Two-dimensional spatial distribution and production mechanism of H⁻ ions in two-chamber inductively coupled H₂ discharges

In this work, radio frequency (RF) inductively coupled negative hydrogen ion sources (NHIS) with two-chamber have been investigated using a two-dimensional (2D) self-consistent fluid model. This paper focuses on the influences of gas pressure and expansion chamber radius on the spatial distribution and production mechanism of H⁻ ions. The results show that H⁻ ions are strongly accumulated near the chamber axis, and their accumulation region shifts from the driver chamber where the RF power is deposited to the expansion chamber with the decrease in gas pressure. This can be attributed to the spatial variation of the dc electric field, which pushes H⁻ ions produced over the whole chamber to its minimum intensity location. Furthermore, the spatial variation of the dc electric field is caused by the variation of the spatial distribution of the charged particles. Due to the larger electron energy relaxation length at low pressures, more electrons can reach the expansion chamber before their energy is exhausted. Thus, the spatial distribution of the dc electric field will be changed according to the Poisson equation. In addition, a large expansion chamber radius also favors the shift of the accumulation region of the H⁻ ions toward the expansion chamber. The accumulation of the H⁻ ions in the expansion chamber can reduce the loss of the negative ions, and it also facilitates their survival and extraction. The results in this work could lead to a deeper insight into the spatial profile of the H⁻ density in two-chamber inductively coupled NHIS, which may be very helpful to extract a high density of H⁻ ions.