Plasma structures of single and multiple double layers (Fireballs)

The plasma Double Layers are charge structures that develop into the plasma volume. A plasma potential jump connects two different plasmas and accelerates the charged particles of opposite signs. This situation is represented in the drawing and an actual experiment with double layers is shown in the photograph.

The acceleration of electrons upstream increases their collisions with neutral atoms of the gas where a fraction of electron energy is transformed in visible light. The result is the development of bright structures within the plasma volume visible to the naked eye, also denominated fireballs in the scientific literature.

In this experiment, a metallic plate is positively polarized with respect to an ambient plasma (evidenced by the blue background glow) and in consequence an electron current is drained. The bright plasma cloud attached to the plate develops for bias voltages exceeding a critical onset where the drained current by the plate exhibits a sudden jump. The collected current by the plate becomes greatly increased because additional charges are produced by local ionization of the background neutral gas inside the plasma cloud.

The double layer plasma potential jump is located in the border of the bright cloud glued to the metallic plate and its thickness is very small, in the order of a fraction of millimeter. On the contrary, the bright plasma cloud is typically of 1-2 centimeters in diameter, as it can be observed in the photograph.

The plasmas have been generated in the laboratory by means of electric discharges since early times of Plasma Physics research. The glow discharges are produced by applying a high voltage between two electrodes placed into a low pressure wessel.

In spherical glow discharges with spherical symmetry, successive double layers may be produced forming a stair step plasma potential spatial profile. In the experiment of these photographs, a glow discharge was produced between the walls of a metallic plasma chamber and the inner surfaces of a cavity formed anode that is located at the extreme of the mast that can be observed in the pictures. The electron current is focused into this electrode and each bright plasma shell corresponds to a different plasma separated by double layers. The plasma potential jumps are precisely located at the borders that separate each glowing plasma shell, where the bright changes. According to the different experimental conditions, the number of plasma shells changes always holding this hemispherical symmetry around the collecting electrode.

For further reading: "A review of recent laboratory double layer experiments". C.Charles. Plasma Sources Science and Technology. 16, (4) pp. R1-R25, (4) (2007). doi: 10.10088/0963-0252/16/4/R01