Langmuir probe interpretation for plasmas with secondary electrons from the wall

Abstract

Summary form only given. In a low-pressure hot-filament discharge, the primary electrons induce the emission of secondary electrons from the walls of the plasma chamber. The generation rate of these secondary electrons typically exceeds the rate of ionization. The plasma thus has three electron components: fast electrons from the filament, secondary electrons from the walls, and plasma electrons confined in the potential well of the plasma. The performed experiments show that the latter two components make most of the electron current to a cylindrical Langmuir probe immersed in the plasma. It is shown that this current to the probe may be accurately fit by orbit-motion-limited probe theory applied to both the secondary and the bulk electrons. The analysis takes into consideration that the secondary and plasma electrons have different reference potentials. The secondary electrons originate from the grounded walls and the plasma electrons are born at the more positive plasma potential. A method to evaluate probe traces is presented that allows the separation of the two components and to calculate their parameters. The secondary electrons have a distribution close to a Maxwellian with a temperature /spl sim/2-3 eV. It is shown that the probe current of the bulk electrons alone goes to zero for probe potentials more negative than the wall potential, indicating that there are indeed no bulk electrons with energies exceeding the ambipolar potential.