The results of experiments on the x rays emitted by an energetic vacuum spark are presented. X-ray spectroscopic studies show, in addition to the characteristic x-ray lines of the electrode metal, lines originating from very high stages of ionization of the element. In particular, lines of the 1s2-1snp sequence in the helium-like spectra Ti XXI through Ni XXVII, and the 1s-2p transition in Fe XXVI, were observed. Pinhole photography and time-resolved studies of the total x-ray emission indicate that these highly ionized species originate in a small, hot region between the two electrodes, which is produced by a pinch in a plasma of evaporated anode metal.
LEONARD COHEN, URI FELDMAN, MARVIN SWARTZ, and J. H. UNDERWOOD, "Study of the X Rays Produced by a Vacuum Spark," J. Opt. Soc. Am. 58, 843-846 (1968)