Equilibrium states of anodic double layers

Abstract

Equilibrium states of anodic double layers that form near a positively biased disk-shaped electrode immersed in a partially ionized plasma are studied experimentally using electrostatic probes. When the potential drop from the electrode to the plasma is less than a critical value, φ c , an anode glow is observed where a double layer potential drop is within a few millimeters of the electrode surface. For larger biases an anode spot forms where the double layer potential drop is centimeters from the electrode and the intervening region is a plasma more luminous than the bulk plasma. A theoretical model is developed which predicts that φ c ∝ 1/P + C, where P is the neutral pressure and C is a constant, and it predicts hysteresis in the current-voltage characteristic at the electrode; both effects are observed experimentally. The model also provides an estimate for the distance between the electrode and double layer potential drop that agrees with the measurements. Near small electrodes, anode spots are observed to be 'fireballs,' which are spherical in shape. Near larger electrodes 'firerods' are found instead, which have a cylindrical shape. It is shown that firerods are required by global current balance because they have a smaller effective electron collecting area than fireballs. Experiments also confirm that global nonambipolar flow (Baalrud S D et al 2007 Phys. Plasmas 14 042109) accompanies firerods. In this case all electrons are lost through the firerod to the electrode, while all positive ions are lost to the other plasma boundaries.

Properties of highly electronegative plasmas produced in a multipolar magnetic-confined device with a transversal magnetic filter

“…Positive ion sheath formed when applying a negative bias with respect to plasma potential, V pl , has been extensively investigated starting with the pioneering work of Langmuir. Recently, there is also a growing interest in electron sheath [15] and its transition to anodic glow [16][17][18]. However, there is very limited understanding of the negative charge sheath formed by both electron and negative ion.…”

Measurement of the sheath capacitance of a planar probe

“…11 In our experiments, we also observed that the sheath capacitance was negative when the voltage of the planar probe was biased near the plasma potential. This result was due to the creation of a double sheath, [11][12][13][14][15][16] and this tendency can be explained using Godyak and Sternberg new boundary condition at the sheath edge.…”

Solid components separation from heterogeneous mixtures through turbulence control

“…Here, the mixed modes theoretically described in the above sections were experimentally observed in a complex low-temperature plasma in which a space charge structure develops in form of a fireball [63,[74][75][76][77][78]. The fireball was produced by positively biasing a tantalum disk electrode with 1 cm in diameter, introduced into a discharge plasma under the following experimental conditions: argon pressure p = 7 × 10 −3 mbar, plasma density n pl ≅ 10 8 − 10 9 cm −3 and the electron temperature kT e = 2 eV [74][75][76][77][78].…”

“…The fireball was produced by positively biasing a tantalum disk electrode with 1 cm in diameter, introduced into a discharge plasma under the following experimental conditions: argon pressure p = 7 × 10 −3 mbar, plasma density n pl ≅ 10 8 − 10 9 cm −3 and the electron temperature kT e = 2 eV [74][75][76][77][78]. The fireball suddenly appears in front of the electrode at a critical value of the voltage applied to it V E = 85 V, being in adynamic state, fact emphasized by the presence of the oscillations of the current collected by the electrode (see Fig.…”