Spectroscopy on highly charged ions — recent advances using tokamaks, laser-produced plasmas and straight ion beams

Nuclear Instruments and Methods in Physics Research B 98 (1995) lo-17

Beam Interections with Materials 6 Atoms

ELSEVIER

Spectroscopy on highly charged ions - recent advances using tokamaks, laser-produced plasmas and straight ion beams * E. TrSbert Experimentalphysik III, Ruhr-UniL~ersitiitBochum, D-44 780 Bochum, Germany

Abstract Recent progress in the spectroscopy on highly charged ions has been achieved by employing tokamaks, laser-produced plasmas, electron-beam ion traps, and stored or single-pass fast ion beams. Out of these, the present paper concentrates on tokamaks, on laser-produced plasmas and on single-pass ion beams. The first of these provide time-integrated spectra of high precision for a determination of the atomic level structure which for higher nuclear charges is increasingly sensitive to relativistic and quantum electrodynamical effects. Single-pass ion beams, in contrast, offer the possibility of time-resolved spectroscopy; this in turn permits to determine transition rates which for high nuclear charges show the influence of relativistic and QED effects in ways different from the simple structure observations. Time-resolved spectroscopy is also most helpful for studies of spin-changing and similar higher-order transitions in many-body systems.

1. Introduction

devices like the tokamak to small ones like the gas-puff pinch, the first being notable for the calm conditions of a low-density plasma, the second for reaching surprisingly high charge states (up to Mo4”+) due to microscopic instabilities, and - ion traps of all kinds and sizes, from almost microscopic ones for singly charged ions via desk-top sizes aiming at bare uranium (the electron-beam ion trap EBIT, see contribution by Elliott), to accelerators curving back on themselves and forming storage rings the size of an indoors athletics stadium (see contribution by Kluge). Out of the recent spectroscopic progress achieved with all these devices, I will concentrate on the spectroscopic results from laser-produced plasmas and from time-resolved spectroscopy on fast ion beams, as well as mention recent work on tokamaks. I will, however, discuss some of the lifetime measurements done with EBIT and storage rings, as far as this ties in with my other topics.

Spectroscopy has gone a long a way in the pursuit of spectra of highly charged ionic species. It started out with flames and arcs for neutral and singly ionized atoms. In the 1930es a considerable push came from the improvement of spark discharges (triggered spark, sliding spark, condensed spark, low-inductance spark), particularly noteworthy by Edltn and the Uppsala group. Those researchers reached the apparent limits of such light sources at ionization stages as high as Cu ‘s+ by 1936 [l]. Various types of spark discharges are still in good use for the study of complex spectra of not too-highly ionized heavy atoms, typically up to about q = 8 + [2-41. Since about the 1960es, several new types of light sources made their appearance and competed for the lead in the quest for ever higher ionic charges: - lasers - in the form of systems capable of delivering high power densities so that a sample of material could be transformed into a dense plasma, - accelerators - providing ion beams which by virtue of the high collision velocity could be stripped of many or even all of their electrons upon passage through thin foils, - discharges of novel design - from large plasma

’ Work partially supported by Deutsche Forschungsgemeinschaft (DFG) and German Minister for Research and Technology (BM~). * Tel. +49 234 700 7310, fax + 49 234 7094 172, E-mail: traebert@ep3,ruhr-uni-bochum.de.

2. Spectroscopic

techniques

2.1. Plasma discharges:

tokamak

People involved with controlled-fusion experiments soon realized that they needed information about the plasma they tried to harness. The plasma cannot be perfectly contained in a vessel without leaking to the surfaces. This plasma-wall interaction brings in impurities which are largely unwanted in a burning plasma, because they radiatively transport energy away from the plasma core. The

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Instr. and Meth. in Phys. Res. B 98 (1995) 10-I 7

very same radiation, however, permits spectroscopic studies which tell about elementary composition, density and temperature of the plasma. Therefore impurity atoms are sometimes introduced at will for plasma diagnostics, for example by laser ablation or by atomic or ionic beam injection. One of the more common designs of fusion experiments is that of the tokamak. Tokamaks reach fairly high electron temperatures whilst running at low particle densities; the direction of progress is towards quieter burning conditions with longer pulse duration and eventually reaching a continuous mode. This is very well for spectroscopic studies, and thus tokamaks are most useful spectroscopic light sources [S-8]. These large plasma devices, however, are mostly engineering exercises, and although spectroscopic diagnostics may at times be appreciated, running solely for fundamental interest in spectroscopy is certainly not on. A laudable exception is the TEXT tokamak in Austin (Texas) where a collaboration of Rowan with Kaufman and Sugar from NIST (Gaithersburg) has resulted in a multitude of spectroscopic studies, of many elements, in charge states up to HfJ2+ [9,10]. Higher charge states would require higher temperatures, as reached in some of the other devices like JET, PLT, Alcator-C, J-60, TFTR, or envisaged for the next step fusion machine, ITER. On the other hand, spectroscopy on some lower charge state ions has recently been revived at JET as it gives clues not only to static plasma properties, but also to plasma dynamics. 2.2. Laser-produced

