Forbidden line wavelengths and transition probabilities measured using an electron beam ion trap (EBIT)

Several coronal lines posed a long-standing riddle to earth-bound spectroscopists, until - following up on a suggestion by Grotrian (1937) - B. Edlen (1942) confirmed that their wavenumbers indeed corresponded to fine structure intervals in the ground configurations of highly charged ions like Fe X and Fe XI. This in turn caused turmoil in solar physics, because the corona must be much hotter than the underlying chromosphere in order to produce such ions. X-ray and EUV spectra of the sun became available after World War II, by observations from sounding rockets and satellites. These spectra confirmed the presence of the highly charged ions. Laboratory observation of the (electric-dipole) forbidden lines, however, had to wait for the development of low-density plasma discharges like the tokamak fusion experiments, because in regular light sources, collisions would likely quench such long-lived levels. Since then, a fair number of forbidden transitions has been observed in the laboratory, and forbidden lines are being valued for plasma diagnostics. While forbidden transitions in light ions are often found in astrophysical light sources, similar transitions in highly charged heavy ions like Kr will be important for plasma machines like ITER, in which Kr will likely be used for radiativemore » cooling and will therefore also be available for detailed diagnostics.« less