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X-ray observations of 2l-nl' transitions and configuration-interaction effects from Kr, Mo, Nb and Zr in near neon-like charge states from tokamak plasmas

X-ray spectra of 2l-3l' transitions in the elements krypton (Z = 36), zirconium (Z = 40), niobium (Z = 41) and molybdenum (Z = 42) from charge states around neon-like have been observed from Alcator C and Alcator C-Mod plasmas. Accurate wavelengths (±0.5 mÅ) have been determined with reference to neighbouring aluminium, silicon, sulfur, chlorine and argon lines with relatively well known wavelengths. Line identifications have been made by comparison to ab initio atomic structure calculations, using a fully relativistic, parametric potential code. Calculated wavelengths and oscillator strengths are presented for 2l-3l' transitions in neon-like ions and neighbouring Na-, Mg-, F- and O-like satellites. Electric quadrupole transitions in neon-like ions of the form 2s-3d and 2p-3p are found to be relatively bright, and their measured intensities agree well with the results of collisional-radiative modelling. The 2p-3s magnetic quadrupole transition is also relatively intense in the absence of collisional de-excitation at tokamak densities (1013-1015 cm-3). For krypton, transitions with upper levels up to n = 9 have been observed, and the measured wavelengths and intensities have been compared with calculations. For some neon-like transitions with nearly degenerate upper levels, there can be substantial configuration interaction which alters the line intensities.


Introduction
There has been considerable interest in x-ray transitions from high-Z atoms with charge states around the neon-like isosequence, attained in pulsed power [1][2][3][4], tokamak [5][6][7][8][9][10] and laser produced [11][12][13] plasmas. X-ray lasing [14,15] has been demonstrated in neon-like ions, and a need to understand the kinetics of this system has motivated development of very precise collisional-radiative modelling tools [16]. The identifications of many x-ray lines from neonlike ions allow high-resolution experimental data to be used for benchmarking multi-electron atomic structure calculations [7][8][9][17][18][19][20][21]. Collisional-radiative modelling of line intensities from neon-, fluorine-, sodium-and magnesium-like ionization states has demonstrated the importance of excitation-autoionization in overall charge state balance in tokamak plasmas [22]. Rates for this process, in conjunction with the latest dielectronic recombination rates [23], have led to a reassessment of the importance of molybdenum radiation in energy balance [24] in tokamak plasmas. Most of the work that has been done in the past has been limited to 3-3, 2-3 and 2-4 transitions in the Ne I iso-electronic sequence and adjacent charge states. Recently, high-n lines in neon-like iron have been measured for application to astrophysical data [25]. High-temperature, optically thin tokamak plasmas enable the measurement of many lines originating in transitions from levels having n 5; in fact, all of the transitions in the 2p-nd series in Mo 32+ lying under the ionization potential have been measured [8].
The availability of a large number of transitions in several adjacent elements provides the opportunity to study the systematics of configuration-interaction (CI) effects [9]. This paper demonstrates numerous instances of different transitions that are enhanced or suppressed by CI. It has also been observed [7][8][9] that a systematic uncertainty enters the calculation for the wavelengths of transitions with a 2s hole in the upper level through the treatment of quantum electrodynamics (QED) contributions in the calculation of these level energies. The same discrepancy is noted in this paper for all n when a relativistic parametric potential code (RELAC [26,27]) is used for the level energies. The tendency to overestimate level energies in multielectron atoms based on hydrogenic calculations with QED corrections [28] has been noted in comparisons between precise measurements of the level structure in neon-like ytterbium [17] and calculations from other relativistic codes which use the same procedure as RELAC. Issues relating to the calculation of level energies for states with a 2s hole have been investigated based on observations of the level structure of neon-like gold [18]. Furthermore, the spectra observed in this paper allow the study of the intensity of dipole-forbidden E2 and M2 lines. These lines are potentially useful as diagnostics of local temperature and density conditions.