plasmas

One of the problems of the classical spark light sources was the method of how to convert stored electrical energy quickly and efficiently into as many and as highly charged ions as possible. The advent of the laser opened new vistas for this field, too. Lasers were not (and are not) able to achieve multiple ionization-cum-excitation of individual atoms, preferably even reaching selected levels. By virtue of its monochromaticity, however, laser light can be well focussed, and there are pulsed lasers that release large amounts of energy in a very short time. Thus the power density in the focal plane of a suitable lens can be enormous, and any material exposed to such power densities will not only vapourize but become a dense and hot plasma (Fig. 1). The plasma will expand and thus cool; ionization by electron-ion collisions proceeds in many stages, however, and that takes time. It is no surprise then that not only the power density of the delivered laser light matters, but also the size and the shape of the plasma produced. Consequently, the highest-power laser facilities of their time have always been interesting places for spectroscopy, too. After all, spectroscopists would be able to tell from spectra of the laser-produced plasma plume which ionization stage had been reached, which tells about the converted energy and thus is the proof of success in this game.

11

/VI Laser light

-Line of sight / Focus line

Target surface

Fig. 1. Schematics of laser-produced plasma spectroscopy using a line focus. Observation along the focus line reduces the Doppler broadening of the spectral lines by decreasing the relative contribution of the fringe regions.

One of the very big lasers in the inertial fusion program, NOVA at Livermore (LLNL), was the first to initiate what is called an X-ray laser (see Matthews in these proceedings). In terms of spectroscopy of highly charged ions, NOVA (two laser beams) and the multi-beam Omega laser at Rochester (as well as many smaller devices) have been put to excellent use. For example, Cu-like Ue3+ has been produced [ll] as well as Na-like Gd53+ [12] and many ions in lower charge states (see, e.g., Refs. [13-191). Thus data for a number of important isoelectronic sequences have been gathered which can also serve in tests of atomic structure theory (see below). For such achievements, the spectroscopy of such laserproduced plasmas had to address inherent problems of the light source. From Fig. 1 it is evident that the line of sight of the spectrograph cuts through an expanding plasma plume. The plume will be hottest near its base, and that is also (roughly) the region of the highest charge states. Farther out, the plasma ions will tend to recombine with available electrons. Where the diameter of the plume is small, the density of the plasma is high, and the plasma may be optically thick. Then the ions observed will have a velocity component towards the spectrometer which will result in a Doppler shift of the observed spectral lines. This scenario has been invoked to explain the systematic deviations of some recent data on Na-like heavy ions [12] from the theoretically expected trend [20] which was also corroborated by a high-Z data point from a stationary light source, the LLNL EBIT [21]. Farther away from the surface of the laser target, the expanding plasma becomes less opaque and therefore less prone to Doppler shifts. Recent work, for example by Ekberg, Reader and others, therefore has changed the previously used observation close to the target (often as close as a fraction of a millimeter) to a line of sight farther away from it, say at 2 mm. The gain in accuracy thus achieved goes along with a certain loss in intensity and in maximum charge state reached.

1.

FUNDAMENTAL ASPECTS

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E. Trabert / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 10-I 7

-

Foil

, ~ectrolls

7

Fig. 2. Schematics of beam-foil spectroscopy. MCP indicates a microchannelplate multichannel detector. The sheet metal protruding from the spectrometer slits towards the ion beam reduces the background in the spectra by blocking 6 electrons before they can strike the spectrometer slit jaws.

