Tsutomu Takahashi

Vacuum UV Absorption Spectra of Liquid Water and Ice

Fundamental absorption bands of water are studied in its various phases. Cubic ice has a well-defined absorption band at about 8.7 eV, while hexagonal and amorphous ices show only a gradual increase of absorption intensity toward the higher energy in the range from 7 to 10 eV. Absorption coefficient of liquid water is found to comply with Urbach rule at the low energy tail of the fundamental absorption.

Mechanism of Nitrogen Incorporation into Amorphous-CN_x Films Formed by Plasma-Enhanced Chemical-Vapor Deposition of the Doublet and Quartet States of the CN Radical

High-resolution CN(B2Σ+–X2Σ+) emission spectra were observed for the various processes to form amorphous-CNx (a-CNx) films using the plasma-enhanced chemical-vapor deposition of the CN radical produced from the dissociative excitation reactions of cyanides. A strong correlation was confirmed between the electronic states of CN in the plasma and the bonding states of nitrogen atoms in the films. The 4Σ+ and 4Π states of CN were the precursors of the one- and/or two-dimensional C=N and C–N network structures of the films with high nitrogen content, [N]/([N]+[C]) ≤0.5. The CN(X2Σ+) state formed the C≡N terminations primarily, a part of which changed to the one-dimensional C=N network from the additive reactions. The above correlation was fully explained by the molecular orbitals and the electronic configurations for the relevant electronic states of CN.

Production of a low density field reversed configuration plasma

The plasma density of a field reversed configuration (FRC) needs to be decreased below the present experimental regime in order to heat the plasma and sustain the configuration by a high energy neutral beam in an FRC reactor. However, as the plasma is produced in a linear vacuum vessel, there exists a severe breakdown limit at a low fill pressure as compared with a toroidal system. A method to form FRCs beyond the breakdown limit is proposed here. The preionized plasma is compressed by a strong bias field to enhance the plasma flow from the confinement region to the outside region and is then diluted before the start of the confinement field on the NUCTE device. The use of a diluted plasma enables the critical density of the FRC to be lowered from 1.1 × 1021 to 5.6 × 1020m-3 and the sum of the electron and ion temperatures to be increased from 0.35 to 0.81 keV.

Multichannel optical diagnostic system for field-reversed configuration plasmas

A constructed diagnostic system consisting of a 60-channel set of optical detectors with flexible viewing configurations is realized to investigate three-dimensional magnetohydrodynamic (MHD) motions and the internal structure of a field-reversed configuration (FRC) plasma. The system can detect radiation from the plasma in the wavelength range of 420–820 nm. Optical filters are used to select the wavelength ranges required in the experiment. The sensitivities of all the optical detectors are calibrated using radiation from the FRC plasma at a quiescent phase. Radiation profiles measured by orthogonal viewing configuration of the detectors are shown at three toroidal cross sections. From these profiles, the time evolution of the three-dimensional MHD motion of the plasma is depicted. The radiation profile measured by a one-dimensional viewing configuration yields not only an electron density profile inside the separatrix but also the width of an edge-layer plasma. A bright halo around the edge-layer plasm...

Limitation of Nitrogen Incorporation into the Hydrogenated Amorphous Carbon Nitride Films Formed from the Dissociative Excitation Reaction of CH3CN

High-resolution CN(B2Σ+–X2Σ+) and CH(A2Δ–X2Π) emission spectra were observed for the dissociative excitation reaction of CH3CN with the microwave-discharge flow of Ar for synthesizing hydrogenated amorphous carbon nitride (a-CNx:H) films. The simulation analysis of these spectra revealed that the relative number density of CH(A) to that of CN(B), NCH(A)/NCN(B), was strongly dependent on the pressure of Ar, PAr, in the range of PAr=0.1–0.8 Torr. The PAr-dependence of NCH(A)/NCN(B) showed a strongly negative correlation with that of the [N]/([N]+[C]) ratio obtained in our previous structural analysis of the films [Saitoh et al.: Jpn. J. Appl. Phys. 39 (2000) 1258]. This correlation was fully explained in terms of the consumption of the CN radical by the hydrogen-abstraction reaction from the film surface, preventing the incorporation of the nitrogen atoms into the a-CNx:H films.

Separatrix shapes and internal structures of a field-reversed configuration plasma

The separatrix shape of a field-reversed configuration plasma is determined in comparison with measured fluxes surrounding the plasma and the solution of the Grad–Shafranov equation with an edge-layer plasma in the open field region. A reconnection point of the bias field, an outgoing flow of torn plasmas and a large amplitude ripple on the separatrix surface are clearly observed at the formation phase. A smoothed separatrix shape having definite ends is observed at the quiescent phase. It is also estimated that the beta value at the separatrix and the thickness of the edge-layer plasma are, respectively, βs=0.5–0.7 and 4–6 ρi (ρi: ion gyroradius). The magnetic structure inside the separatrix is investigated by solving the Grad–Shafranov equation with the measured separatrix shape and βs. It is found that magnetic islands are formed near the magnetic axis not only at the formation phase but also at the quiescent phase. The appearance and coalescence of the islands are repeated during the discharge.

Control of a global motion on field-reversed configuration

An n=1 mode global motion on a field-reversed configuration (FRC) plasma is observed by means of a newly developed optical system. The deviation of the FRC from the coil axis reaches 20%–40% of the plasma radius. In order to push back the FRC to the equilibrium position, a multipole field (quadrupole or hexapole field) is applied. The n=1 motion can be easily controlled by the quadrupole field, the critical field strength of which is required to be about 15% of the confinement field. It is found that the n=2 rotational instability can also be stabilized by strength of the same order. The critical strength for the n=1 motion is theoretically obtained from a model such that the driving energy of the motion given at the formation phase balances with the work done by the multipole field. The theoretical estimation agrees within a factor of 2 with the experimental results.

New type of pulsed ion source with cryogenic anode

A magnetically insulated diode with a cryogenically refrigerated anode is proposed and a prototype of such a diode is constructed. H2O ice is produced on an anode which is cooled with liquid nitrogen. A small machine, consisting of a marx generator and a blumlein line, is used to extract ion beams from the diode. Proton beams of about 50 A and 70 keV are obtained with good reproducibility. Diode electrical characteristics and the extracted beam performance are also examined.

Sources of fluctuating field on field-reversed configuration plasma

Fluctuating azimuthal fields, B θ , are detected during a lifetime of a Field-Reversed Configuration plasma. The maximum strength reaches to 70 G which corresponds to about 1.5% of a confining field. Sources of the fields are discussed with relation to such plasma motions as radial shifting and tilting motions, and elliptical deformation of the plasma column triggered by a rotational instability. Analytic formulas of the B θ are derived from disturbance of the confining field due to these motions. Estimated values from the formulas agree well with field profiles obtained from a magnetic probe array.

Effects of background neutral particles on a field-reversed configuration plasma in the translation process

A field-reversed configuration (FRC) plasma was translated into a weakly ionized plasma and the effects of heating and particle buildup of the FRC plasma due to the background neutral particles and plasma injection in the translation process were investigated. Improvement of the particle and poloidal flux confinements and delay of onset of n=2 rotational instability were observed in the translation process. It was found that the internal structure of the plasma pressure (plasma temperature and density) at the separatrix and field null was deformed by the particle injection. FRC plasma translation through the background particles was equivalent to an end-on particle beam injection to the FRC plasma. Particles and energy were supplied during the translation. The results obtained for the phenomena of particle supply and plasma heating were also supported by the results of two-dimensional particle simulation. The effects of background particle injection appear to be a promising process for the regeneration of...