The electric field in the cathode fall region of an Ar hollow cathode discharge has been measured by the use of optogalvanic spectroscopy in a gas pressure range from 1 to 6 Torr and at a discharge current from 12 to 20 mA. The radial variation of the field strength follows the mathematical expression derived from the space charge distribution function of ρ(<i>r</i>) = −<i>Ar</i> + <i>B,</i> where <i>r</i> is the radial distance from the cathode axis. In this equation the parameters <i>A</i> and <i>B</i> have been found by curve fitting to follow the formula <i>B</i> = <i>aA</i> + <i>b,</i> in which <i>a</i> is constant at −2.57 ± 0.1 mm when <i>A</i> and <i>B</i> are expressed in units of C/m<sup>4</sup> and C/m<sup>3</sup>, respectively, while <i>b</i> is directly related to the gas pressure. In addition, the current- and pressure-dependent behavior of the cathode fall voltage and the electric field at the cathode surface have been examined.

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