In this paper, measured wavelengths and brightnesses, and calculated wavelengths, line intensities and oscillator strengths for 2p-3d, 2p-3s, 2s-3p, 2s-3d and 2p-3p transitions in the neon-like ions of Kr, Mo, Nb and Zr, in addition to associated 2p-3s, 2p-3d and 2s-3p satellites from magnesium-, sodium-, fluorine-and oxygen-like ions are presented. Measured and calculated lines from higher-n transitions in neon-like krypton are also presented. In section 2 the experimental set-up and wavelength calibration are described, and in section 3 a discussion is given of the atomic structure codes used and modelling issues are outlined. In section 4 the 2-3 experimental results and comparison with theory are presented. This includes extensive line tables, a Z scaling of satellite wavelengths and modelling of magnetic and electric quadrupole lines. In section 5 experimental results for high-n ground state transitions in Kr and Mo are compared with theory, with numerous examples of configuration-interaction effects being demonstrated.

Experiment description
The x-ray observations described here were obtained from the Alcator C [29] and Alcator C-Mod [30] tokamaks. Alcator C operated from 1978-1986, with toroidal magnetic fields up to 12 T, a molybdenum poloidal limiter and with central electron density and temperature in the ranges of 5 × 10 12 to 5 × 10 15 cm −3 and 600 to 2500 eV, respectively. Alcator C-Mod is a compact high-field device with all molybdenum plasma-facing components. For the measurements presented here, plasma parameters were in the range of 1.4 × 10 14 n e0 3.4 × 10 14 cm −3 and 1100 T e0 3000 eV. Molybdenum is an intrinsic impurity and krypton was introduced through a piezo-electric valve in both devices. A laser blow-off impurity injection system [31], which has been utilized to study impurity transport, was used to inject niobium and zirconium into Alcator C-Mod plasmas.
The 2l-3l spectra of krypton from Alcator C were obtained with a von Hamos-type spectrometer equipped with a pentaerythritol (PET) crystal (2d = 8.742 Å), and with wavelength coverage between 6 and 8 Å. The active area of the position-sensitive proportional counter detector was large (12 cm), which allowed for up to 400 mÅ coverage in one wavelength setting, but non-uniformities along the length of the Be entrance window led to sensitivity variations, so some line ratios from opposite ends of the detector may be up to 25% off. The x-ray spectra from Alcator C-Mod were recorded by a five-chord, independently spatially scannable, high-resolution x-ray spectrometer array [32]. Four of these von Hamos-type spectrometers have quartz crystals with 2d spacings of 6.687 Å, and wavelength coverage from 2.7 to 4.1 Å, 120 mÅ at a time. The remaining spectrometer has an ammonium dihydrogen phosphate (ADP) crystal with a 2d spacing of 10.640 Å, and a wavelength range from 4.3 to 6.5 Å. Wavelength calibration [5][6][7][8][9] has been achieved by determining the instrumental dispersions in reference to H-and He-like argon, chlorine, sulfur, silicon and aluminium lines and previously measured molybdenum [8] lines. The argon was introduced through a piezoelectric valve and chlorine and sulfur are intrinsic impurities. Si and Al were also injected into Alcator C plasmas with a laser blow-off system. Lines from hydrogen- [33,34] and helium-like [35][36][37] charge states are taken to have well known wavelengths, either measured or calculated. Experimental wavelengths were determined by performing a least-squares fit to observed spectra using Gaussian lineshapes; measured linewidths are dominated by Doppler broadening. The wavelengths of the H-and He-like calibration lines were taken to have an uncertainty of ±0.1 mÅ. For the 2-3 spectra of krypton, a quadratic fit was performed for the experimental dispersions, using an iterative procedure for neighbouring spectra, whereas for all other transitions, a linear dispersion was determined. The overall accuracy of experimental wavelengths in the tables is ±0.5 mÅ, although for spectra with good statistics and several calibration lines, the accuracy is ±0.3 mÅ.

