Akira Yonesu

Anisotropic Optical Spectra of α-Al2O3 Single Crystals in the Vacuum Ultraviolet Region. I. Spectra of Absorption Tail and Reflectivity

The spectra of absorption constant in the intrinsic absorption tail region of α-Al 2 O 3 single crystals at 297 K, 78 K and 10 K are presented for the electric fields of incident light parallel to the optic axis of α-Al 2 O 3 and normal to it. The absorption tail spectra obey the Urbach rule for each polarization, indicating that absorptions in the tail region result from the direct interband transitions. The optical anisotropy characterized by the Urbach rule is interpreted in terms of the bond length O 2- –Al 3+ participating in the absorption processes. The spectra of reflectivity are presented in the range of photon energies from 4.6 eV to 120 eV for electric fields parallel to and normal to it. Spectral positions of main maxima in the reflectivity spectra for each polarization are determined at 297 K and 10 K; the spin-orbit doublet of the Al 3+ L 2,3 is revealed at 78.67 eV and 79.12 eV for the first time in the case of α-Al 2 O 3 .

Anisotropic Optical Spectra of α-Al 2O 3 Single Crystals in the Vacuum Ultraviolet Region. II. Spectra of Optical Constants

The spectra of optical constants of α-Al 2 O 3 single crystals at 297 K and 10 K are presented over the range of photon energies from 6 eV to 120 eV for the electric fields of incident light parallel to optic axis and normal to it. The optical constants include the complex index of refraction, conductivity, complex dielectric constant, loss functions and effective number of electrons per unit cell. The spectra in E ≥ 78 eV are interpreted in terms of the interband transitions from the O 2p and 2s bands to the Al conduction bands and the free electron-like absorption of photons by the O 2p band electrons. The volume energy loss function has a prominent peak at E ∼26.4 eV for the plasma oscillation dominantly of the 2p electrons. The spectral features in E ≤78 eV resulting from electronic transitions from the Al L 2,3 levels are presented with the L 2,3 exciton doublet, e.g. , at 78.72 eV and 79.15 eV for e 2// .

Optical Spectra of Y3Al5O12 (YAG) Single Crystals in the Vacuum Ultraviolet Region

The reflectivity spectrum of Y 3 Al 5 O 12 (YAG) single crystals at 297 K is presented in the region of 6.734 eV ≤ E ≤116 eV. The spectra of optical constants derived from the reflectivity by the Kramers-Kronig analysis are tentatively interpreted in terms of the energy levels of the constituent atoms of YAG. Plasma resonance peaks are found at 23.3 8 eV and 36.2 0 eV in which the valence-band electrons and valence- as well as full-band electrons participate, respectively. The structure of absorption due to excitations of the electrons in the Al 3+ L 2,3 levels is found in the region of E ≥76 eV. The free electron-like absorption calculated by the Drude formula by use of the line shape parameters of the plasma resonance line is compared with the dielectric constant in the region of E ≥24 eV. The effective optical dielectric constant calculated from the optical constant e 2 is found to be in good agreement at 116 eV with the optical dielectric constant derived from the dispersion formula.

Production of a Large-Diameter Uniform ECR Plasma with a Lisitano Coil

A large-diameter uniform plasma was demonstrated to be produced by electron cyclotron resonance heating with a slotted Lisitano coil of 40 cm diameter. The diameter of the realized uniform plasma was found to be almost equal to that of the Lisitano coil. It was also suggested that the microwave is propagated in the whole region inside the Lisitano coil, and produces the uniform plasma.

Ce3+ centres in YAlO3 (YAP) single crystals

Spectra of optical absorption (3 eV ∼ 6 eV) and emission (2.9 eV ∼ 4.1 eV) have been investigated on Ce 3+ centres in YAlO 3 :Ce 3+ single crystals at 297 K; excitation spectrum for 360 nm light in the Ce 3+ emission band region has also been measured in the region of 3.5 eV ∼ 6.2 eV. Measurement of excitation spectrum for Ce 3+ centres in YAlO 3 :Ce 3+ powder phosphor has been made at room temperature in the region of 3 eV∼ 11 eV. The excitaticn spectra in the air UV region consist of five bands assignable to the five levels of the Ce 3+ 5 d ; they are expressible by Gaussian functions, respectively, and therefrom the crystalline field splitting 10 D q =1.12 8 eV is derived. The excitation spectrum in the VUV region has a great peak at 8.05 eV which originates from the valence-band excitons. It is confirmed that the luminescence band expressed in terms of quanta per unit photon-energy interval can be fitted by two Gaussian components.

