VUV Radiometry with Hydrogen Arcs. 1: Principle of the Method and Comparisons with Blackbody Calibrations from 1650 A to 3600 A.

A method is described that utilizes the continuum emission from a wall-stabilized arc discharge as a radiometric standard in the vuv. Ultimately, this standard will cover the wavelength range from 500 A to 3600 A. Results of a first experiment comparing this method to two other calibration methods in the region above 1650 A are presented. A calibrated tungsten strip lamp is used between 2500 A and 3600 A the method of blackbody limited lines is applied at two wavelengths in the vuv. The hydrogen arc method depends upon the fact that the continuum emission coefficient for a hydrogen plasma at typical arc temperatures of about 14,000 K is calculable to within a few percent since the essential spectroscopic constants, continuum absorption coefficients, and transition probabilities are exactly known. The accuracy of the method depends primarily on the capability of spatially resolving in an end-on measurement the nearly homogeneous plasma layers near the axis of the cylindrically symmetric arc column.