Radial variation of electron temperature gradient in a double plasma laboratory device

Abstract

Radial control of the electron temperature gradient is demonstrated in a double plasma device by making use of segmented grid biasing. The plasma produced in the source region is allowed to diffuse into the target region through a single grid as well as through the cassette of multiple grid assembly, under different grid bias conditions. Both electron heating and cooling are observed radially at one location in the target region when a single grid is used. The electrons are cooled down to a temperature of 3.3 eV from 5.1 eV when the grid bias is raised from −25 to 0 V. Similarly, during heating, the electron temperature increases from 4.8 to 7.3 eV when the grid bias is varied between 0 and +20 V. Two different transparencies of grids, 45% transparency (mess-size, m = 0.8 mm ∼ λDe) and 75% transparency (mess-size, m = 2.4 mm > λDe), are used, where the value of λDe ≈ 0.8 mm. The obtained electron energy distribution function suggests that a grid with less transparency is more effective in cooling the electrons because of insignificant energetic electron–neutral collisions in the target region as a sheath in the close vicinity of grid allows only the high energetic electrons to pass through it. The higher transparent grid, on the other hand, produces electron heating as it exerts a negligible influence on the free flow of accelerated high energy electrons to target plasma due to insignificant thermalization. We expanded this concept and, for novelty, applied it to a radially segmented grid assembly of electrically isolated grids, for effectively charging different plasma regions with differently various potentials for exerting a radial control on electron temperature. The results obtained show that a significantly sharp electron temperature gradient is obtained with a typical gradient scale length of LTe∼10 cm in the target plasma region. The outcome of this study may be useful both in plasma processing applications and for studying plasma turbulence in unmagnetized plasmas.