ABSTRACT
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The history of spectroscopic studies of highly charged ions (HCIs) goes back to the 1930s. At that time, HCIs were produced using vacuum spark techniques. With such techniques, the charge state distribution was rather broad and not easy to control, but the precise spectroscopic studies of vacuum spark led to the first identification of HCIs in astrophysical plasma, for example, highly charged Fe in the solar corona [1]. These are very important studies as they dispelled the ideas concerning the elements coronium and nebulium and revealed the corona temperature to be several millions of degrees instead of several thousands of degrees believed at the time. Since then, many techniques for producing HCIs have been developed, among which the most successful one was the beam-foil (BF) method. The BF technique was developed independently by Bashkin [2] and Kay [3] and pioneered by several groups. In this technique, an ion beam from an accelerator passes through a thin foil (usually
Ion
a carbon foil), where the ions are stripped and excited. Higher incident ion beam energies will lead to higher average charge state distributions of the ions passing through the foil. Other techniques, such as various spark forms (sliding, vacuum, etc.) and laser-produced plasmas, have also made important contributions to the knowledge of HCI spectra and structure. In recent years, the interest for HCIs has been growing due to the efforts to produce controllable fusion power. HCIs from wall materials would degrade the fusion plasma operation, as the photon radiation of the HCIs exhausts large amounts of the energy of the plasma, and the radiation power increases rapidly with increasing atomic number of the HCIs. On the other hand, the photons emitted from the HCIs provide valuable diagnostics of the plasma temperature, plasma density, and plasma movement. Being both a blessing and a menace for the success of controlled fusion experiments, detailed knowledge of the HCIs involved in fusion plasmas is imperative. Laser plasma spectra often suffer from many charge states, satellite line contamination, and Doppler shift and broadening. BF spectroscopy of HCIs requires access to large accelerator facilities and hence expensive.
