Cathode configuration changes mode structure of transparent cathode discharges

A transparent cathode discharge is a self-sustained direct-current plasma that occurs at low pressures in inertial electrostatic confinement devices with a hollow grid cathode.

Depending on certain conditions, the generated plasma may organize in different modes of operation, which appear as different patterns of optical emission. Little previous research has examined the mechanisms underlying the various transparent cathode discharge modes. To better understand these mechanisms, Hardiment and Bowden investigated the influence of cathode configuration.

The authors studied an open and enclosed cylindrical hollow grid cathode. When they closed the open cathode end with wire gridwork, the low-pressure range for one mode, known as the cathode-confined mode, extended into the range at which a different mode, called the beam mode, usually occurs.

The authors found the cathode-confined mode is sustained by the hollow cathode effect and may occur at a significantly lower pressure due to the uniformity of an enclosed cathode. Overall, their results confirm cathode geometry influences the mode structure of transparent cathode discharges.

“We show how a clearly-recognizable hollow cathode-type mode will occur widely for transparent cathode discharges,” said author Tom Hardiment. “This work also adds to the wider literature on hollow cathode discharges in modified configurations.”

Enclosing the cathode also changed the appearance of additional optical features that occur in different modes. Different potential distributions cause a plume-shaped glow to appear for the cathode-confined mode, and an axial beam for the beam mode. When the cathode is closed, these extend through a grid hole to contact the interior glow and may improve understanding of charge diffusion within the cathode.

Next, the authors will study different modes and continue to explore applications for transparent cathode discharges.

Source: “Influence of cathode grid geometry upon mode structure of a transparent cathode discharge,” by T. Hardiment and M. D. Bowden, Physics of Plasmas (2020). The article can be accessed at https://doi.org/10.1063/1.5143310 .