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.
Suppression of Radial Divergence of Extremely Low Energy Ion Beam by an Electron Beam Injection to a Grounded Electrode
Transition to a highly focused state is observed in a hydrogen ion beam extracted from an ion source using concave-shaped electrodes. The beam has a low energy (∼95 eV) and a high current (∼60 mA) with a ∼60 mm diameter. The transition is induced by electron beam injection (1 keV and up to 50 mA) to a grounded electrode of an ion source from a direction obliquely opposite to the ion beam velocity. Probably, secondary electrons emitted from the electrode play an effective role in cancelling the beam ion space charge, since they are produced in the core region of ion beam immediately after the extraction. Comparison between the cases with and without an electron beam exhibits great improvements not only in a total ion beam current reaching a target, but also in the focusing of ion beam. The sudden transition to this highly focused state, which is triggered by a low electron beam current (∼9 mA), suggests that some kind of a nonlinear process plays an essential role in this focusing.
ELECTROMAGNETIC DESIGN OF A RADIO FREQUENCY QUADRUPOLE FOR THE FRONT END TEST STAND AT RAL
The goal of the RAL front end test stand is to demonstrate cleanly chopped bunches of a 60mA H − ion beam at 3MeV. The acceleration of the H − ions from 65keV to 3MeV will be done using a radio frequency quadrupole (RFQ) operating at a resonant frequency of 324MHz. The two types of RFQ considered were a 4-vane and a 4-rod. The 4-vane has a higher Q-value but the post-production adjustment is limited. The 4-rod design is easier to manufacture but requires complicated cooling at 324MHz. The results of electromagnetic simulations using CST Microwave Studio are presented for the 4-vane type and 4-rod type RFQ.