|
内容記述 |
Various phenomena due to negative ions in plasmas are widely studied by many researchers in astrophysics, atomic physics, chemistry, and plasma physics. These fields sometimes involve curious phenomena that can not be explained without any consideration of the inherent properties of negative ions. Hence, negative ion diagnostics are essential to understanding of behavior of negative ions and interactions among plasma particles. The photodetachment technique becomes a powerful diagnostic tool for determination of negative ion densities and its temperatures in plasmas.<br /> In the first part of this thesis, studies on plasma responses after a photodetachment reaction have been carried out both theoretically and experimentally. In the second half, the diagnostic techniques for negative ion densities and temperatures measurement are applied to an actual H- ion source. In particular when cesium vapor is introduced into a hydrogen discharge, an anomalous enhancement of H- is observed. The combination with photoelectron current measurements on a plasma electrode and the other plasma parameter measurements, including the measurements of negative ion densities, allows us to discuss the enhancement mechanism of H- in a cesium seeded case.<br /> First stage of the study on the laser photodetachment technique is the investigation of the influence of the energies of photodetached electrons upon the determination of the H- density and the H drift velocity in a hydrogen plasma by using the fundamental frequency (1064nm), the 2nd harmonic (532nm) and the 3rd harmonic (355nm) of a Nd-YAG laser. It is found that the δIp-Vp characteristics, where δIp is the photodetachment current and Vp is the probe voltage, do not depend on the photon energies. The effective temperature of photodetached electrons Teff is close, within 40%, to that of background electrons. As a result the measurements of the H- density and the H- drift velocity are not affected by photon energies.<br /> Next, the entire dynamics and physical picture of plasma response after laser photodetachment have been studied theoretically and experimentally. The hybrid fluid-kinetic model of Friedland et al., where negative ions are treated in a ballistic approximation, electrons in a linearlized Boltzmann's relation, and positive ions as a fluid, is extended to the analysis of perturbed densities inside and outside a laser beam. From the results of calculations, the time evolution of perturbed electron current is in good agreement with experimental data. The overshoot in the perturbed electron current, which is frequently seen under some experimental conditions, has been studied. From the comparison of overshoot signals between the observation and the calculation, the way of the determination of positive ion temperature is proposed.<br /> The rest in this thesis concentrates on the cesium effect of negative ion densities both in driver region and in extraction region of an ion source of volume production type, because it has been reported that beam intensity of negative ions from a negative ion source is enhanced drastically after a cesium injection into an H- ion source. In a cesium seeded hydrogen discharge, the H- density and temperature by use of the laser photodetachment technique are investigated in the driver region of tandem H- source, in which the plasma is not connected directly with the wall of the ion source. The amount of cesium in the ion source is monitored by spectroscopic measurement. The plasma space potential, Vs, obtained from the probe I - V characteristics has increased by Cs injection. However no clear changes in ne, n-, Te, T+, and T- are observed. This result suggests that the wall surface in the extraction region influence the H- production in the Cs/H2 discharge.<br /> For the purpose of studies on the cesium effect in an extraction region of an ion source, the work function of the plasma electrode surface is measured photoelectrically using an Ar+ ion cw laser. The mount of alkali metal vapor is measured by spectroscopic method, and negative ion densities are measured by photodetachment method. The measured work function is correlated with the H- density measured simultaneously by the laser photodetachment technique. As the work function of the plasma electrode is reduced due to Cs seeding, the ratio of H- density to electron density, n-/ne, increased gradually. The n-/ne ratio becomes maximum when the work function reaches at the minimum value. The same result is obtained in the case of Rb seeding. These results are discussed with the help of rate equations in the plasma, taking the surface effects into account.<br /> This work can be summarized as the following; H- ions and other plasma species in negative ion sources can be diagnosed by means of the laser photodetachment technique. From the agreement between the theoretical calculations and experimental results, it is found that the hybrid-fluid kinetic model is valid under the conditions of the collisionless, unmagnetized and low n-/ne ratio, described here, and that it is useful to determine the H- density, H- temperature, and positive ion temperature. It is confirmed that the work function of the surface of the plasma electrode of a cesium seeded H- volume source influences the n-/ne ratio in the extraction region. |
要旨・審査要旨 / Abstract, Screening Result (342.6 kB)
