Investigation of the current density properties of an ion beam extracted from a low pressure wire discharge

Summary form only given. In sources are nowadays used for numerous applications. The various requirements - in terms of beam energy, type of particle, beam spot size - of these applications led to diverse ion production physical mechanisms: glow discharge, laser, microwave, cyclotron resonance, hollow cathode... Nevertheless, this wide range of concepts makes ion sources capabilities quite specific and thus hardly transposable from one application to another. An ion source derived from a wire plasma source [1], in which ions are extracted from the plasma source and then accelerated by means of a biased electrode in a way much similar to secondary electron guns [2], have been shown to generate narrow band, variable energy beams of various ionic species [4]. One of the main advantages of this system is the ability to finely tune the beam energy on a wide range (200 eV to 5 keV) by varying the accelerating electrode bias, and that without increasing the beam energy spread (-10 eV). In this paper, we investigate the properties of the axial ion beam current density of such an ion source. RPA measurements show that the axial current density exhibits a maximum for a given accelerating voltage which depends on the total ion beam current. This behavior is reproduced numerically by coupling the potential and ionization fields inside the wire plasma source obtained by 2d/3v PIC simulations with a 3D electrostatic particle tracking algorithm. Numerical simulations underline the influence of the space charge electric potential existing inside the WIPS. Besides, by modeling the accelerating stage of the system as an electrostatic lens, the accelerating voltage corresponding to the current density maximum is identified as a trade off between the current extracted from the source and the divergence of the beam imposed by the electrostatic lens focal length. Such an understanding of the beam current density properties would allow an optimization of the system such that the nominal operating conditions for a given application would coincide with the maximum current density.