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To clarify the property of tritium removal from carbon codeposition by using pulsed-laser induced desorption, hydrogen removal from co-deposits on JT-60 divertor tile using a fourth-harmonic emission (266 nm) of a nano-sec Nd:YAG laser was demonstrated. The threshold laser fluence for ablation was ∼0.3 J/cm 2 , which was slightly higher compared to that of the pico-sec laser irradiation. The energy absorption coefficient for the nano-sec laser, which was obtained by fitting the removal rate by so-called Beer’s law, was larger than the pico-sec laser. Since the time constant of thermal wave propagation into the target is of the order of nano-sec, such differences between nano- and pico-sec lasers could be attributed to thermal effects. The ablation threshold of the deposited layer was lower than that of a pure graphite, which could be attributed to the difference of thermal conductivity between deposited layer and pure graphite. The removal rate of the nano-sec laser was higher than that of pico-sec laser in the fluence range of >∼0.5 J/cm 2 . On the other hand, the production ratio [hydrocarbon species]/[H 2 ] continuously increased with the laser fluence, and no significant ionization of carbon was observed in this fluence range. This indicated that the fluence range in this study was “weak”-ablation range, which was still not sufficient to minimize the hydrocarbon production.

For comparative assessment of tokamak and helical reactors, two base case reactor designs with 1 GW electric power output are introduced; high-field high-beta compact tokamak reactor TR-1 (major radius R = 5.2 m, magnetic field B = 7.1 T) and high-beta helical system HR-1 (R = 12.1 m, B = 4.6 T). Operational boundaries of both reactors are compared by the zero-dimensional energy balance model, which clarifies the difference between the higher temperature operational regime of TR-1 and the lower temperature regime of HR-1. These operational scenarios are reconfirmed by the toroidal transport analysis linkage (TOTAL) code coupling with two-dimensional equilibrium in the TOTAL-T tokamak code and three-dimensional equilibrium in the TOTAL-H helical code. The beta effect on confinement degradation and the impurity radiation effect on energy balance are found to make the ignition difficult in both systems.

Properties of high-density nitrogen plasmas with excitation of the m=0 mode helicon wave has been studied for reactive plasma sputter synthesis of carbon nitride films. High-density nitrogen plasmas with densities of 4×1012 cm−3 were obtained in nitrogen at ∼0.1 Pa. Optical emission spectroscopy showed that the line emissions of atomic nitrogen (NI) and atomic-nitrogen ions (NII) were considerably enhanced in the helicon wave-excited high-density plasma, whereas, the spectra measured from the induction-mode (non-wave excitation) low-density plasma were dominated by those of the molecular band emission associated with the first positive system of N2. Carbon nitride films have been deposited on Si (100) substrates by reactive sputtering of carbon target with the helicon wave-excited high-density nitrogen plasmas at ∼0.1 Pa. Compositional characterizations of the CN films were performed using Rutherford backscattering spectrometry (RBS). The RBS analysis showed that the N/C ratio of ∼1.3 was achieved by depositing the CN films at plasma densities as high as 1×1012 cm−3, where the line emissions of atomic nitrogen were significantly higher than the molecular band emissions. Increase in the plasma density and/or the emission-intensity ratio of the atomic nitrogen (NI) to the molecular band in the vicinity of the substrate was found to directly contribute to the N/C composition ratio in the CN films. Structural analysis by Fourier transform infrared spectroscopy (FTIR) showed that the bonds associated with hydrogen impurity could be effectively eliminated by increasing the substrate temperature during film depositions.

Large-area and high-density RF plasmas at 13.56 MHz have been produced in a discharge chamber with 400 mm inner diameter and 200 mm height by inductive coupling of an internal-type double half-loop antenna (320 mm diameter). The present study has been carried out to develop the basic discharge techniques that can be applied to the production of high-density and large-volume plasmas for a variety of plasma-based ion implantation (PBII) processes. The plasma source could be operated stably at RF input powers up to 2.5 kW to attain a plasma density as high as 5×1011 cm−3 at argon pressures of approximately 1 Pa. It has been demonstrated that a high-plasma density can be obtained using the low-inductance internal antenna configuration for effective suppression of the electrostatic coupling. Discharge experiments in nitrogen and hydrogen resulted in a radially uniform plasma production with density as high as 5–9×1010 cm−3.

Large-area high-density RF plasmas at 13.56 MHz have been produced by inductive coupling of an internal-type double half-loop antenna (320 mm diameter), which was immersed in the discharge chamber with 400 mm inner diameter and 200 mm height. The plasma source could be operated stably at RF input power of up to 2.5 kW to attain plasma densities as high as 5×1011 cm−3 at argon pressures around 1 Pa, simultaneously achieving effectively suppressed electrostatic coupling. Azimuthal distribution of the plasma density showed the nonuniformity with m=2 mode, which was found to be controlled by the RF voltage distribution along the antenna and the gas pressure.