Calculation of energy levels, oscillator strengths and line intensities
Ab initio atomic structure calculations for the aluminium-through oxygen-like isosequences (ground states 2p 6 3s 2 3p to 2s 2 2p 4 , respectively) have been performed using the fully relativistic RELAC code [26,27]. RELAC solves the Dirac equation by optimizing a parametric potential in order to minimize the average energies of a set of user-specified configurations. Because a different potential is found for each set of configurations, the calculations are not rigorously self-consistent. For each set of configurations in the problem, the fine structure level energies are then computed in intermediate coupling on jj -coupled basis functions. Contributions to level energies from the Breit operator and vacuum polarization effects [38] are also computed. RELAC computes the self-energy of a bound electron due to the emission and re-absorption of a virtual photon by determining an effective Z which satisfies the requirement that the expectation value of the relativistic subshell r nlj agrees with the corresponding hydrogenic value and then, in the case of an s-or p-orbital, interpolating the result on the hydrogenic values tabulated in [28]. In a previous paper that discussed the structure of neon-like molybdenum ions [8], part of the systematically larger difference between observed and calculated 2s-np transition energies and the more accurately predicted 2p-nd transition energies was ascribed to RELAC's use of the effective Z method for QED corrections to the binding energies of L-shell electrons. Furthermore, the assumption of a point nucleus (rather than a finite nuclear charge distribution) introduces a systematic error in the calculated wavefunctions for orbitals with large probability near r = 0. A quantitative discussion of the observed and calculated 2s-3p neon-like transition energies is given below.
Synthetic spectra were calculated for each ion in this paper using the HULLAC atomic physics package. The energy levels and radiative transitions rates (for electric and magnetic dipole and quadrupole transitions: E1, M1, E2 and M2) [39], along with autoionization rates [40], are calculated by RELAC. All levels with principal quantum number n 10 have been allowed to interact in the calculation of the structure and rates for each ion. Electron impact excitation collision cross sections between all levels of each ion are computed semi-relativistically in the distorted-wave approximation using RELAC's multi-configuration wavefunctions. The rapid calculation of so many collision strengths is made possible by the factorization theorem and interpolation of radial integrals technique of Bar-Shalom et al [41]. Collisional excitation rate coefficients γ are found by averaging the collisional cross sections vσ (v) over a Maxwellian velocity distribution. All level energies and rate coefficients have been entered into the collisional-radiative (CR) rate matrix for the ions of each element, and the resulting steady-state level populations are then found according to where n j is the population in level j , and R i,j is the rate at which the population leaves level i and goes to level j , possibly belonging to a neighbouring ion. (Rates R are related to rate coefficients γ by R j,i = n e γ j,i where n e is the electron density.) The resulting relative intensity for an observed line is given by where A j,i is the radiative transition rate for some multipole. Thus, for levels connected to the ground only by slow (non-electric dipole) decays, the resulting emission can be strong if the upper level's population is large. Examples of bright E2 and M2 lines will be discussed below. These lines are sensitive to increasing electron density through the competition between radiative decays and collisional destruction of the upper level. For the high atomic number elements studied here, the critical densities above which collisions suppress the observed emission are higher than can be achieved in tokamak plasmas. RELAC's wavefunctions are computed taking full account of CI between levels with the same parity and the same total angular momentum. The result is that the physical state of the upper and lower level in each observed transition can have components from many configurations. The CI can have many effects on an ion's spectrum: in the case of Na-and F-like ions, classically forbidden transitions involving two jumping electrons are enabled (see, for example, table 3). Also, when two interacting levels become closely separated in energy, strength is transferred from one transition (potentially quenching it fully) to the other [42,43].
