Effects of external magnetic filter on behaviors of fast primary electrons in a volume negative ion source.
The relationship between enhancement of hydrogen negative ion H(-), volume production, and behaviors of fast primary electrons is studied theoretically. Trajectories of fast primary electrons are calculated, including collisional effects with hydrogen molecules. Active region of fast electrons depends strongly on the field intensity of the magnetic filter (MF), B(MF), and filament position. Therefore, spatial distributions of ionization and vibrational excitation collision points are also affected with changing B(MF). Although number of vibrational excitation collision points is much smaller than that of ionization collision points due to different mean free path, the relative intensity is varied by changing energy of fast electrons. According to the results and discussions including transport of produced vibrationally excited molecules, for enhancement of H(-) production, it is desired that fast electrons should approach the MF position and move into the downstream region as deep as possible without destruction of H(-) ions. To realize these conditions, good combination between filament position and the intensity of the MF should be required.
Experimental study on spatial uniformity of H- ion beam in a large negative ion source
Abstract The origin of the beam non-uniformity, that is one of the key issues in JT-60 negative ion sources, was experimentally examined by measuring correlations between the intensity of the H− ion beam and plasma parameters, such as an electron temperature and plasma density in the JAERI 10 A negative ion source. The ion source was operated without cesium. Although a uniform filter field was applied, the electron temperature near plasma grid (PG) was higher than 1 eV in the upper half region of the longitudinal direction, where the spatial beam intensity decreased rapidly. This indicates that the beam non-uniformity is caused by a higher electron temperature than 1 eV, namely, by a leakage of the high-energy electrons toward the PG. To investigate the path of high-energy electrons leaking into the PG, a metal plate of 50 mm × 50 mm was placed in the plasma. By placing the plate in a weak magnetic field of
Electron energy dependence of vacuum ultraviolet emission and H− production in a low-pressure hydrogen plasma
A two-step process of pure H− volume production via vibrationally excited ground state molecules H2(v″⩾5) is discussed. The principal source of this vibrational excitation is through singlet excitation by fast primary electrons followed by vacuum ultraviolet (VUV) emission. The VUV emission from a low-pressure hydrogen plasma has been used to study the production mechanism of H2(v″⩾5) molecules and its electron energy dependence, in a double plasma source where both energy and density of fast electrons are well controlled. The fast electron energy and density strongly affect the VUV emission intensity at wave lengths of, for example, 123 and 124.9 nm. There exists a certain threshold in fast electron energy for singlet excitation and then for H2(v″) excitation. Fast electrons with energy in excess of 15–20 eV mainly contribute to the VUV emission intensity and are sufficient to increase H− production.