Investigation of Radiofrequency Ion Heating in the magnetoplasma of an ECR ion trap

Abstract

The Electron Cyclotron Resonance Ion Sources (ECRIS) are nowadays the most effective devices to provide relatively intense currents for highly charged ions devoted to particle accelerators for nuclear science and applications. In ECRIS, the main mechanism of microwave-to-electrons energy transfer is the Electron Cyclotron Resonance heating, when electrons gyrofrequency $\boldsymbol{f}_{\boldsymbol{ce}}=\boldsymbol{eB}/(2\boldsymbol{\pi} \boldsymbol{m}_{\boldsymbol{e}})$ equals the frequency $\boldsymbol{f}_{\boldsymbol{\mu} \boldsymbol{wave}}$ of the injected microwaves. In ECR plasmas, because of the low electron-ion collision cross-section, the ions remain cold (few eV or less). A direct mastering of the ion temperature through Ion Cyclotron Resonance Heating (ICRH) could be relevant both for improving the performances of the ECRIS as well as for fundamental Physics. In this latter case, the aim is to investigate nuclear decays as a function of the ionization state or the ion temperature. In this paper, the modeling of Radio Frequency (RF) wave-plasma interactions in the ICRH range is investigated for the first time in a compact ECR, B-minimum plasma trap. RF field computation - based on a 3D full-wave FEM-based code - is able to predict wave propagation and power absorption in a non-uniform “cold” anisotropic plasma bounded by a metallic resonator and immersed in a magnetic configuration typical of ECR ion sources. Moreover, some technological aspects and a conceptual design of the RF antenna and related systems delivering multi-kilowatts of power to the plasma ion component are discussed.