size tokamak.