Spectroscopic determination of the cold electron population in very low pressure ECR discharges in N2/He mixtures

We describe a spectroscopic study of the electron distribution functions (EDFs) in two types of electron cyclotron resonance (ECR) discharges at very low pressure: a discharge in a simple magnetic mirror trap (ECRD) and a discharge in a sheared minimum-B-trap used as ion source for multiply charged ions (ECRIS). The discharges were run in mixtures of helium and nitrogen at about 7 mPa. Special attention was paid to electrons with kinetic energies below 10 eV. Their contributions were attained by a quantitative analysis of the vibrational structures in the emission spectra of nitrogen, the analysis of processes of step-wise excitation or an analysis of the rotational populations of nitrogen. The models for evaluating the EDFs are discussed in detail. Besides collision processes in the volume of the discharge processes occurring at the chamber walls contribute significantly to the particle balance and are included in the model. In all cases considered low energy electrons contribute more than 90% to the electron population of the discharges. For every discharge EDFs are measured for two benchmark conditions at low and high discharge power. For the ECRIS other conditions are covered by a measurement of the plasma potential, which turned out to be below 30 V for the conditions considered. This low value is a good indicator for the presence of low energy electrons. The plasma potential was obtained from a retarding field analysis of the energy of ion beams extracted from the plasma. Besides the EDFs, our study yields insight into the important collision and wall processes in these low-pressure discharges, including the first observation of rotational cooling of desorbing N-2 molecules in an ECRIS.