Three measured examples of this phenomenon on strong lines in the neon-like spectra of Kr and Mo are presented in section 5, below. The strength of a line is defined as the square of the reduced dipole matrix element where ψ (ψ ) are the physical wavefunctions composed of many basis states, the sum runs over all N electrons of the atom (or ion) and r i is the radial position of the ith electron in Bohr units. S is related to the radiative decay rate according to where g u is the statistical weight of the upper level of the transition, a 0 is the Bohr radius and σ is the wavenumber of the spectral line in cm −1 . The ultimate contribution of each basis component to the matrix element in equation (3) is determined by the product of the mixing coefficient (with a definite phase) and the radial transition integral involving one-electron orbital functions nlj r n l j = (−1) l [(2l + 1)(2l + 1)] 1/2 l 1 l 0 0 0 ∞ 0 rP nl P n l dr.
The superposition of positive and negative terms in equation (3) can result in constructive or destructive interference, thus transferring line strength from one transition to another [42,43].
Hence, no general statement can be made concerning the direction of the strength transfer (i.e. from lower-to higher-energy levels) and the full multiconfiguration calculation must be done in each case.

2-3 experimental spectra and interpretation
Shown in figure 1 is the 2-3 spectrum of neon-like krypton (Kr 26+ ), with satellites; previous observations can be found in [2,3,12,13]. This is a composite spectrum obtained by scanning in wavelength during a sequence of reproducible Alcator C discharges, with a peak electron density of 2.6 × 10 14 cm −3 and a peak electron temperature of 1450 eV. The spectrum is dominated by the neon-like electric-dipole transitions 3C (2p − -3d − ) and 3D (2p + -3d + ). (The standard notation for Ne-like transitions [44] is used.) The 3F and 3G (2p ∓ -3s + ) transitions, including the magnetic quadrupole line (M2, 2p + -3s + , J = 2) at 7519.2 mÅ, are also intense. Transitions with a 2s hole (3A, 2s-3p + and 3B, 2s-3p − ) are bright, including the electric quadrupole 2s-2d + line at 6103.8 mÅ. Also apparent are F-, Na-and Mg-like satellites. This composite spectrum is comprised of six individual spectra from different discharges, and considering non-uniformities along the detector (as mentioned above), the relative intensities of lines separated by more than 150 mÅ have a 25% uncertainty. Shown in the bottom of figure 1 is a synthetic spectrum, which is in good qualitative agreement. Transition designations, experimental and theoretical wavelengths and calculated oscillator strengths for all of these lines are presented in tables 1-4. Most observed transitions are within 3 mÅ of the calculated wavelengths, or about 0.04%. The synthetic spectrum was computed for T e = 1600 eV and n e = 1.0 × 10 14 cm −3 . An expanded view of this spectrum in the vicinity of the 3C transition at 6697.3 mÅ, which was obtained during a single discharge, is shown in figure 2. Several bright Na-like satellites are present, along with a Mg-like transition, on the long-wavelength side; on the short-wavelength side are several less intense F-like satellites. The calculated spectrum is overlaid, and in good agreement (see tables 1-4). The helium-like Si 12+ resonance and forbidden lines used for the wavelength calibration are indicated by the vertical dotted lines.
The neon-like 3C spectrum and long-wavelength satellites have been obtained for several row 5 transition metals [4,12]. A comparison of these spectra is shown in figure 3, and all are qualitatively similar. (The Pd lines were previously reported [9,45].) This allows for the Table 5. 2-3 transitions in neon-like Mo 32+ and associated satellites. In the first and second columns are the measured and calculated wavelengths (in mÅ), in the third column is the isoelectronic sequence (with the neon-like transition label), in the fourth column are the calculated oscillator strengths and in the last column are the transition designations (j -j coupling).   study of relative satellite transition wavelengths as a function of atomic number. Transition designations, experimental and theoretical wavelengths and calculated oscillator strengths for neon-like 2l-3l lines, along with nearby satellites, are given in tables 5-7 for molybdenum, niobium and zirconium. These wavelengths may be compared with previous measurements [4,5,12]; in [5], wavelengths of the Cl and S calibration lines were only given to the nearest mÅ, and were based on older, less accurate calculations; in [4] there were significant Doppler shifts due to the expanding source, leading to substantial uncertainties in the measured wavelengths; in [12] there was no independent wavelength calibration and all lines were given with reference to the calculated neon-like 3C and 3D lines. Shown in figure 4 are the measured wavelength differences between the 3C resonance lines and the three brightest satellites considered, the 2p − -3d − J = 1 2 sodium-like transition, the 2p − -3d − J = 3 2 sodium-like transition and the 2p − -3d − J = 1 magnesium-like transition (normalized to the resonance line wavelength), as a function of the atomic charge of the neon-like ion. There is a systematic trend for the satellites to approach the neon-like line in wavelength for higher atomic number, for the three brightest satellites considered. This trend is also apparent in the calculations, as shown by the full lines in figure 4. The Mg-like Pd line at 3763.6 mÅ was mistakenly identified with the unclassified feature at 3774.6 mÅ in table 5 of [9].
