A Dual Recording, Variable Range, Rapid-Scan Spectrometer; Comparison of Simultaneously Recorded Stark-Broadened Hα and Hβ Line Profiles

An ultrarapid scanning spectrometer has been constructed that records spectra at a constant angle of diffraction. The spectral scanning is accomplished by the rotation of a high-speed rotating mirror (7.6 cm diam) that causes a collimated light beam to sweep across a plane grating, changing the angle of incidence. The full-aperture scanning range depends upon the over-all grating width, upon the grating spacing, and upon the orientation of the grating with respect to the high-speed mirror. For example, a 12.5-cm, 1200-lines/mm grating gives a scanning range of approximately 2000 A on each of two separate detector systems, or a total range of 4000 A that can be centered at any wavelength between 4000 A and 8000 A. This grating provides a theoretical resolving power of approximately 85,000 in the first order, a reciprocal dispersion of 17.5 A/mm at f/5, and a spectral scan speed of 115 A/microsec at 1100 rps. This spectrometer has been used to record, simultaneously, Stark-broadened Halpha and Hbeta emission line profiles from a transient plasma generated in the impulse tube. Electron densities calculated from the half-width of the Hbeta profiles are in good agreement with values obtained by fitting the line wings of the self-absorbed Halpha profiles to the theoretical profiles of Griem, Kolb, and Shen.