A small radius hydrogen discharge: An effective source of volume produced negative ions

Abstract

Free-fall regime maintenance of hydrogen discharges is analyzed based on numerical solutions of a set of equations involving the balance equations of the charged particles [electrons, the three types of the positive ions (H+, H2+, and H3+), and negative H− ions] and of the neutral species (hydrogen atoms H and vibrationally excited molecules), the momentum equations of the positive ions, the electron energy balance equation, and the Poisson equation, all together 25 differential equations. The obtained results for varying discharge radius show strong accumulation of the negative ions in the on-axis region of the discharge when the discharge radius is small, which leads to a concept for a design of a volume-production based source as a matrix of small radius discharges. The variation in the negative ion density with changing gas pressure and electron density at the discharge axis is also analyzed.

Negative hydrogen ion maintenance in small radius discharges: Two-dimensional modeling

“…2͑a͔͒ display strong accumulation of the negative ions at the discharge center. The radial accumulation of the ions at the axis ͑at r =0͒ confirms the results from the previous 1D models, 25,26 which show that small radius discharges ͓with radius of 2-3 cm͔ are efficient sources of volume-produced negative ions sustaining high density of the ions in their on-axis region. In addition to the results from the 1D models, the present 2D model shows that the ions are also axially accumulated ͓Fig.…”

“…Based on the results from the 1D models of such a discharge 25,26 showing high concentration of the negative ions when the discharge radius is small ͑R =2-3 cm͒, the radius of the discharge is fixed at R = 2.25 cm ͑Fig. 1͒.…”

“…On the other hand, recent one-dimensional ͑1D͒ models 25,26 of hydrogen discharges reveal the presence of high concentration of negative ions at the discharge axis in a single-chamber rf driven source, and the lower the radius, the stronger the effect is, with an optimum at radii of 2-3 cm. The reason for this accumulation of the ions in the on-axis region of the discharge is in the flux-in the radial dc electric field-of the negative ions produced all over the discharge cross section and in the possibility for the ions to reachwithout being destroyed in collisions-the discharge center, which is only possible when the discharge radius is small.…”

“…This study is an extension of the previous 1D models 25,26 toward 2D modeling of small radius hydrogen discharges. At the current stage, diffusion-controlled discharges are considered.…”

Semi-analytical modeling of the NIO1 source

“…The hypothesis of a single ion species is generally not true, especially in molecular gases, but this approximation, also made in previous works [1] is kept in order to simplify the problem. The gas pressure and temperature are considered as external parameters (in particular p gas = 1 Pa and T gas = 400 K unless otherwise stated), thus defining the gas density; the ion temperature is taken equal to the neutral gas temperature, T ion = T gas [2].…”