Investigation of the electron energy distribution in an ECR discharge by means of Thomson scattering

In this paper time averaged and time resolved measurements of the electron energy distribution function (eedf) in an argon ECR discharge by Thomson sc attering are reported. Various plasma conditions could be studied using a c lassical 90 degrees -scattering set-up, which collects scattered photons at 11 spectral positions of the spectrum simultaneously so that short data ac quisition times are achieved. Due to the scattering geometry only the energ y distribution perpendicular to the magnetic field could be measured. Our r esults show that in low-density plasmas the eedf significantly deviates fro m Maxwellian for electron energies larger than approximately 20 eV. We foun d a highly-energetic tail whose development can be explained by the heating of the electrons by the ECR process and the trapping of these hot electron s in the magnetic mirror field. The experimental results are compared with results of a numerical simulation of the electrons based on a particle-in-c ell model.