Determination of the EEDF by Langmuir probe diagnostics in a plasma excited at ECR above a multipolar magnetic field

In order to better understand the mechanisms of plasma production above multipolar magnetic fields via electron cyclotron resonance, the electron energy distribution function (EEDF) of an argon plasma in the magnetic field of a planar magnetron-like structure is determined by using optical emission spectroscopy and a cylindrical Langmuir probe. After a brief recall of the validity conditions for probe measurements in a magnetic field, probe characteristics generally allow the determination of the whole EEDF while emission spectroscopy can only provide the integral of the distribution function above the threshold energy of the selected optical transitions. The probe results show that the EEDF in fact appears as the sum of two Maxwellian electron populations. The first one is the population of fast electrons, accelerated at electron cyclotron resonance and which produces the plasma, and the second one corresponds to the cold, plasma electrons produced by the fast electrons. The variations in the parameters which characterize these two electron populations, i.e. density and electron temperature, as a function of the position in the multipolar magnetic field clearly demonstrate that the fast electrons remain trapped in the magnetic field close to the multipolar structure while the population of the slow, cold plasma electrons diffuses away from the magnets with a nearly constant electron temperature. The variations as a function of external parameters, gas pressure, microwave power, microwave frequency or magnetic field configuration are also discussed. In all cases, the maximum of optical emission corresponds to the region in the magnetic field where the fast electrons, accelerated at electron cyclotron resonance, are trapped and oscillate within two field lines between two mirror points in front of two adjacent poles of opposite polarity. Finally, simulation of the plasma production, as deduced from the experimental values, allows the determination of the ionization frequency of fast electrons. The results are perfectly consistent with Literature data on magnetron plasmas.