Extreme Ultraviolet Spectra of Few-Times Ionized Tungsten for Divertor Plasma Diagnostics

Abstract

The extreme ultraviolet (EUV) emission from few-times ionized tungsten atoms has been experimentally studied at the Livermore electron beam ion trap facility. The ions were produced and confined during low-energy operations of the EBIT-I electron beam ion trap. By varying the electron-beam energy from around 30-300 eV, tungsten ions in charge states expected to be abundant in tokamak divertor plasmas were excited, and the resulting EUV emission was studied using a survey spectrometer covering 120-320 Å. It is found that the emission strongly depends on the excitation energy; below 150 eV, it is relatively simple, consisting of strong isolated lines from a few charge states, whereas at higher energies, it becomes very complex. For divertor plasmas with tungsten impurity ions, this emission should prove useful for diagnostics of tungsten flux rates and charge balance, as well as for radiative cooling of the divertor volume. Several lines in the 194-223 Å interval belonging to the spectra of five-and seven-times ionized tungsten (Tm-like W VI and Ho-like W VIII) were also measured using a high-resolution spectrometer.

Extreme ultraviolet spectra of multiply charged tungsten ions

“…Figure 1 shows the EUV spectra obtained at electron beam acceleration voltages of 100 to 280 V. For simplicity, here we refer to the acceleration voltage (multiplied by the elementary charge) as "electron energy" although the actual electron energy is generally lower than the acceleration voltage by few to few tens of eV. From the comparison with the previous observations [4,6], emission lines at 21.6 and 26.1 nm observed at 90 eV can be identified to be 5p 1/2 − 5d 3/2 and 5p 3/2 − 5d 5/2 lines in W 6+ , respectively. The features observed at around 20 nm in the 115 eV spectrum (the lines indicated by the square bracket) are consistent with those observed in the spectra obtained by Clementson et al with the Livermore EBIT [4], and thus can be assigned to 5p − 5d and 5p − 6s lines from W 7+ by following their identification.…”

“…In particular, it has been pointed out that the diagnostics and control of the edge plasma are extremely important for the steady state operation of high-temperature plasmas [2,3]. Thus the atomic data of relatively low charged tungsten ions are of great significance to the ITER plasma diagnostics [4]. Nevertheless, only few experimental data are available at present.…”

“…W 6+ and W 7+ , experimental data obtained with spark plasma are available for the vacuum ultraviolet to extreme ultraviolet (EUV) region [6,7,8]. Recently, observation with an electron beam ion trap (EBIT) has also been done [4,9]. For higher charge states, e.g.…”

Guest Editor’s Notes on the “Atoms” Special Issue on “Perspectives of Atomic Physics with Trapped Highly Charged Ions”

“…A low electron beam energy thus enables the selection of a low maximum charge state, and a controlled variation that adds one charge state at a time to a spectrum excited. The contribution by Joel Clementson, Tomas Lennartson and Peter Beiersdorfer to this special issue [25] pushes the low electron beam energy range even further down (this time for the EBIT-I device), to about 30 eV. At such low energies, the achievable electron beam current is low, too, but the excitation cross sections are favorably large so that the photon signal can still be sufficient for statistically meaningful data.…”

“…By appropriately small beam energy increments, one can track the appearance of new lines and associate them with specific ion charge states, and all that under low-density conditions. This technique has been exercised before especially for iron [24], and it has now been employed by Clementson et al for tungsten, as is described in their contribution to this Special Issue [25]. (Of course, electron beam ion traps are not the only tools used in this research; this journal carries a Special Issue on Atomic Data for Tungsten [26] which describes also very different research methods.…”