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内容記述 |
High energy neutral hydrogen atom beam is used in the Neutral atom Beam Injection (NBI) for effective plasma heating in nuclear fusion research. There are two methods for production of neutral hydrogen atoms; one is electron capture process of positive hydrogen ions (protons) and the other is electron detachment process of negative hydrogen (H<SUP>-</SUP>) ions. However, at high energies (more than 100 keV), the single electron detachment from H<SUP>-</SUP> ions is much more efficient because the electron detachment cross sections from H<SUP>-</SUP> ions are far larger than the electron capture cross sections of protons. <br /> In the present thesis, I perform some theoretical analysis on the single electron detachment processes of H<SUP>-</SUP> ions in collisions with neutral atoms and also with positive ions. <br /> There are two electrons with different ionization potentials in a H<SUP>-</SUP> ion in the ground state. I treat independently two electrons with different binding energies: one is the tightly bound 1s electron (ionization energy <u>～</u> 13.6 eV) and the other is the loosely bound 1s' electron (ionization energy<u>～</u> 0.75eV) in the Hylleraas-Eckart function. Therefore, I take into account two different detachment processes, namely i ) detachment of the tightly bound (1s) electron and ii) detachment of the loosely bound (1s') electron. To describe more detail of the electron detachment processes, the binding energies of each electron are taken into account in the detached electron wave functions. By these methods, I calculate the cross sections for production of hydrogen atoms not only in the ground state, H<SUP>0</SUP>(1s), but also in the excited states, H<SUP>0</SUP>(2s) and H<SUP>0</SUP>(2p), in the exit channel.<br /> In collisions with neutral atoms including rare gases and atomic hydrogens, I use the first Born approximation to calculate the electron detachment cross sections from H<SUP>-</SUP> ions. I also consider the target excitation in the exit channel. However as it is too much time- consuming to calculate all the possible states of target in the exit channel, I use the closure approximation for the target excitation.<br /> First, I present the calculated results of the total and partial single-electron detachment cross sections from H- ions in collisions with He and H neutral atoms. Then, I compare the calculated total cross sections of electron detachment with experimental data available and find reasonable agreement between them.<br /> The calculated partial cross sections for the production the ground state hydrogen atoms, H<SUP>0</SUP>(1s), are found to be dominant in all the collision energy range (10 keV ～ 10 MeV) investigated in the present study. The cross sections for H<SUP>0</SUP>(2s) formation are found to account for about 20 % and those for H<SUP>0</SUP>(2p) formation about 10% of total electron detachment cross sections.<br /> The present calculated results show that the dominant contribution to the production of the ground state H<SUP>0</SUP>(1s) comes from the electron detachment of the loosely bound 1s' electron in H<SUP>-</SUP> ions. On the other hand, in the production of the excited state hydrogen atoms H<SUP>0</SUP>(2s) and H0(2p), the detachment of the tightly bound 1s electron in H<SUP>-</SUP> ions is found to be dominant over the detachment of the loosely bound 1s' electron.<br /> In collisions with positive ions, I use the Four-Body Continuum Distorted Wave-Eikonal Initial State (4-B CDW-EIS) method to calculate the electron detachment cross section from a H<SUP>-</SUP> ion because there are strong distortion effects to its electrons in H<SUP>-</SUP> ion due to the Coulomb field of the incident positive ions. Thus I consider the distorted wave and distorted potentials. Though the transition amplitudes are given in two different forms, namely the 'prior' and the 'post' because there are two distortion potentials, I treat the prior transition amplitudes only here. <br /> Then, I compare the calculated total electron detachment cross sections from H<SUP>-</SUP> ions in collisions with protons with experimental data.<br /> I also notice that in general, in collisions with positive ions, the calculated partial cross sections of production of H<SUP>0</SUP>(1s) is far larger (more than two orders of magnitude) than those of H<SUP>0</SUP>(2s) in all the energies investigated. <br /> The present calculated results also show that the total electron detachment cross sections of H<SUP>-</SUP> ions in collisions with protons are roughly one order of magnitude greater than those with neutral hydrogen atoms and a simple scaling which indicates that the electron detachment cross sections at high energies are roughly proportional to the square of the positive ion charge colliding with H<SUP>-</SUP> ions.<br /> These results suggest that H<SUP>-</SUP> ions can be converted into neutral hydrogen atoms with more fraction of the ground state in collisions with positive ions more efficiently than in collisions with neutral atoms, indicating that the plasma neutralizer is indeed more promising than the neutral gas neutralizer in producing neutral hydrogen atoms for NBI from H<SUP>-</SUP> ions. |
要旨・審査要旨 / Abstract, Screening Result (323.9 kB)
