PHYSICAL REVIEW A 86, 062501 (2012) Experimental and theoretical study of the ground-state M1 transition in Ag-like tungsten Z. Fei, 1,2 R. Zhao, 1,2 Z. Shi, 1,2 J. Xiao, 1,2 M. Qiu, 1,2 J. Grumer, 3 M. Andersson, 1,2 T. Brage, 3 R. Hutton, 1,2,* and Y. Zou 1,2,† 1 The Key Lab of Applied Ion Beam Physics, Ministry of Education, China 2 Shanghai EBIT Laboratory, Modern Physics Institute, Fudan University, Shanghai, China 3 Division of Mathematical Physics, Department of Physics, Lund University, Sweden (Received 13 August 2012; published 3 December 2012) We present an experimental and theoretical study of the 2 F 5/2 → 2 F 7/2 M1 transition in Ag-like W (W 27+ ). The experiments employed the Shanghai permanent magnet electron beam ion trap, which has been developed especially for assisting spectroscopic diagnostics of edge plasmas for magnetic fusion devices. The theoretical value was obtained using the GRASP2K set of computer codes and included a comprehensive correlation study. The experimental M1 wavelength was measured as 3377.43 ± 0.26 ˚ A (3378.43 ˚ A vacuum wavelength), whereas the calculated wavelength is in good agreement at 3381.80 ˚ A. This good agreement shows the importance of fully understanding the electron correlation effects to predict the energy of the fine structure even in this, for tungsten, relatively simple case. DOI: 10.1103/PhysRevA.86.062501 PACS number(s): 32.30.Jc, 32.70.Fw, 31.15.aj I. INTRODUCTION Tungsten, because of its excellent thermomechanical prop- erties and its very low erosion under various physical and chemical conditions, is considered as a strong candidate for the plasma facing material for future fusion devices. But as a heavy element, it exhibits very high radiation power, so a small fraction of W atoms in a fusion plasma could lead to dramatic effects on the plasma temperature and thereby degrade the plasma performance. Much effort is being made by various groups to provide spectroscopic data for W in many charge states and over wide spectral regions [1–3]. Unfortunately, for plasma diagnostic purposes, there is very little spectroscopic data for tungsten in the charge states between W 6+ and W 28+ . The atomic structure for many of the ions in this region is very complicated, and in some cases it is not even possible to easily establish the electronic ground configuration. For example in W 7+ it is not clear whether 4f 13 5s 2 5p 62 F 7/2 or 4f 14 5s 2 5p 5 2 P 3/2 is the lowest level of the ground state—a confusion that arises from the fact that the 4f and 5p orbitals have almost the same binding energy in this ion. The W 27+ ion has a relatively simple 2 F ground state and an M1 line connecting the lowest 2 F 5/2 with the 2 F 7/2 has already been the subject of some theoretical investigations [4–6]. This fine-structure interval is also given in the comprehensive review of available tungsten data by Kramida and Shira [7]; however, as will be discussed below, there are reasons to doubt the value quoted in Ref. [7]. This M1 line is the subject of the work reported here. An M1 forbidden line is also expected to connect the levels in the ground-state doublet, whether 2 F or 2 P , in W 7+ . Accurate measurements of the wavelengths of these lines would give precise values for the ground-state fine structures in these ions. These fine structures would in turn be useful for establishing more reliable theoretical models for these and the more complex close by ions. There are also some previously observed lines from W 13+ in the soft-x-ray region [8]. Similar to the above, accurate wavelength measurements * Corresponding author: rhutton@fudan.edu.cn † Corresponding author: zouym@fudan.edu.cn can help to establish good theoretical models for tungsten ions with charge states around 13+. It is clear that one of the most versatile light sources for studying tungsten spectroscopy, in all of its charge states, is the electron beam ion trap (EBIT); in fact, the data referenced in Refs. [1–3] are all from EBIT facilities. EBITs have been in existence now for over 25 years with the first such device being developed at the Lawrence Livermore Laboratory [9]. Most EBITs use superconducting magnets to provide the field for compressing the electron beam; however, room-temperature EBITs also have a fairly long history [10], where less compression of the electron beam is required. The work presented here focuses on a visible M1 ground- state transition between the 4f 2 F 5/2 and 4f 2 F 7/2 levels in W 27+ . The spectra were recorded using the recently developed Shanghai permanent magnet EBIT (SH-PermEBIT) [11]. We also carried out a theoretical study of the ground-state fine structure of Ag-like ions, including W 27+ , using a revised version of the GRASP2K codes [12]. The fine structure defining the wavelength for this transition has been measured by soft- x-ray spectroscopy [13] and this energy is reported in the compilation by Karamida and Shirai [7] as 33 000 ± 800 cm −1 ; this leads to a wavelength of 3030 ± 75 ˚ A. Recent calculations by Ivanova et al. [4] and Ding et al. [5] give wavelengths for this transition at 3147.8 and 3430.4 ˚ A, disagreements large enough to promote further investigation. The importance of visible forbidden transitions from tungsten ions in ITER plasma diagnostics was discussed in a recent paper by Skinner [14]. Although this M1 line is just outside the visible region of interest reported in Ref. [14], it is important to understand the atomic structure here as a guideline when working with the more difficult W ions in the range of charge states between 7+ and 27+. II. EXPERIMENTAL METHODS We have developed a room-temperature EBIT capable of operating in the range of electron beam energies between 60 and 5000 eV [11]. We can therefore create W ions in charge states from very low, two or three times ionized, up to almost 50 times ionized. The design and operating conditions of the SH-PermEBIT are discussed in some detail in Refs. [11,15]. 062501-1 1050-2947/2012/86(6)/062501(5) ©2012 American Physical Society