Beam driven plasma instabilities in electron beam ion sources

Electron beam ion source (EBIS) is capable of generating highly charged ions by ionizing neutral gas atoms using a high-energy electron beam. The generated ions are confined axially and radially by electric and magnetic fields within the breeding region until they are extracted by lowering the axial potential barrier. However, in various EBIS devices, significant ion heating has been observed, which can lead to stagnation in the ionization process, particularly for heavier ions. This stagnation imposes a practical limit on the maximum achievable charge state. To investigate plasma heating in an EBIS, the role of plasma instabilities induced by a Gaussian electron beam in a magnetically confined plasma is examined in detail. The resulting growth rates and dispersion relations confirm the formation of instabilities in the EBIS devices, which are dominant in the form of lower hybrid waves and may be responsible for the limited performance of EBIS in terms of generating fully stripped or highly charged ions. The resulting instability frequencies from the theoretical calculations lie close to ion plasma frequency, i.e., in the MHz range.