As is apparent from figure 1, the J = 2 2p + -3s + magnetic quadrupole (M2) transition in krypton at 7519.2 mÅ is quite prominent, although the radiative transition rate, relative to the nearby J = 1 2p + -3s + electric-dipole transition (3G) at 7504.4 mÅ, is six orders of magnitude slower (5.4 × 10 6 versus 5.3 × 10 12 s −1 ), while the collisional excitation rates coefficients from the ground level are within a factor of two (1.3×10 10 versus 3.0×10 10 cm −3 s −1 ). The intensity of this transition is enhanced in the collisional-radiative model by a build-up of population in the nearly metastable upper level. The corresponding transition in molybdenum is similarly bright, as can be seen in figure 5. Central plasma parameters for the molybdenum spectrum were T e = 1800 eV and n e = 1.8 × 10 14 cm −3 . The predicted intensities of these magnetic quadrupole transitions in both krypton and molybdenum are in excellent agreement with measurements relative to the nearby electric-dipole lines. The collisional-radiative intensities of these lines are sensitive to the cascade channels from higher-n levels in the model; 93.5% of the population in the upper level of the M2 line comes from radiative cascades through levels with n 9, versus 88% for the upper level of 3G. (These cascades were not properly included in the synthetic spectrum of figure 1.) The relative intensities of the M2/3G lines may be used as an electron density diagnostic, because at higher densities, collisional de-excitation of the M2 line dominates over radiative decay. This is demonstrated in figure 6 where this calculated line ratio is shown as a function of density for molybdenum and krypton. Also included are the measured points deduced from figure 5. Unfortunately, the electron density in tokamak plasmas is about an order of magnitude too low to be able to take advantage of the diagnostic application of this line ratio for these species.      Near the bright Kr 26+ 2p − -3s + transition 3F (7267.5 mÅ) are the J = 2, l = 0 electric quadrupole transitions 2p + -3p + (E22) and 2p + -3p − (E23), at 7208.1 and 7277.5 mÅ, respectively. (These transitions were seen in Ag [6].) These lines are shown in figure 7, along with the corresponding, and previously unreported transitions in Mo 32+ at 5008.4 and 5072.8 mÅ, respectively. Central plasma parameters for the molybdenum spectrum were T e = 1850 eV and n e = 1.6 × 10 14 cm −3 . Also shown in figure 7 are synthetic spectra, again in good agreement with the observations. Similar to the magnetic quadrupole lines, these electric quadrupole lines have radiative transition rates that are at least three orders of magnitude lower than that of 3F, and yet these lines are relatively strong. Again, this is because of the highly populated nature of the upper levels of these transitions. E23 and E22 both have quadrupole transition rates to the ground level of 2.0 × 10 10 s −1 in Mo 32+ (relative to 7.8 × 10 12 s −1 for 3F). The branching ratios of E23 and E22 to the ground level, as opposed to n = 0 dipole decays to 2p 5 3s levels, are 0.63 and 0.34, respectively. The other related electric quadrupole transition E21 (2p − -3p + ) in molybdenum is located very close to the intense 3D transition, while in krypton it is blended with relatively bright F-and Mg-like lines. Similarly, the relatively weak 3E transitions in Mo and Kr are blended with nearby satellites.