An optically thin expanding plasma will permit to observe the full cross section of the plasma plume, with velocity components pointing towards and away from the spectrometer, and will consequentially lead to spectra suffering from Doppler broadening. This broadening can be reduced by using a line focus instead of a point focus of the laser light, and by observing along the extension of the line (Fig. 1) [13,15]. Again, this spreading-out of the laser light intensity reduces the power density of the plasma and thus the maximum charge state present, but at the considerable advantage of narrower spectral lines. Maybe this also reflects a certain maturity of the field, after the realization that one of the (previously elusive) goals for atomic structure studies, H-like U (U91’), will probably not be seen in any laser-produced plasma, but has already been produced and studied in other light sources, like fast ion beams, the Darmstadt ESR storage ring [22] and the LLNL EBIT [23]. 2.3. Time-resolved beams

spectroscopy,

mostly

using

fast ion

The spectroscopy of fast ion beams which are excited by being passed through thin foils (Fig. 2) is now with us since about thirty years. Its early days have been described by the inventors of beam-foil spectroscopy, Kay and Bashkin, at the second of the HCI series of conferences, Oxford 1984, which also incorporated the last of the dedicated meetings on beam-foil spectroscopy. In certain aspects, beam-foil spectroscopy has reached the goals it promised: any spectrum of any (natural) element can be excited by ion beams of suitable energy interacting with a solid target. H-like, He-like and Li-like spectra of U have been studied at Berkeley, Caen and Darmstadt [24-261, and a fair number of other accelerators are being used for studies of few- and many-electron systems of somewhat lighter ions. Beam-foil spectroscopy is still the only general method for measuring the lifetime of atomic levels in highly charged ions, although some recent EBIT and storage ring work does well for some particularly long-lived

levels [27-301. The beam-foil technique offers time resolution and thus permits to discriminate prompt spectra from delayed spectra; in the latter, intercombination transitions and decays of long-lived levels show [31] which are hardly seen in other light sources (except the solar corona) because collisions depopulate such levels. For some studies, the somewhat selective electron capture into fairly slow ions is exploited, e.g. on beams from ECR ion sources at Grenoble, Groningen, Berkeley and Uppsala [32,33]. In other aspects, in particular for precision work, the shortcomings of the beam-foil light source are severe: the excitation is non-selective, and the interaction with the foil goes along with considerable angular scattering, energy loss and energy straggling. The non-selective excitation results in rich spectra in which a line of interest often is blended with unidentified others of no interest. The ion-foil interaction causes a foil-related background (secondaryelectron X-rays, 6 electrons, see Fig. 2) which is particularly worrying in the EUV spectral range where detection efficiencies are low because of instrumentation (grating spectrometers) and detectors. Almost bare ions of any element can by now be produced not only by fast ion beams, but also in stationary EBIT [23]. The low light intensity available from an EBIT requires long signal integration times in spectroscopic studies. This presently restricts practical studies largely to the X-ray range where spectrometers of sufficiently large solid angle and equipped with low-dark rate detectors are available. However, for excitation levels which are sufficiently well separated in energy from their neighbours and which have a lifetime in a suitable range, voltage switching of the electron beam in the EBIT can be used to measure the time behaviour of some radiative emission. In this way, lifetimes of the 1~2s ‘Sr level in He-like ions of Ne and Mg have been obtained with good accuracy [27,28]. Another ion trap, the ion storage ring, is also a contender for improvements beyond what traditional spectroscopy on fast ion beams can achieve. In these devices an electron target (the electron cooler section) or a gas target can be used and those ions be detected which have suffered electron capture and thus have undergone a change of charge. These ions take a trajectory different from the circulating beam and can be detected with unity efficiency. For the same atomic level as studied with the EBIT, the 1~2s 3S, level in He-like ions, the charge exchange in the electron cooler section of an ion storage ring as mediated by the dielectronic recombination (DR) process has been used. Rather valuable lifetime data have been obtained with this technique for C and N [29], with this method which tells about atomic levels without resorting to photon spectroscopy. However, radiative capture processes in ion-atom or ion-electron collisions can be observed, too, and a coincidence measurement of the photon and the very ion may result in exceptionally clean spectra. This was first achieved with an internal gas target at ESR [34,22]. Taking this a step further, collisions with “free” electrons

