Ion emission from plasma in the presence of charge transfer

Abstract

Usually particles do not collide in the accelerating gap of ion sources, because the gas pressure in the gap should be low to provide a high electric strength. However, the gas discharge systems of the sources may allow for such gas conditions that the ion free path becomes comparable with dimensions of the systems. This is due to the pressure differential between the accelerating and gas-discharge systems of the sources and also to the fact that dimensions of the gas-discharge chambers may by far exceed the length of the accelerating gaps. Given these conditions, the relationships derived for the collisionless regime cannot be used to evaluate the ion current flowing from the discharge system to the accelerating gap. These relationships need be refined allowing for charge transfer. The goal of this study was to ascertain how collisions affect the value of the ion emission current over the pressure interval where the collisionless mode of the ion motion (the ion velocity is determined by the potential difference passed) is replaced by motion in a strong electric field (the ion velocity is proportional to the square root of the electric field intensity).