On constriction and striation of a diffuse DBD in large gap filled with moderate pressure helium diluted by the air and easily ionized impurity

The experimental results on studying the plane-to-plane dielectric barrier discharge (DBD) excited in a very large volume (120 x 120 x 120 mm(3)) are presented. The DBD was powered by sinusoidal voltage with a frequency of 100 kHz. The plasma-forming gas is helium, which is slightly diluted by the air and trace impurity of an easily ionized gas. The total pressure of the working gas was varied within of 20-100 Torr. A moderate gas pressure allowed one the creation of non-typical DBD with a very long plasma column of 12 cm in length. Such a long gap DBD exhibits new features in its behavior compared to the well-known DBD in short gaps of several millimeters in length. Immediately after the discharge ignition, a long and uniform diffuse plasma column occurs that occupies the entire space of the inter-electrode bulk. However, this transversally homogeneous plasma mode is unstable. The ionization-thermal instability develops in the plasma owing to gas heating by the discharge. Therefore, the transversally uniform plasma breaks into several separate narrow current channels that move chaotically relative to each other. After a while, they eventually integrate into a single constricted plasma column located in the center part of the discharge volume. At the very beginning of its formation, the constricted plasma column is longitudinally uniform. But soon, it becomes streated. The strata are almost standing and their shape differs from that in a DC glow discharge at low pressure in a sealed tube. The revealed features in the slow spatial-temporal behavior of the long gap DBD are discussed with the involvement of mathematical analysis describing the small plasma disturbance development in the linear approximation.