glowdischarge

Glowdischarge (commonly glow discharge) is a type of plasma formed when an electric current passes through a low-pressure gas between electrodes. It is characterized by visible luminescence and distinct spatial regions, including cathode dark spaces, the negative glow, Faraday dark space, positive column, and anode glow. The appearance and color of the discharge depend on the gas composition and excitation states of atoms and molecules.

The discharge is sustained by electron-impact ionization and secondary electron emission from the cathode. Free electrons accelerated in the electric field ionize neutral atoms, producing ions that drift to the cathode and liberate additional electrons on impact, creating a self-sustaining cycle. Most of the voltage drop occurs across thin cathode sheaths, while the bulk plasma remains quasi-neutral. Typical operating pressures range from millitorr to a few torr, with voltages and currents set to maintain a glow rather than an arc; if current becomes too large, the discharge transitions to an arc with higher temperature and lower voltage.

Glow discharges have many applications: lighting (neon and fluorescent lamps), thin-film deposition and sputter cleaning, materials analysis (glow discharge optical emission spectroscopy and mass spectrometry), surface modification, and plasma etching. They are also used in laboratory ion sources and plasma physics studies. Practical considerations include gas choice, pressure control, electrode materials, and safety measures for high voltage and ultraviolet emissions.