Suppression of Radial Divergence of Extremely Low Energy Ion Beam by an Electron Beam Injection to a Grounded Electrode

Abstract

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.

Measurements of beam current density and space potential in a highly focused ion beam of extremely low energy

“…The experimental apparatus (figure 1) includes an ion source, extracting electrodes, an ion-beam propagation chamber, and measurement systems including a pair of double probes and three Faraday cups. The setup is almost the same as that used in our previous experimental work [4][5][6][7][8].…”

“…A weak electron beam with a beam current of I eb ∼3 mA and energy E eb =1 keV is injected on the surface of the ground electrode (the final electrode that faces the propagation chamber) in the direction which is nearly opposite to the ion beam direction [7]. The electron beam is extracted from the LaB 6 cathode with a mesh grid and is focused using several focusing electrodes to inject into the central region of the electrode.…”

“…In experiments, focusing of the low-energy ion beam with high intensity (less than 200 eV and up to 100 mA) was obtained by injecting an electron beam (∼2-10 mA and 1 keV) into the final electrode (the ground electrode) in the ion beam source [7]. This electrode faces the propagation chamber of the ion beam.…”

“…To survey the possible mechanisms underpinning the rapid transition to the highly focused state, we improved to some extent measurement taking and the methods of analysis from previous work [4][5][6][7][8]. We tried selective use of a double probe, in which one electrode (P 3 ) is shielded by another electrode (P 1 ) with respect to the ion beam trajectory.…”