Kiyoshi Kadota

Plasma production by helicon waves

High density plasma production using m=+1 and m=-1 helicon waves is studied. Characteristics of cylindrical helicon waves including effects of a vacuum gap between the plasma and the conducting wall and of a non-uniform density profile are presented. The m=+1 and m=-1 helicon modes are separately excited by a helical antenna, and the dependences of plasma density and antenna loading resistance on RF power are different for these modes.

Plasma diagnostics by neutral beam probing

A new method using both techniques of a neutral beam and spectroscopy has been developed to measure the spatial electron density profile of a plasma. This neutral beam probing technique utilizes the photons emitted by impact excitation of the probing lithium beam with plasma electrons. The spatial electron density distribution of the plasma of Te approximately 10 eV, ne approximately 1011 cm-3 has been determined from the measurements of the local photon flux and beam intensity by using the excitation rate coefficient calculated from the available cross-section data. The comparison of these results with those of conventional methods proves this new method is of practical use in the space-resolved measurements of plasma parameters, especially when a Langmuir probe is no longer usable. The cross sections for excitation of the Li beam in collisions with He, Ar and H2 have been also measured and used to estimate the neutral lithium beam intensity.

Helium I line intensity ratios in a plasma for the diagnostics of fusion edge plasmas

Helium I line intensity ratios obtained by a collisional radiative model, including new recommended excitation rate coefficients and the effects of hot electrons, enable us to measure electron density (ne) and temperature (Te) in high density plasma. Measured ne and Te using 492.2 nm (4 1D→2 1P)/471.3 nm (4 3S→2 3P), 504.8 nm (4 1S→2 1P)/471.3 nm, and 492.2 nm/504.8 nm line intensity ratios are in good agreement with the Langmuir probe results in the helium discharge plasma in the NAGDIS‐I linear device (Nagoya University Divertor Simulator) for ne and Te regions of 1011–4×1012 cm−3 and 5–20 eV. Hot electrons in the plasma are important for the He I line emissions when Te is below the excitation energy ≊20 eV. A resonance scattering effect included in the calculation accounts for the experimental result of the enhanced 501.6 nm (3 1P→2 1S) line emission.

Distributions of C2 and C3 radical densities in laser-ablation carbon plumes measured by laser-induced fluorescence imaging spectroscopy

We measured temporal variations of the distributions of C2 and C3 radical densities in carbon plumes produced by laser ablation of graphite in ambient He gas. Laser-induced fluorescence imaging spectroscopy was used for the measurement. The temporal variations of total numbers of C2 and C3 contained in plumes were evaluated by integrating the density distributions. The experimental observations have shown that the gas-phase production of C2 is comparable to the direct production from the target, while C3 is mainly produced in gas phase by three-body reactions between C and C2. In addition, we have discussed a scenario for the temporal evolution of heavy clusters (Cn with n⩾4). The present results are useful for understanding initial formation processes of carbon clusters in laser-ablation plumes.

MEASUREMENTS OF NEGATIVE ION DENSITY IN HIGH-DENSITY OXYGEN PLASMAS BY PROBE-ASSISTED LASER PHOTODETACHMENT

Probe-assisted laser photodetachment has been developed and applied to measure the negative ion density (n−) in high-density plasmas. Temporal variation of n− is obtained in high-density and low-pressure oxygen plasmas generated by helicon wave excitation. Negative ions are not observed in the active discharge phase and rapid increase of n− is seen only in the afterglow phase. This efficient production of negative ions is considered to be due to dissociative electron attachment to metastable molecular states, O2M(A3Σu+, A′3Δu, and c1Σu−), located at 4–5 eV above the ground state.