E. Triibert / Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 10-I 7

either in a single-pass channeling experiment [35] or in the electron cooler of the storage ring, where “monochromatic” electrons of adjustable relative energy are available for capture processes, have already been used for bare Au and U ions [36,37] and their great potential for precision studies been noted. However, there is life with lighter ions than uranium also, and in particular with more than one or two electrons. Precision studies of atomic structure of these intermediateheavy, few-electron systems aim at tests of QED corrections in a range of Z in which nuclear size effects are not important [38-401. Besides the structural aspect there is dynamics: level lifetimes [31,41-521, relative line intensities [53,54], branching ratios and the like. In measurements of atomic lifetimes (in the picosecond to nanosecond range even for highly charged ions), time resolution is of prime importance and is achieved by good spatial resolution at the ion beam after benefitting from the “instantaneous” excitation by the interaction of fast ions with a thin foil. Examples are discussed below. 2.4. Spectral analysis and combination

of methods

One of the problems of spectroscopy is the complexity of the spectra once several electrons are involved. Only high-resolution, high-precision data can lead to a reliable analysis. As long as lines are blended or not well enough measurable, considerable ambiguity remains. For neutral atoms and ions in low charge states, classical light sources like hollow cathode discharges and spark discharges in combination with spectrometers of high resolving power (like Fourier transform spectrometers which work in the infrared, visible and UV ranges) yield such quality data. The light sources for highly charged ions and the available spectroscopic equipment (most of the interesting lines will be in the VUV, EUV and X-ray range) regularly do far worse. The light sources may be feeble (ion beams, EBIT) and thus for sufficient signal require wide spectrometer slits which result in poor spectral resolution; the emission lines may suffer Doppler shifts and broadenings (ion beams, laser-produced plasmas), and the light sources often are hard to get at (much sought-for accelerator, tokamak or power laser time), pre-empting many systematic checks and. thorough studies. Notably the ion-foil interaction populates many levels not reached in classical light sources. Consequently lines appear in the spectra which are not easily understood, and considerable amounts of data need to be combined to extract the basic information on level schemes and transition probabilities. A modern tool for this, an interactive data analysis program package, has recently been developed by Azarov [55,56]. This powerful package assists the spectroscopist by graphical presentations, built-in checks for internal consistency and cross checks with data bases. It can also be used in combination with the output of atomic structure computations [3,4,57]. Such a process helps to detect misidentifications in avail-

13

able data as well as it then helps to direct the search for decisive clues in spectra. On the experimental side, a number of recent studies have taken into account information obtained from a variety of light sources, in order to make use of the benefits of each and to overcome the shortcomings of individual measurements. For example, there are spectral data on highly charged ions from the solar corona (transitions between some low-lying levels, including those of low transition probabilities, forbidden lines (Ml and E2 transitions)), from tokamaks (precise data on a few transitions from low-lying levels), laser-produced plasmas (more lines, in particular of transitions with higher probabilities, but typically not those from long-lived levels) and EUV data from foil-excited ion beams (time-resolved measurements, not resolving the multitude of lines from short-lived levels, but showing and thus identifying the decays of long-lived levels in delayed spectra). JupCn et al. have combined these data and thus practically completed the level schemes of low-lying levels in a number of medium-high ionization stages of Fe [58]. Other experimenters have combined tokamak with beam-foil data [59,60], spark discharge data for an isoelectronic series with the information from the time-resolved beam-foil data for a single spectrum [61], or beam-foil with laser-produced plasma data [62]. Wherever such combinations can be made, they will be the method of choice. For archival data bases like the reference publications presented by NIST [63], only well-filtered and cross-checked data are deemed acceptable anyway. Of course, the reviews at this meeting do not cover everything: in spectroscopy, too, there are interesting studies being done which transgress the pre-set pattern. The very conference contributions show that EBIT is being used for straightforward spectroscopy [64,65], as well as the excitation of gases by ion impact or the electron capture by ions in a gas target [66-691 provide worthy spectroscopic venues beyond the collision aspects involved.

3. Examples 3.1. H-like/He-like

ions

Excitation levels in ions with one or two electrons can be calculated with high precision. For highly charged ions, these levels will be strongly affected by relativity and by quantum electrodynamical effects. The leading terms of both are well established, but higher order terms are testable by precise measurements and sometimes then theory is found to be incomplete. One example is the He isoelectronic sequence in which recent spectroscopic data on n = 2 triplet levels indicated a missing term by the systematics (Z dependence) of a slight mismatch between experiment and theory for the ls2p 3Pl level [70,71]. Other points of interest are particular decay modes, like 1. FUNDAMENTAL

