Rei Okasaka

Spectroscopic determination of hydrogen and electron densities in plasma in the ionizing phase: Application to WT-III

The Balmer line intensities of atomic hydrogen from the WT-III tokamak plasma have been observed and interpreted in terms of the collisional-radiative model. The atomic and molecular hydrogen densities are deduced together with the electron density. Going from the plasma centre to the edge region along the minor radius, the atomic hydrogen density increases slightly and molecular hydrogen is found only in the periphery. In the edge region, the atomic hydrogen density reaches 1 × 1016 m−3 and the molecular hydrogen density 4 × 10l7 m−3. The electron density distribution is consistent with that determined by infrared laser interferometry.

Excitation cross section in He+-He collisions. I. Excitation function and potential curve crossing

The authors measured the excitation cross sections of He I 31,3S, 21P, 31,3P, 41,3P, 31,3D, 41,3D, 51,3D and He II 22P and 32P levels at ion energies from 50 to 2*104 (lab) eV. Two types of structures have been recognised on the energy dependence of the cross sections, the origins of which are ascribed to excitation from the incoming channel by radial and rotational couplings through potential degeneracies at small internuclear distances. Profiles of the excitation functions depend systematically on angular momentum and spin multiplicity; the outgoing channels of the Sigma state strongly populate the triplet levels of the He atom, following the statistical weights of the spin multiplicity and those of the Pi state mainly populate the singlet levels. Rotational coupling is important for P-level excitation, as is radial coupling for D-level excitation. The characteristics of the polarisation of emitted light are consistent with our assignments for the excitation processes. The two-level model calculations give excitation functions which agree qualitatively with the observed ones.

Line Broadening Measurement in High Density Plasma I. Hα and Hβ

The profiles of H α and H β , were measured in a shock heated plasma of high electron density of 1 to 3×10 17 cm -3 . The measured profiles are compared with the theoretical ones, and some discrepancies are pointed out; the measured half width is larger than the theoretical one for H α , and the measured depth of the central dip of H β is smaller than the depth calculated by theories. The dependence of the measured depth of the dip on electron density agrees with the extrapolation from the experimental result by Wiese et at. in electron density less than 10 17 cm -3 , which indicates the effect of the ion motion. The measured value of the degree of asymmetry of H β profile, i.e., the ratio of the difference between the blue and the red peak intensity to the blue one, is approximately predicted by the theory including the effect of the quadrupole interaction between a radiating atom and the nearest ion.

Stepwise Excitation and Local Thermodynamic Equilibrium on Excited Hydrogen Levels in Helium-Hydrogen Plasma behind Shock Front

The calculation is carried out on the ionization relaxation as well as on the time developments of population densities of excited hydrogen levels for helium-atomic hydrogen mixtures (H: 0∼3 atomic %) behind the reflected shock front under the conditions of the experiment with an electromagnetic T-tube conducted by the present authors. The result gives the shortening of ionization relaxation time depending on mixing ratio of hydrogen and the characteristic initial hump in time development of population density, which is remarkable for low-lying excited levels of hydrogen in a mixture of low hydrogen mixing ratio. The temporal changes of the population and depopulation rates in the relaxation zone are presented for several excited hydrogen levels so that the existence of stepwise excitation between the coronal equilibrium and LTE is illustrated and the validity criterion is proposed for the establishment of LTE.

Line Broadening Measurement in High Density Plasma. II. Neutral Helium Lines

The profiles of neutral helium lines at λ 4471, 4921, 5016, 6678, and 7065 A were observed in a plasma of electron density 1.0 to 3.5×10 17 cm -3 generated in an improved T-tube. Comparison of the measured profiles with the profiles predicted by line broadening theories shows that the λ 7065 A line is well described by the isolated line approximation but the λ 5016 and 6678 A lines should be treated by the overlapping line theory in a plasma of electron density of 3.0×10 17 cm -3 . On the profile of the λ 4471 A line which has intense forbidden components, disagreement between the observed and the theoretical result is seen on the wavelength separation between the allowed and the forbidden component. The observed ratio of the maximum intensity of the forbidden component to that of the allowed component is larger than the theoretical ratio.

BEHAVIORS OF ELECTRON CAPTURE CROSS SECTIONS IN THE INTERMEDIATE VELOCITY REGIME ON ION-ATOM COLLISIONS

The behavior of electron capture cross sections in the intermediate velocity regime are classified into two types: one showing moderate monotonic change against collision velocity and the other showing noticeable change with a clear maximum in the intermediate velocity regime. In order to reproduce the two types of velocity dependencies for the collision system of low-ionization-stage ions and light atoms, we proposed a velocity matching model for an electron capture process in the intermediate velocity regime, taking the characteristics of the intermediate velocity collisions into consideration; i) collisions with a small impact parameter are effective. ii) Finite relaxation time necessary for electron capture reduces the cross section proportionally to 1/V, as collision velocity V increases. The limitations of the application of this model are also discussed.

Radial distribution of ion temperature in a WT-III tokamak plasma, as determined from Abel inversion of impurity line profiles

Abstract By using a spectrometer fitted with a multichannel detector, we have observed a chord dependence of emission-line profiles of OV and CV impurities in a WT-III tokamak plasma in a Joule-heating mode. After Abel-inversion of the observed intensity for each channel of the detector, we obtained a Doppler-line profile as a function of radius. Ion temperatures derived from the two lines agree with each other and are consistent with the energy distribution of neutral particles emanating from the plasma.

Spectroscopic Studies on Relaxation Process of Excited Levels in Shock Heated Plasma

The time developments of emission spectra and electron density of the plasma generated by an electromagnetically driven shock wave were observed. Helium mixed with a small amount (0.01 to 3%) of H 2 , Ar, Kr or CO was used as the test gas. From a comparison of the experimental results with the calculation based on collisional-radiative model it has been concluded that an excited level of atoms or ions goes through the following states of excitation with the progress of ionization relaxation: (I) coronal state, (II) partial stepwise excitation state, (III) complete stepwise excitation state and (IV) partial LTE. The critical electron densities corresponding to the transitions from state II to III and that from III to IV were determined experimentally. The influence of radiation from driving discharge plasma on the relaxation process was also made clear in experiment.

Improvement of Shock Wave Generation in a T-Tube

In order to eliminate complex structure which is often found in shock wave region in a T-tube, three types of modifications were tried on an ordinary T-tube; (i) converging test section of the T-tube, (ii) application of a strong magnetic field to primary shock, and (iii) L-shape electrodes. In each case the behaviors of driving discharge plasma and the shock wave trajectories in front of a reflector plate were observed with a streak camera. In the experiment with an ordinary T-tube, discharge plasma was separated away from the electrodes at an early time in the first half cycle of discharge and this gave rise to the complex structure in shock wave region. In the experiment with the L-shape electrodes the complex structure was completely eliminated and the turbulence in the discharge plasma was suppressed to a considerable extent.

Behaviors of Excitation Cross Sections in the Intermediate Velocity Regime on Ion-Atom Collisions

We studied the behaviors of excitation cross sections and electron capture cross sections as functions of collision velocity using previous data of various collision systems. The structures peculiar to the intermediate velocity regime were recognized on the excitation cross sections. Close correlation of the excitation cross section to the electron capture cross section and a relaxation time longer than that for the electron capture process characterize the excitation process in the intermediate velocity regime. A model consisting of a two-step electron transfer process well explains the characteristics of the excitation cross sections in the intermediate velocity regime.