Characterization of the electron distribution function in an electron-cyclotron driven toroidal plasma

The local properties of the electron energy distribution function are investigated in the electron-cyclotron wave driven toroidal device TORPEX (Fasoli et al 2006 Phys. Plasmas 13 055902). The electron energy distribution function is measured by means of a time-resolved electrostatic energy analyser. The measurements indicate that electrons are accelerated to different energies at the electron-cyclotron and upper-hybrid resonances. Electrons from the upper-hybrid resonance have higher energies and can be described by a secondary maxwellian energy distribution function. The electron energy distribution function is found to be invariant with respect to experimental parameters such as the neutral gas pressure, the magnetic field configuration and the injected microwave power. The implications for a correct modelling of the particle source term for TORPEX plasmas are discussed.