Optimisation of volume-produced H- ions is studied by using a set of particle balance equations in a steady-state hydrogen plasma with a single-chamber system. The dependence of production of both H- ions and vibrationally excited hydrogen molecules H2* (vibrational level V") on plasma parameters (i.e. electron temperature Te, electron density ne, hydrogen gas pressure p, density ratio nfe/ne of fast primary electrons ef to slow plasma electrons e, energy of fast electron Efe, etc.) is explored because it is expected that H- ions are produced by the following two-step process, i.e. H2+ef to H2*(V")+Ef; H2*(V")+e to H-+H. Particular attention is also paid to wall effects, i.e. neutral particles-wall interaction, on H- production. So, a wall recombination coefficient gamma 1 for H and a wall de-excitation collision parameter gamma 2 for H2*(V") are treated as numerical parameters. It is confirmed that most H- ions are produced by the above-mentioned two-step process, and that the presence of ef with energies in excess of 40 eV is reasonable for H2*(V") production. With increasing Te (above 1 eV), H- yield decreases monotonically. Besides, ne, nfe/ne and p have some optimum values for H- production. However, the optimum condition for H- formation is not compatible with that for H2*(V") production. Another significant point is that the ion species ratios depend strongly on the wall parameters, i.e. gamma 1 and gamma 2. For H- production, the optimum condition is that gamma 1 approximately=1 and gamma 2<<1.

Isotope effect on H− / D − volume production is studied by measuring both VUV emission and negative ion density in the source. In a double plasma type source, under some discharge conditions, extracted D − currents are nearly the same as H − currents, although VUV emission intensity (corresponding to production of vibrationally excited molecules) in D2 plasmas is slightly lower than that in H2 plasmas. Considering the factor √ 2 due to mass difference, D − ion density in the extraction region of the source is higher than H − ion density. In another experiment with a rectangular arc chamber, axial distributions of H − / D − ion densities in the source are measured directly using a laser photodetachment method. Relationship between H − / D − production and plasma parameter control with using a magnetic filter (MF) is discussed. Furthermore, relative intensities of extracted negative ion currents are discussed compared with the negative ion densities in the source. Production and control of D2 plasmas are well realized with the MF including good combination between the filament position and field intensity of the MF. Extracted H − and D − currents depend directly on negative ion densities in the source.

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Ion species ratios in a hydrogen plasma are calculated systematically as a function of plasma parameters, i.e. the electron density, the electron temperature. the pressure of hydrogen gas and the plasma volume. Furthermore, in the present analysis. the recombination factor for hydrogen atoms at thewall surface of a vacuum vessel is treated as another plasma parameter.The most significant point is that ion species ratios depend strongly not only on plasma parameters but also on the recombination factor. The proton ratio increases with decreasing the value of the recombination factor. Primary electrons also play an important role for ion species ratios, and the presence of primary electrons causes the proton ratio to decrease.

An asymmetric plasma divided by a magnetic filter is numerically simulated by the one-dimensional particle-in-cell code VSIM1D ͓Koga et al., J. Phys. Soc. Jpn. 68, 1578 ͑1999͔͒. Depending on the asymmetry, the system behavior is static or dynamic. In the static state, the potentials of the main plasma and the subplasma are given by the sheath potentials, M ϳ3T Me /e and S ϳ3T Se /e, respectively, with e being an electron charge and T Me and T Se being electron temperatures (T Me ϾT Se ). In the dynamic state, while M ϳ3T Me /e, S oscillates periodically between S,min ϳ3T Se /e and S,max ϳ3T Me /e. The ions accelerated by the time varying potential gap get into the subplasma and excite the laminar shock waves. The period of the limit cycle is determined by the transit time of the shock wave structure.

Although optimizing the magnetic filter position and the plasma grid potential is one of the most effective factors to enhance H− yield, details concerning their roles are not now clarified well. In this article, spatially resolved measurements of the electron energy distribution function, plasma fluctuations, and plasma parameters are presented. On the basis of these experimental results, we will discuss the roles of both the magnetic filter and the plasma grid biasing voltage Vb on enhancement of H− production and extraction of H− ions.

Trajectories of Hions are calculated numerically by solving the 3D motion equation, including effects of collisional destruction, elastic collisions and charge exchange collisions. According to these trajectories, extraction probability of Hions produced at any location inside the source and energy of extracted Hions are discussed as a function of gas pressure. Effects of production zone and filter magnetic field on extraction probability are also discussed. The probability for surface produced Hions keeps nearly the constant value, and that for volume produced Hions decreases with gas pressure. The kinetic energy of extracted Hions is reduced mainly by charge exchange collision. We also discuss the characteristics of extracted negative ion current combining the present numerical results and the results of the model calculation with the zero-dimensional code.