The photo shows electric discharges in air at an insulating surface. The experimental setup is used for testing the resistance of different insulation materials against discharges.
The test object is a plate of insulating material (usually 1-3 mm thick) placed on top of a disc-shaped ground electrode. A high voltage electrode is placed just above the test object (1 to 2 mm). When energized, the surface of the test object is subjected to electrical discharges, which over time will break down the insulation material. The test is used both under normal conditions and pressurized to a few bar gas pressure.
The method was developed in the project: “Electrical Insulation materials and Insulation Systems for Subsea High Voltage Power Equipment“, funded by the Norwegian Research Council and Norwegian and foreign industry. (The model in the photo is slightly modified to make it more suitable for photography; the method is based on the CIGRE Method II).