Speaker
Description
Plasma instabilities, particularly electron driven instabilities, play a critical role in limiting the performance of advanced ion sources used for particle accelerators. In systems such as Electron Beam Ion Sources [1] and Electron Cyclotron Resonance ion sources [2], energetic electrons interacting with magnetized plasmas can excite collective modes due to strong anisotropies in the electron energy distribution, with perpendicular to parallel temperature ratios typically in the range of two to ten, steep density and potential gradients, and intense electromagnetic fields with magnetic field strengths commonly between about half a tesla and six tesla. These electron beam driven instabilities strongly influence energy transport, particle confinement, and charge state evolution. In EBIS devices, high current density electron beams, with beam currents of order one tenth to ten amperes, electron energies of a few to several tens of kiloelectronvolts, and current densities reaching hundreds to thousands of amperes per square centimetre, can drive instabilities that lead to anomalous ion heating to energies of a few electronvolts and reduced confinement times, thereby limiting the achievable charge states, especially for heavy ions. In ECR ion sources, radio frequency generated energetic electrons, with hot electron populations extending from roughly ten to one hundred kiloelectronvolts at microwave frequencies in gigahertz, can excite low frequency drift type instabilities, typically observed in the kilohertz to megahertz range, resulting in enhanced cross field transport and fluctuations in the extracted beam intensity at the level of a few percent. A detailed understanding of electron driven instability mechanisms, including their characteristic frequency ranges and dependence on operating parameters, is therefore essential for optimizing ion source stability and performance.