The J = 2, l = 2 electric quadrupole transitions (2s-3d + ) in Mo 32+ and Kr 26+ , at 4276.2 and 6103.8 mÅ, respectively, are relatively bright, in spite of their slow radiative transition rates, in a similar fashion to the 2p-3p lines. X-ray spectra in the neighbourhoods of these lines are shown in figure 8, which also includes the 2s-3p + transitions 3A. Core plasma parameters for the molybdenum spectrum were T e = 1800 eV and n e = 1.4 × 10 14 cm −3 . The 4276.2 mÅ line was not observed in a previous study of the 2-3 transitions in molybdenum because it was outside the spectral range of the instrument [5]. Shown for comparison are the simulated spectra; the calculated neon-like lines appear systematically on the short-wavelength side. This shift has been noted earlier [7][8][9]25] and is due to a combination of factors. In the model potential, a point nucleus has been assumed instead of a distribution of charge over a finite radius, and this has a larger effect on orbitals with a large probability near r = 0, like 2s. In addition, QED effects are strongest for s-orbitals and are only estimated in the calculations. The relative intensities of the neon-like lines are in fairly good agreement with the calculations, although E2s is observed to be slightly brighter than calculated relative to 3A for both molybdenum and krypton. The reason that these electric quadrupole lines are so strong in spite of their slow transition rates can be seen in the competition between the E2 channel and the allowed n = 0 (3d → 3p) E1 decay channel. The branching ratios for the E2s decays are around 0.4 and 0.3 for Mo and Kr, respectively; at the relatively low densities of tokamak plasmas, these forbidden lines are quite bright. An x-ray spectrum of molybdenum at slightly longer wavelength, for a higher-temperature plasma (3000 eV), is shown in figure 9. This spectrum is dominated by the fluorine-like 2p − -3d − J = 5 2 transition at 4498.3 mÅ. In addition to 3A and 3B, another intense line in this spectrum is the oxygen-like 2p + -3d + (J = 2 to 3) transition [10] at 4535.8 mÅ. The corresponding oxygen-like 2p + -4d + (J = 2 to 3) transition at 3483.2 mÅ has previously been observed in Alcator C-Mod plasmas [9]. Similar to the magnesium-like transitions, the oxygen-like lines are observed at systematically longer wavelength than the predicted lines. This is due to the configuration interaction between the J = 0 levels in the ground and other J = 0 levels up to and into the continuum.

High-n transitions and configuration-interaction effects
Higher-n transitions have been observed for neon-like krypton, similar to those seen in Mo 32+ [8,9]. Shown in figure 10 are the 4D (2p + -4d + ) and 4C (2p − -4d − ) lines of Kr 26+ at 5396.4    A composite spectrum of high-n transitions in Kr 26+ , up to n = 9, with Fl-like satellites, and Cl 15+ calibration lines is shown in the top frame. In the bottom frame is a synthetic spectrum, with Ne-like shown as the black curve, Na-like as the red dotted curve and Fl-like as the purple chain curve. and 5278.7 mÅ, respectively, along with neighbouring satellites and the neon-like 4F (2p − -4s + ) line at 5407.3 mÅ. Central parameters for the plasma from which this spectrum was obtained were T e = 1150 eV and n e = 1.6 × 10 14 cm −3 . These transitions have been studied extensively in molybdenum [8,22]. Also shown in figure 10 is a synthetic spectrum, which is in good agreement with the observations, although there is a ∼−10 mÅ shift of the calculated wavelength for the magnesium-like 2p − -4d − transition at 5436.6 mÅ. The 4F transition at 5407.3 mÅ is readily noticeable, having about 10% of the intensity of the 4D transition; the 4F line in Mo 32+ at 3705.6 mÅ was too weak to be reported in [8], but has been