ASPECTS

14

E. Triibrrt /Nucl. Ins&. and Meth. in Phys. Res. B 98 (1995) 10-l 7

the two-photon decay of the 2s ‘S1,2 and 1~2s ‘St, levels. which have been studied in H-like ions of Kr [72] and in He-like ions of Ni and Br at the ATLAS accelerator [41,42]. The triplet level of the same electron configuration, 1~2s 3S,, decays by Ml transition because of a purely relativistic interaction. The transition rate scales with Z”‘, and the aforementioned lower-Z data obtained with storage ring (Z = 6,7) [29] and EBIT (Z = 10,121 [27,28] are complemented by fast-ion beam lifetime measurements for Z = 36 [73] and Z = 41 [74], extending and improving on the available data. In the same two-electron system, the ls2p ‘P;’ level is rather long-lived, because its only regular decay is to the 1~2s 3S, level, and this transition rate is low because of the small energy interval (transition rates scale with (A.#) and because it scales only linearly with Z. (The scaling with energy was used some time ago for a Lamb shift measurement in He-like U”“+ [24].) In isotopes with hyperfine structure, however, there is mixing of hyperfine components of the ls2p 3Pz and 3Pp levels. The latter is short-lived because of the intercombination decay branch to the ground state which scales roughly as Z”‘. A measurement of the 3Pi level lifetime in a suitable odd isotope can then be interpreted in terms of the level structure (‘Pi- 3Pp splitting) or the hyperfine interaction [43]. This quenching by hfs interaction has also been used to reduce the cascade repopulation when measuring the decay properties of the 1~2s 3S, level in Nb39+ [74]. The game of playing with this hfs-induced mixing seems to be restricted to the He isoelectronic sequence where the level to mix with, ls2p 3Pp, is very short-lived for high Z. In the Be and Mg sequences, the corresponding nsnp ‘Pp level decays by An = 0 transition, the probability of which scales only with Z’, and any hfs-induced lifetime effects will be very difficult to measure [75,76].

Na-like ions with 11 electrons, only one of which is outside the closed shells formed by the others, are work horses of spectroscopy. The n = 3, An = 0 resonance lines are easily excited and have been studied for many elements, up to Gd (Z = 64) using laser-produced plasmas [82,12], for Pt (Z = 78) using an EBIT [21], and for Pb (Z = 82) on a fast ion beam [83]. Measurements of transition rates, however, did not spread higher than Z = 29 until very recently. The latest data, gained with fast ion beams of Nb [45], Xe and Au [44], therefore present a major step forward. The data for Xe (Z = 54) are very sensitive to higher-order relativistic effects: the precision on the 3p,,, level lifetime is comparable to the relativistic effect on the transition matrix element. The new lifetime results (Figs. 3, 4) confirm only those theoretical calculations which match the experimental level structure well enough [84-861. Measurements at higher nuclear charge than Z = 54 would be desirable, but there the 3~-3p,,~ transition gets too short in wavelength for grating spectrometers and the level lifetime too short for meaningful measurements with present techniques.

3.2. Be-like ions

3.4. Ne-, Mg-, Al-, S-like

Four-electron systems feature displaced terms, and therefore their structure is much more complex than that of three- or fewer-electron ions. For some lighter ions (of Al to Ar [77] and Si [57], respectively) two independent studies, being done in parallel and using partly different data sources and methods of analysis, arrived at very much the same structural results for the many levels up to it = 6. This agreement and mutual corroboration is a very satisfying example of present-day analytical possibilities. Beyond structure there is dynamics: the resonance and intercombination transition rates in Be-like ions are of fundamental ,and astrophysical interest. The intercombination transition rate appears to be well known for high-Z ions [78], but there are problems at low Z which are presently being tackled by storage ring work [30]. For the resonance line, there is interest in extending the systematits to ions beyond Fe. Measurements on n = 2, An = 0 transitions in Kr3” have been done at GSI Darmstadt this spring [79]. The EUV spectrum is known from tokamak

In ions with several electrons in the valence electron shell, intercombination (spin-changing) transitions occur. Their wavelength permits to locate the relative positions of

and fast-ion beam work; unfortunately the resonance line (decay of the 2s2p ‘Pp level) coincides in wavelength and decay rate with the strongest cascade repopulating this very level. The preliminary lifetime results are in the range predicted by MCDF calculations [80]. For the intercombination decay branch from the 2s3p 3Pp level in Be-like ions, there has been progress by extending the measurements up to Si’“+ [Bl]. The experimental problems, however, leave the results for Na to Si at a less than satisfying level of accuracy. 3.3. Na-like ions

10000

ions (rt = 3 shell)

I Na

Mg+AI

x 2.5

18

I 12

9

Wavelength

I

13

I

14

(nm)

Fig. 3. Spectra of foil-excited Xe at a specific ion energy of 5.9 MeV/amu in the wavelength regions near the 3s-3p and 3p-3d transitions of Xe4) + [44,47].

