EUV spectra of highly-charged ions W-W relevant to ITER diagnostics

We report the first measurements and detailed analysis of extreme ultraviolet (EUV) spectra (4 nm to 20 nm) of highly-charged tungsten ions W to W obtained with an electron beam ion trap (EBIT). Collisional-radiative modelling is used to identify strong electric-dipole and magnetic-dipole transitions in all ionization stages. These lines can be used for impurity transport studies and temperature diagnostics in fusion reactors, such as ITER. Identifications of prominent lines from several W ions were confirmed by measurement of isoelectronic EUV spectra of Hf, Ta, and Au. We also discuss the importance of charge exchange recombination for correct description of ionization balance in the EBIT plasma. PACS numbers: 32.30.Rj, 32.70.Fw, 31.15.Am, 52.50.Hv Submitted to: J. Phys. B: At. Mol. Opt. Phys. EUV lines from W ions 2 Future fusion reactors, such as ITER, are to reach temperatures of about 20 keV to 25 keV [1]. While light elements (e.g., D, T, and C) will be completely ionized in the plasma core interior, heavy impurity ions will still possess a number of electrons. This could result in strong line emission, primarily in the x-ray region. Such radiation is the source of power losses that represent one of the major concerns for sustainable fusion. On the other hand, high resolution x-ray spectroscopy of impurities offers reliable measurements of important plasma parameters such as ion temperature Ti, rotation velocities in toroidal and poloidal directions, and electron temperature Te [2]. Among all possible impurities in ITER, tungsten is expected to be the most abundant, since according to current planning the front surface of the divertor will be made of this element. Extreme ultraviolet (EUV) line emission of impurities has attracted less attention than the x-ray lines, in part because the flux of emitted photons in the EUV region is typically smaller due to lower transition probabilities. Feldman et al [3] recently pointed out that a number of EUV lines from highly-charged tungsten (ion charge z & 53) may be reliably recorded in the ITER plasma and used for diagnostics of plasma temperature and impurities transport. They proposed, in addition to the traditional grazing incidence spectrometer, a system of segmented multilayer-coated telescopes for registration of EUV lines, similar to what is used in solar corona diagnostics (see, e.g., [4]). Other possibilities for registration of EUV lines in ITER are also discussed in the literature [2]. The objective of the present work is to investigate the EUV spectra from those highly-charged W ions that will be abundant in the plasma of ITER. The measurements of the spectra were performed with the Electron Beam Ion Trap (EBIT) at the National Institute of Standards and Technology (NIST). EBIT is a versatile light source, capable of producing nearly any ion charge state of nearly any element. A fine control of the charge state distribution in the trap is due to a very narrow electron energy distribution function (EEDF) of the beam (width . 60 eV) [5]. A detailed description of the NIST EBIT can be found elsewhere [6]. The ions under study were injected into the EBIT using a multi-cathode, metal vapor vacuum arc (MEVVA) [7] designed for a rapid change of the injected element without downtime or alteration of experimental conditions. The EUV spectra between 4 nm and 20 nm were recorded with a grazingincidence spectrometer described in detail in [8]. The instrument’s resolution is about 350, corresponding to a resolving limit of about 0.03 nm. The W spectra were calibrated with lines of highly-charged ions of Fe, with wavelengths taken from the NIST Atomic Spectra Database [9]. The measured spectra for six nominal electron beam energies EB varying from 8.8 keV to 25 keV are presented in figure 1. A typical value of the beam current was about 150 mA. Unlike our recent work on low-energy EUV tungsten spectra [10], here we did not use a zirconium foil to filter out light from wavelengths above 25 nm; this resulted in significantly higher signal-to-noise ratio near the edges of the spectral range. For easier visual identification of the second-order lines, the shifted spectra (red line) EUV lines from W ions 3 show the same experimental spectra with wavelengths multiplied by 2. The spectra also show a number of impurity lines, mainly from oxygen. The oxygen lines at 13.3 nm, 15.0 nm, 17.2 nm, and 17.3 nm are marked by crosses in the spectrum for EB = 9.3 keV (figure 1). As with our previous studies of x-ray and EUV spectra of tungsten ions with lower charges (z = 37 to 47) [10, 11], identification of the spectral lines relies upon collisional-radiative modelling performed with the non-Maxwellian code NOMAD [12]. The Flexible Atomic Code (FAC) [13], based on the relativistic model-potential method and jj-coupling scheme, was used to generate all relevant atomic data such as energy levels, wavelengths, transitions probabilities, and collisional cross sections for the ions W through W. Previous comparisons with the EBIT tungsten spectra at lower beam energies [10, 11] showed that FAC provides sufficiently accurate results for highlycharged ions. Using the FAC data, NOMAD calculates the ionization balance, level populations, and line intensities for the EBIT conditions. The EEDF in our simulations was modelled as a Gaussian peak with a full width at half maximum of 60 eV, and the electron density was set at Ne = 2×10 11 cm. The calculated spectra were convolved with a Gaussian instrumental function and relative efficiency curve of the spectrometer [8]. The identifications of the spectral lines, the measured and calculated wavelengths, transition types, and calculated radiative decay rates are presented in table 1. The total uncertainties in the wavelengths reported in the table are typically dominated by the uncertainty in the spectrometer calibration, rounded up to one significant digit. An additional statistical uncertainty due to fitting the reported line centers has been added in quadrature. To facilitate classification, for each state we also report a calculated level number in the energy-ordered list of levels within the corresponding ion stage. All identified spectral lines correspond to ∆n = 0 transitions within the n = 3 shell. The LS-coupling identifications of the upper and lower levels that were obtained using the Cowan code [14] are only approximate due to strong spin-orbit interaction. A straightforward method to match experimental and calculated line intensities consists in varying the theoretical beam energy until a reasonable agreement is achieved. While the calculated wavelengths agree well with the measurements, our modelling required significantly lower beam energies to reproduce the observed relative line intensities. For instance, for the highest nominal beam energy of EB = 25 keV, the best-fit theoretical energy was only 10.5 keV, while for EB = 8.8 keV the fit energy was about 6.2 keV. Although space-charge effects in the trap may effectively reduce the beam energy, for the present values of the nominal voltage and beam current this would result in a less than 300 eV shift. Calculation of ionization balance in an EBIT plasma was a subject of several studies (e.g., [15, 16, 17, 18] and references therein). Each ion is normally modelled as one state with no internal structure, and approximate formulas, such as Lotz formula for ionization cross sections, are used for description of atomic processes. In constrast, our collisional-radiative modelling emphasizes the atomic physics component of simulations EUV lines from W ions 4