Reconstruction of edge-plasma density profiles by neutral beam probe spectroscopy

Thermal neutral beam probing combined with a spectroscopic technique is numerically developed to reconstruct electron-density profiles of edge plasmas. Although the reconstruction has been performed by assuming all beam atoms to have a thermal velocity, the influence of a beam-velocity distribution has not received as much attention. It is shown that the method which neglects the beam-velocity distribution reconstructs electron densities lower than the true values by a factor of ~2 in the density range up to ~1×1013 cm-3, while the procedure which takes into account the velocity distribution yields true density profiles in the same density range. Some results of the beam probe mounted in the Heliotron-E device are also described, and good agreement with Langmuir probe measurements is found.

Anisotropic Optical Spectra of YAlO3 (YAP) Single Crystals in the Vacuum Ultraviolet Region. II. Spectra of Reflectivity

Anisotropic reflectivity spectra of YAlO 3 single crystals at 297 K measured with the light polarized parallel to each of the a , b and c axes are presented in the range from ∼6.5 eV to ∼120 eV. A comparison of the reflectivity spectra of YAlO 3 with those of Y 2 O 3 , Y 3 Al 5 O 12 and α-Al 2 O 3 single crystals reaffirms that the peaks at ∼35 eV are mainly associated with the Y 3+ 4 p 6 and the features beyond ∼76 eV with the Al 3+ L 2,3 . Anisotropic reflectivity spectra at 10 K are shown in the region from ∼7 eV to 15 eV. Effective optical dielectric constants calculated by the sum rule including the imaginary part of the dielectric constant up to ∼120 eV are seen to saturate at the values corresponding to 98.0% ∼ 99.7% of the literature values of the optical dielectric constant.

Production of a large‐diameter uniform plasma by electron cyclotron resonance heating with a small‐diameter Lisitano coil

A large‐diameter uniform plasma is produced by electron cyclotron resonance heating with a slotted Lisitano coil of 9 cm in diameter by locating the resonance apart from the Lisitano coil. Although the plasma production with a Lisitano coil has been performed extensively by placing the resonance near the Lisitano coil, the influence of the resonance location has not received as much attention. When the resonance is located further than 8 cm from the Lisitano coil, the uniform plasma of ∼40 cm in diameter at densities of ∼1.2×1011 cm−3 is produced over the vacuum chamber with an inner radius of 46 cm. The microwave is propagated in the whole space between the resonance and the Lisitano coil, and spatial electric‐field distributions of the microwave play an important role on forming the radially uniform plasma.

Anisotropic Optical Spectra of YAlO3 (YAP) Single Crystals in the Vacuum Ultraviolet Region. III. Spectra of Optical Constants.

The Kramers-Kronig derived complex dielectric constant, energy loss functions and effective number of electrons of YAP are presented in the region of 6 eV∼120 eV for the light polarized parallel to each of the three crystallographic axes. The peak value of e 2 spectrum at 33.34 eV, the highest-energy component of the bands associated with five resonance transitions from the Y 3+ 4 p 6 level, is found to be proportional to the site number of Y 3+ /cm 3 in the lattices of YAG, YAP and Y 2 O 3 . The maxima due to plasma oscillations are derived at ∼23 eV and ∼36 eV in each volume loss function. The energy dependence of the dielectric constant is found to be in substantial agreement in the region of 50 eV∼120 eV with that calculated by the Drude formula with the use of the peak energy and half value width of the loss maximum at ∼36 eV. The above continuum is superposed on the features led by doublet lines at 78.4 eV and 78.8 eV which originate from the transitions from the Al 3+ L 2,3 levels.

Flute Stabilization of a Mirror-Confined Plasma by a Positive Ambipolar Potential

A positive axial ambipolar potential is found experimentally to play a role in the stabilization of flute instabilities driven by unfavorable magnetic field curvature in a simple mirror geometry. The stabilization mechanism, which does not depend on line tying to end walls, is the result of electrons which are confined by the positive ambipolar potential.