The electric field above the cathode surface of an Ar hollow cathode discharge has been measured by optogalvanic Stark spectroscopy. The field depends on the radial position of illumination and describes the radial distribution of the space charge density, ρ(<i>r</i>), as ρ(<i>r</i>) = −<i>Ar</i> + <i>B</i>, where <i>r</i> is the radial position and the parameters <i>A</i> and <i>B</i> are positive. The values of <i>A</i> and <i>B</i> increase with the discharge current, but the width of the cathode fall region remains nearly constant at about 1.02 ± 0.03 mm for a brass cathode of 5 mm i. d.
Takashi Masaki, Akihide Wada, Yukio Adachi, and Chiaki Hirose, "Space Charge Distribution in the Cathode Fall Region of an Ar Hollow Cathode Discharge," Appl. Spectrosc. 42, 49-51 (1988)