Effects of a dielectric material in an ion source on the ion beam current density and ion beam energy.
To understand a strong focusing phenomenon that occurs in a low-energy hydrogen ion beam, the electron temperature, the electron density, and the space potential in an ion source with cusped magnetic fields are measured before and after the transition to the focusing state using an electrostatic probe. The experimental results show that no significant changes are observed before or after the transition. However, we found unique phenomena that are characterized by the position of the electrostatic probe in the ion source chamber. Specifically, the extracted ion beam current density and energy are obviously enhanced in the case where the electrostatic probe, which is covered by a dielectric material, is placed close to an acceleration electrode.
Self-focusing of a high current density ion beam extracted with concave electrodes in a low energy region around 150 eV.
Spontaneous self-focusing of ion beam with high current density (Jc ∼ 2 mA/cm(2), Ib ∼ 65 mA) in low energy region (∼150 eV) is observed in a hydrogen ion beam extracted from an ordinary bucket type ion source with three electrodes having concave shape (acceleration, deceleration, and grounded electrodes). The focusing appears abruptly in the beam energy region over ∼135-150 eV, and the Jc jumps up from 0.7 to 2 mA/cm(2). Simultaneously a strong electron flow also appears in the beam region. The electron flow has almost the same current density. Probably these electrons compensate the ion space charge and suppress the beam divergence.
Ion beam extraction with ion space‐charge compensation in beam‐plasma type ion source
The beam‐plasma type ion source that we have developed has an original ion‐ beam extraction mechanism. An ion beam is formed with an ion space‐charge compensation by electrons which are distributed by force in the extraction region. The electrons are used in order to ameliorate an ion space‐charge effect, which might be an obstacle for high‐current ion‐beam extraction. The space charge of ions is effectively neutralized by many kinds of energetic electrons; that is, a primary electron beam, a secondary electron beam, and high‐energy plasma electrons. When the ion space‐charge compensation by these electrons is effective, theoretically, the divergence of an ion beam is suppressed, the space‐charge limitation of an extractable ion current is relaxed, and the shape of the ion‐emission surface is improved. Therefore, a high‐current ion beam with good quality can be extracted, even at a low‐extraction voltage. In the experiment, the current of more than 0.1 A at the extraction voltage of 1–5 kV, which was more...