Development of high density RF plasma sources at low pressures is described based on the production of inductively-coupled plasmas (ICP) for the application to varieties of physical vapor deposition (PVD) processes. Two types of plasma sources are studied; one is an ICP with a conventional helical antenna for magnetron sputtering assistance and the other is a large-diameter ICP sustained with a low inductance double half-loop antenna. In both systems the RF antenna is located within the vacuum chamber and is terminated to the grounded earth potential via a blocking capacitor. By a simple circuit analysis it was found that the floating antenna configuration effectively suppressed the anomalous rise of the plasma potential, which has been verified experimentally. A large-diameter ICP production with a double half-loop antenna of 320 mm in diameter was successfully employed for reducing the antenna inductance, and full coverage of the antenna with an insulator tubing exhibited a significant effect on the suppression of the electrostatic coupling. Azimuthal distribution of the plasma density in this source showed a non-uniformity with m =2 mode. Dense plasmas with plasma density ranging 10 11 –10 12 cm −3 were sustained in both sources. These types of plasmas are expected to be applicable as a tool for enhanced ionization in the sputtering process and for three-dimensional ion based PVD processes.

The production of high-density plasmas with excitation of the m =0 mode helicon wave has been studied for the highly reactive sputter synthesis of carbon nitride films in N 2 and N 2 +Ar mixtures in the pressure range of ~0.1 Pa. High-density plasmas of 10 12 –10 13 cm −3 were produced even in pure N 2 by using a helical antenna with an azimuthal mode number of m =0. The helicon-wave-excited plasmas have been investigated by wave-magnetic-field measurements and optical emission spectroscopy. The plasma density achieved in 0.26 Pa nitrogen was 4×10 12 cm −3 at a radio frequency (RF) power of ∼3 kW and a static magnetic field strength of ∼1000 Gauss. The wave-magnetic-field measurements confirmed excitation of the m =0 mode helicon wave to propagate in the plasmas at the onset of attaining the high-density mode. Significant enhancement of line emissions of atomic nitrogen (N I) and nitrogen ions (N II) was observed in the helicon-wave-excited plasma, whereas the optical emission spectra in the induction mode were dominated by those from the molecular bands of nitrogen. Carbon nitride films have been deposited on a Si(100) substrate by using reactive sputtering of a cylindrical carbon target with the wave-excited high-density plasmas in N 2 and N 2 +Ar mixtures at around 0.1 Pa. Compositional and structural characterization of the CN films was performed by X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS), showing that the N/C ratio in the CN films could be controlled up to 1.3. Chemical and structural analysis by XPS has been performed. The XPS analysis showed the existence of peaks indicating a C=C bond and two C=N bonds where nitrogen is bonded to carbon atoms in sp 2 - and sp 3 -hybridized states. Nanoindentation measurements showed that hardness values as high as ∼20 GPa were attained at a substrate bias of −200 V.

Studies directed towards the reduction of particle and heat fluxes to plasma facing components by the application of ponderomotive forces generated by radio frequency (RF) are being conducted in TEXTOR. A modified poloidal limiter is used as an antenna with up to 3 kW of RF power; the data obtained show that the plasma is repelled by the RF ponderomotive potential. The density is reduced by a factor of 2–4 and the radial decay length is substantially altered. The density near the limiter decays exponentially with RF power. The electron temperature profile changes, with the decay length becoming longer (almost flat) during the RF. The temperature in the scrape off layer (SOL) increases and its increase is roughly proportional to the RF power until it saturates, suggesting that the heating efficiency drops with power, and that improved performance is to be expected at higher powers.

Azimuthal periodic breakup of a radially modulated 0.35 μm laser beam has been inferred in plasmas produced from solid targets. The breakup is more severe in gold plasmas compared to glass or aluminum plasmas and occurs at rather modest laser intensities of ∼5x10 12 W/cm 2 . Thermal filamentation is suggested as the mechanism for the observed beam breakup.

Summary This paper deals with the solution of optimum control problems, using Pontryagin's maximum principle. Since an optimum control system which completes the transient response in minimum time behaves like a relay servo, the important task of the controller is to reverse the sign of the manipulated variable at the proper instants according to the operating condition of the system. A set of points at which the sign of the manipulated variable is to be reversed forms a switching surface in the phase space. In this paper the optimum switching surface of a linear third-order system is investigated. Also dealt with is the optimum switching curve of a non-linear control system governed by van der Pol's equation with an additional forcing term.