E. Triibert /Nucl. Instr. and Meth. in Phys. Res. B 98 (1995) 10-I 7 120 MeV Br -P

the partial level schemes, and their transition probability indicates the extent of higher-order contributions to atomic structure, like the gradual change from LS- towards jjcoupling. Intercombination transition data, mostly for atoms in intermediate ionization stages, have been reviewed only recently, so that the reader may be referred to Ref. [31], except for the latest high-Z studies. The isoelectronic sequences of interest which have lines in the same range as the resonance lines in the Na-like spectra are the Ne-, Mg-, Al- and %-like spectra. When using multiplex detection (position sensitive detectors on optical spectrometers [87]), a number of such transitions often appear on the same exposure (Figs. 3, 5). Interesting data have been obtained recently for Br [48], Xe and Au [44,47]. 3.5. &-like

ions, n = 4 shell ions

Cu-like ions with a single valence electron in the n = 4 shell are in a way related to the Na-like ions. Laser-produced plasma work in the soft-X-ray spectral range has produced almost all of the IZ= 4, An = 0 transitions, but the resonance transition 4~-4p,,~ was out of the spectral range. It is, however, the only one in the range of GSI’s 5 m grazing-incidence spectrometer, and measurements have been attempted on Cu-like U63+ [79]. A number of lines were seen (among the others probably were intercombination lines in Zn- and Ga-like spectra), but the product of beam intensity and time was not sufficient to obtain meaningful decay curves. Ions with several electrons in the n = 4 shell (Zn-, Ga-, Ge-sequences) show similarity with the aforementioned

Au Nb Ni Ti

1.0

u

Xe

II

‘,

cu

,‘1

Fe

1,

Cl Ar S

Si

Mg Al

Na

, 1 ,’ , 1, ,I

0.6 txf

15

Wavelength

C

(nm)

Fig. 5. Combined display of delayed spectra obtained of foil-excited bromine ions 1481. The individual spectra have been recorded at different foil positions using a position-sensitive detector. The spectrum close to the foil is crowded with lines from decays of short-lived levels; farther away from the foil the lines from slowly decaying levels dominate. At wavelengths near 25.5 and 26.0 nm there are intercombination lines.

Mg, Al and Si sequences. Recent beam-foil data have been obtained and systematized for Nb [46], MO, Rh and Ag (see Ref. 1311. The necessity of good structure calculations, for example allowing for core polarization effects, is obvious from those results. Besides the singly-excited levels, Cu-like spectra show levels from configurations with an excited 3d electron, that is then an open 3d-shell. These levels are essential for the Cu-vapour laser as well as for the Morley-Sugar laser scheme for highly ionized ions in the Cu sequence [88]. Beam-foil spectroscopic measurements for Nb, MO, Rh and Ag confirm the atomic structure data for the laser scheme as obtained from Cowan-code calculations. The apparent lifetimes of the laser levels to be, 3d9 4s’ ‘D, however, are much longer than the predicted ones, because the real ones are masked by a multitude of cascades from the 3d9 4s4p levels. This observation ties in with data from other sources which reach from Zn+ to Iz4+.

-

4. Conclusion

Fig. 4. Oscillator strength data of the resonance lines in Na-like ions. The solid curve represents the semi-empirical calculations by Theodosiou and Curtis [84]; this curve agrees with the results of ab initio Dirac-Fock calculations for 2 > 55 [86] and of ManyBody Perturbation Theory calculations for Z < 30 [85]. The most recent experimental data are those for Au and Xe [44] and for Nb 1451.

The field of spectroscopy of highly ionized atoms seems alive and rather well, although the community appears to be shrinking because of funding shortages. The multi-purpose light sources tokamak, laser-produced plasma and foil-excited ion beam are being used to produce many useful reference data, in particular by combining results from a variety of sources with different merits. There are, however, a number of severe problems to be overcome for precision work, and for some of these problems very recently valuable progress has been made by using other light sources, for example an EBIT (stationary

1. FUNDAMENTAL

ASPECTS

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E. Triibert /Nucl.

Instr. and Meth. in Phys. Res. B 98 (1995) 10-l 7

light source) or an ion storage ring (better control over the excitation process, cleaner spectra) instead of the less selective plasma and fast-ion beam light sources described here.

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1. FUNDAMENTAL

ASPECTS