Spatial and temporal variations of CF and CF2 radical densities in high-density CF4 plasmas studied by laser-induced fluorescence

Laser-induced fluorescence spectroscopy has been employed for the measurements of the ground-state CF and CF2 radical densities in low-pressure and high-density CF4 plasmas generated by helicon wave discharges. In the pulsed operation (5 Hz, 10 ms duration), the radial profiles of the CF and CF2 densities were hollow shape in the discharge phase, which indicates that both radicals were desorbed from the wall and were lost in the plasma column. In the continuous-wave operation, roughly uniform radial profiles were observed for both radicals. Therefore the desorbed radicals in the pulsed discharge seem to originate from the adsorbed species in the afterglow of the previous discharge.

Comparison of the Fluorine Atom Density Measured by Actinometry and Vacuum Ultraviolet Absorption Spectroscopy

The validity of actinometry for measurement of fluorine atom density was examined by comparing the results with those obtained by vacuum ultraviolet absorption spectroscopy. The experiments were carried out in high-density CF4 and C4F8 plasmas excited by helicon-wave discharges. As a result, the error in the relative F atom density measured by actinometry was within a factor of ~2 for the same working gas. On the other hand, the value of K in the actinometry relation n F = Kn Ar (I F/I Ar) was considerably different between the CF4 and C4F8 plasmas. Hence evaluation of the absolute F atom density by actinometry is generally difficult, except for rough order-estimation by substituting K 2.

Kinetics of fluorine atoms in high-density carbon–tetrafluoride plasmas

Reaction processes of fluorine (F) atoms in high-density carbon–tetrafluoride (CF4) plasmas were investigated using vacuum ultraviolet absorption spectroscopy. A scaling law nF∝(nenCF4)0.5–0.7 was found experimentally, where nF is the F atom density and ne and nCF4 stand for the electron and parent gas (CF4) densities, respectively. The lifetime measurement in the afterglow showed that the decay curve of the F atom density was composed of two components: a rapid decay in the initial afterglow and an exponential decrease in the late afterglow. The decay time constant in the initial afterglow τ1 satisfied the scaling law τ1∝(nenCF4)−(0.3–0.4), which is a consistent relationship with the scaling law for the F atom density. The two scaling laws and the lifetimes of CFx radicals suggest that the major loss process of F atoms in the initial afterglow is the reaction with CFx radicals (probably, x=3) on the wall surface. The loss process in the late afterglow was simple diffusion to the wall surface. The surface...

Surface production of CF, CF2, and C2 radicals in high-density CF4/H2 plasmas

Surface production of CF, CF2, and C2 radicals in high-density CF4/H2 plasmas was examined using laser-induced fluorescence spectroscopy. No significant amount of surface production was observed in pure CF4 plasmas. The addition of H2 into CF4 plasmas enhanced the surface production of CFx and C2 from fluorocarbon film deposited on the chamber wall. The characteristics of the surface production in cw discharges are reported, in comparison with surface production in pulsed discharges. In addition, it has been found that the surface production rates are determined not by the partial pressure but by the flow rate of H2, suggesting the significant consumption of feedstock H2 in discharges. The surface production of CFx and C2 indicates that these radicals are not the precursors for the deposition of fluorocarbon film in the CF4/H2 plasma. The deposition mechanism of fluorocarbon film in the CF4/H2 plasma is discussed, taking into account the surface production of CFx and C2.

Surface Kinetics of CFx Radicals and Fluorine Atoms in the Afterglow of High-Density C4F8 Plasmas.

Temporal variations of absolute densities of CF, CF2 and atomic fluorine (F) were measured in the afterglow of high-density C4F8 plasmas generated by helicon-wave discharges. Laser-induced fluorescence (LIF) spectroscopy was adopted for CF and CF2 radicals, while vacuum ultraviolet (VUV) absorption spectroscopy was employed for the F atom. CF and F densities gradually decreased for 20–80 ms after the extinction of the rf power, while CF2 density steadily increased during the same period. This slow increase in CF2 density can be explained by surface kinetics of the radicals. In the afterglow of discharges with a high degree of dissociation, the increase in CF2 density is approximately equal to CF density at the beginning of the afterglow. The mechanism for the surface production of CF2 in the afterglow is discussed based on the close relationships between the temporal variations of CF and CF2 densities.