observed here, with about 1% of the intensity of 4D. The reason the 4F line is so intense in Kr 26+ is because of its close proximity to the 4D line; the upper levels are separated by only 4.6 eV, and significant configuration-interaction results that transfers strength from 4D to 4F. This phenomenon has been seen between the 7D and 6C levels in Mo 32+ [8,9,45], but in contrast here, the longer wavelength line is the beneficiary of the enhanced intensity. The energy level diagrams for n = 4 and 5 transitions in neon-like molybdenum and krypton are shown in figure 11. As can be seen, the upper levels for the 4D and 4F transitions in Kr 26+ are very close, while in Mo 32+ these levels are 31 eV apart, too far away for any significant configuration interaction. The calculated oscillator strengths of the 2p − -ns + (nF) transitions as a function of n for Mo 32+ , Nb 31+ , Zr 30+ and Kr 26+ are shown at the top of figure 12. Only in the case of Kr 26+ are the 4D and 4F upper levels close enough to interact; as such the intensity of the 4F transition is enhanced by a factor of ∼ 50. The calculated and measured brightness ratios between the 4F and 4D transitions are shown in figure 13, as a function of the level separation of the upper levels for neon-like ions of the elements Br through Mo. The value of this ratio for Y 29+ is suppressed by three orders of magnitude and does not appear on the plot. The agreement between theory and experiment is good and observations of the 4F/4D ratio in Rb 27+ are planned; in the case of molybdenum, the 4F line is very weak, less than 1% of the intensity of the bright 4D line, but in good agreement with the calculations.
The Kr 26+ 2p-nd series with n between 5 and 9 is shown in figure 14; this is another composite spectrum taken during a series of similar discharges, with n e = 1.8-2.8×10 14 cm −3 and T e = 1200-1700 eV. Also visible in this spectrum are the Kr 26+ 4A, 4B, 5A and 5B transitions and two strong Kr 27+ lines, 2p − -4d − at 5055.2 mÅ, and 2p + -5d + at 4701.0 mÅ. High-n krypton lines are summarized in table 8. Similar high-n transitions have been seen in molybdenum, niobium and zirconium [9,45]. Apparent from figures 1, 9 and 14 and the tables is that the 2s-np series is comprised of pairs of transitions, nA and nB, that have an intensity ratio of about 2.5 and upper levels which become closer together as n increases. The measured and calculated energy separation of these pairs of levels as a function of n is shown in figure 15 for Mo 32+ and Kr 26+ . The agreement for the level separation is quite good, although the calculated wavelengths for these lines are significantly shifted from the measured values [8,9], for the reasons mentioned above. The measured transition pairs for n 7 [9] are actually too close together to resolve. (In table 2 of [9] the calculated pairs of wavelengths for n = 7, 8 and 9 should be switched.) The intensities of the 2s-np series transitions decrease strongly with increasing n. Shown in figure 16 are the calculated oscillator strengths of the 2s-np transitions in neon-like molybdenum, niobium and krypton. The oscillator strengths decrease roughly as 1/n 3 , maintaining a ratio of about 2.5 for the two series. The 4B transition in Kr 26+ (at 4968.2 mÅ) does not follow this trend and is about a factor of four lower. The reason for this can be seen in the energy level diagram, figure 11: the upper level for 4B in krypton is within 2.2 eV of the upper level for 5G, there is significant configuration interaction and the intensity of 5G increases at the expense of 4B. For molybdenum, the upper level of 5G is 11.0 eV away from the upper level of 4A and 23.4 eV away from the upper level of 4B, on the short-wavelength side of both, and there is very little interaction.
