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.
Volume production of D− negative ions in low-pressure D2 plasmas—negative ion densities versus plasma parameters
Pure volume production of D− negative ions is studied in a rectangular arc chamber. Production and control of D2 plasmas are tested by varying the intensity of the magnetic filter MF. The values of electron temperature Te and electron density ne in D2 plasmas are slightly higher than those in H2 plasmas. Te in D2 plasmas cannot be decreased below 1 eV in the extraction region with the same MF conditions for optimizing H2 plasmas. A stronger MF field is required for controlling Te in D2 plasmas. Therefore, the transport in or the production of H2 and D2 plasmas is different, i.e. there is an isotope effect, a consequence of which is that the H− and D− densities have different spatial distributions. Extracted H− and D− currents are primarily determined by H− and D− densities in front of the extraction hole, respectively. According to the estimated rate coefficients and collision frequencies of main collision processes, it is reconfirmed that Te in the extraction region should be maintained below 1 eV with ne kept high for the enhancement of H− and D− production. For the enhancement of D− production, preliminary results on VUV emission measurement are presented and discussed briefly.
Effects of the energy distribution of fast electrons on H2 vibrational excitation in a tandem negative ion source
The population distribution of vibrationally excited hydrogen molecules H2(v‘), and the H− production are investigated theoretically by solving numerically a set of particle balance equations in a steady‐state pure hydrogen plasma. In particular, the enhancement of the H2(v‘) distribution is discussed for different energy distributions of fast electrons ef. Whether the energy distribution for ef is a delta function type or a plateau type, collisional excitation of H2(v‘) caused by ef is very effective to enhance the H− production and a resultant vibrational distribution becomes the plateau distribution. Besides these, if ef with energies higher than 30–40 eV are present, both H2(v‘) excitation and then H− yield hardly depend on the form of the energy distribution of ef.