These predicted configuration-interaction effects are borne out by the observations, as can by seen in figure 17, where the spectra in the vicinity of the 4A and 4B transitions in molybdenum and krypton are shown. Central plasma parameters for the molybdenum  spectrum were n e = 1.6 × 10 14 cm −3 and T e = 1850 eV, and for the krypton spectrum were n e = 1.8 × 10 14 cm −3 and T e = 1250 eV. Also shown are the synthetic spectra, which are in good agreement with regard to line intensities, although the 4A and 4B lines are at significantly longer wavelengths than the predictions. In spite of the presence of three sodium-like lines between the Mo 32+ 4A and 4B lines, their ratio is close to what is expected from figure 16. Their separation also agrees well with the predictions of figure 15. The weak 5G transition at 3428.7 mÅ is also visible. For the case of krypton, shown at the bottom of figure 17, the situation is different: the 4B transition at 4968.2 mÅ is relatively weak. The identification of this feature is based on the separation from the 4A transition at 4958.5 mÅ, and from the trends in figure 15. The 5G line at 4972.6 mÅ is relatively bright due to configuration interaction with the upper level of 4B. This enhanced intensity for 5G is apparent from the bottom of figure 12, as manifested by the enhanced oscillator strength, and is reflected in the synthetic spectrum of figure 17. This selective configuration-interaction effect in the Kr 26+ 4B upper level is emphasized in figure 18, where the brightness ratios of nA/nB are shown  as a function of n for molybdenum and krypton, with excellent agreement between theory and experiment. In general, the ratios are between 2.5 and 3, with the exception of Kr 26+ 4A/4B, due to the configuration-interaction effect described above. Spectra including 5A and 5B in molybdenum and krypton are shown in figure 19. Central plasma parameters for the    Yet another manifestation of the configuration interaction, this time in Mo 32+ , is also demonstrated in figure 19. The interacting states in this case are the upper levels for 7D and 6C. As was shown previously [8,9], the 6C transition in Mo 32+ is suppressed relative to 7D. This is not the case for Kr 26+ where the upper levels are 20 eV apart, as can be seen at the bottom of figure 19. Nearby satellites in both the molybdenum and krypton spectra contaminate the 6C lines, but the effect is clear. The brightness ratios for these lines as a function of level separation are shown in figure 20, which is similar to figure 5 of [45], but with the inclusion of the krypton points. The agreement between experiment and theory is quite good; it would certainly be gratifying to verify that this effect changes sign in Tc or in Ru. It is noteworthy that in the case of the 2p-nd configuration interaction, as in figures 19 and 20, the 'beneficiary' of the enhanced intensity is on the short-wavelength side (higher-energy side), whereas in the case of 2p-ns level enhancement, the 'beneficiaries' are on the long-wavelength side (lower-energy side) of the 'donor' transition.
Finally, it is interesting to note that there is no configuration interaction between the upper levels of E23 and 3F, even though these levels are only 2.3 eV apart in Kr 26+ , as can be seen in figure 7. This is because the upper levels are of opposite parity and cannot interact. Shown in figure 21 are the calculated intensity ratios for E22/3F and E23/3F as a function of upper level energy separation in Kr, Zr, Nb and Mo; also shown are the measured ratios for Kr and Mo, in excellent agreement with theory. In contrast to the cases of figures 13 and 20, here there is no resonant behaviour within a few eV of zero-energy separation for these transitions.

Conclusions
2-3 transitions in neon-like Kr, Mo, Nb and Zr have been observed from Alcator C and Alcator C-Mod plasmas, in addition to associated fluorine-, sodium-and magnesium-like satellites. Accurate wavelengths have been determined with reference to nearby calibration lines from hydrogen-and helium-like ions. Measured wavelengths and line intensities have been compared with atomic structure calculations and collisional-radiative modelling from the RELAC code, with very good overall agreement. A Z-scaling of the wavelengths of the brightest Na-and Mg-like satellites also shows good agreement with theory. Electric and magnetic quadrupole transitions are found to be relatively intense (compared with bright allowed transitions in neon-like ions) for tokamak plasma conditions, again in good accord with the modelling. High-n transitions (up to n = 9) in neon-like krypton and molybdenum have also been recorded. Configuration-interaction effects in certain line intensities have been observed, for transitions with nearly degenerate upper levels; comparison of observed line intensities with theory is excellent.