R. Radtke

Spectroscopy of highly-charged tungsten ions relevant to fusion plasmas

The Berlin electron beam ion trap (EBIT) has been established by the Max-Planck-Institut fur Plasmaphysik to generate atomic physics data in support of research in the field of controlled nuclear fusion by measuring the radiation from highly charged ions, particularly tungsten ions, in the x-ray, extreme ultraviolet and visible spectral ranges. With EBIT a selected ensemble of ions in specific charge states can be produced, stored and excited for spectroscopic investigations. Employing this technique, we have investigated the soft x-ray lines at 0.56 nm from Cu-like W45+ to V-like W50+ ions originating from 3d–4f transitions, which also are observed in the high-temperature core plasma of the ASDEX Upgrade tokamak. A further study focuses on Si-like W60+ to Ne-like W64+ tungsten ions predicted to dominate the core plasma of the International Thermonuclear Experimental Reactor (ITER) and radiate strongly at 0.13 nm from n=3–2 transitions.

Disentangling the emissions of highly ionized tungsten in the range 4–14 nm

In this work EBIT measurements in the spectral range of 4-7 nm with electron energies from 0.7 keV to 4 keV are compared in detail with tokamak spectra from ASDEX Upgrade. The energy and temperature dependence of spectral lines and features are investigated and an identification of the charge states W 39+ -W 45+ for spectral lines between 4.5 nm and 6.5 nm is possible by taking cross sections from atomic data produced by plane-wave Born calculations via the Cowan code. In the range 12-14 nm the identification of spectral lines emitted by tungsten is improved even though no EBIT measurements are available, as special tokamak discharges have provided ideal possibilities for analysis. These discharges show spectra which are dominated by the emissions from a small region with a narrow electron temperature range. Therefore, only very few ion states contribute to the emission in contrast to the usual case where in the tokamak spectra the emissions of a larger number of ion states are superimposed. Tokamak spectra for varying electron temperature are also used to investigate the correlation to EBIT spectra for the quasicontinuum feature at 5 nm which is emitted by ion states W 27+ -W 35+ . We conclude that the narrower features in the EBIT spectra at a certain electron beam energy result from the smaller number of coexisting ion states.

First results from the Berlin EBIT

A new Electron-Beam-Ion-Trap (EBIT) has been installed at the Max Planck Institute of Plasmaphysics Berlin branch. The experimental technique consists in producing and trapping highly charged ions inside the space charge of an electron beam and measuring the emitted x-ray radiation. During the first test phase x-ray spectra of highly charged Ba, W and Ar-ions were observed.

X ray and EUV spectroscopic measurements of highly charged tungsten ions relevant to fusion plasmas

Using high-resolution x ray and extreme ultraviolet (EUV) spectrometry, the line emission of W28+ - W50+ ions was measured at the Berlin Electron Beam Ion Trap (EBIT). Our study encompasses a wide range of wavelengths (5-800 A) and includes the observation of electric and magnetic dipole lines. The results of our measurements are compared with predicted transition wavelengths from ab initioatomic structure calculations.

EBIT spectroscopy of Pm-like tungsten

Methods of VUV electron beam ion trap (EBIT) spectroscopy are applied to the study of Pm-like tungsten (W13+). These data show that theory appears well capable of dealing with these multi-electron (61) ions, at least for high ionization stages. A comparison of other spectroscopic methods applied to the study of other ions of the Pm I sequence is also given, and finally a search for the Pm-like W lines at the ASDEX Upgrade Tokamak is mentioned

EBIT: An electron beam source for the production and confinement of highly ionized atoms

Highly ionized atoms can be conveniently produced by an electron beam in an EBIT device. We give a survey on the technical operation, the physical principles involved in production and confinement of the ions, and the possible applications in atomic and plasma physics.

Dielectronic and radiative recombination of Si- to N-like tungsten ions

We have performed x-ray spectroscopic measurements of the dielectronic recombination resonance strength for the LMn (n = 3, ..., 10) series of Si-like W60+ to N-like W67+ tungsten ions. Highly charged tungsten ions were produced, stored and excited with the Berlin electron beam ion trap and the emitted radiation was analyzed with a solid state detector. Information on the charge state abundance in the trap was extracted from a fit of the theoretical radiative recombination intensity to measured values. The fit procedure was only feasible when the fine structure, angular momentum of the recombination channels is taken into account. Our measurement of x-rays from n = 2-3, 2-4 and higher DR resonance transitions was compared to relativistic calculations of the DR cross sections and rate coefficients calculated with the Hebrew University Lawrence Livermore Atomic Code (HULLAC). The previous theoretical predictions for Ne-like tungsten (W64+) [3] were extended with calculations for ions in adjacent charge states and compare well with the observed DR resonance structure.

Charge exchange of highly charged argon ions as a function of projectile energy

X-ray emission of highly charged argon ions following charge exchange collisions with argon atoms has been measured as a function of projectile energy. The ions are extracted from the Electron Beam Ion Trap (EBIT) in Berlin and selected according to their massto-charge ratios. Experiments focussed on hydrogen-like and bare argon ions which were decelerated from 125q eV/amu to below 0.25q eV/amu prior to interaction with an argon gas target. The x-ray spectra recorded probe the cascading transitions resulting from electron capture into Rydberg states and are found to vary significantly with collision velocity. This indicates a shift in the orbital angular momentum of the capture state. Hardness ratios are observed to increase with decreasing projectile energy though at a rate which differs from the results of simulations. For comparison, measurements of the x-ray emission following charge exchange within the trap were carried out and are in agreement with the findings of the EBIT group at LLNL. Both of these in situ measurements, however, are in discrepancy with the results of the experiments using extracted ions.

Extraction of highly charged ions from the Berlin Electron Beam Ion Trap for interactions with a gas target

Highly charged ions are extracted from the Berlin Electron Beam Ion Trap for investigations of charge exchange with a gas target. The classical over-the-barrier model for slow highly charged ions describes this process, whereby one or more electrons are captured from the target into Rydberg states of the ion. The excited state relaxes via a radiative cascade of the electron to ground energy. The cascade spectra are characteristic of the capture state. We investigate x-ray photons emitted as a result of interactions between Ar17+ ions at energies ⩽5qkeV with Ar atoms. Of particular interest is the velocity dependence of the angular momentum capture state lc.

Sawtooth Oscillations in EBIT

The dynamics of mixed ensembles of highly charged argon/xenon and krypton/xenon ions in an electron-beam ion trap (EBIT) was studied by recording the characteristic x-ray emission of the trapped ions. Sawtooth-like signatures manifest in the x-ray spectra for a variety of trap parameters. The effect can be understood as arising from the feedback between low-Z and high-Z ions. In this paper we report on a new and unexpected effect that is triggered by the non- linear dynamics and interaction of ions confined in electron beam ion traps and was recently demonstrated for the first time at the Berlin EBIT facility (1,2). The effect manifests by sawtooth-like oscillations of the trap plasma and has the potential to serve as benchmark for testing of theoretical EBIT models. It is caused by the feedback between low-Z and high-Z ions when, in addition to a heavy component, a light element is injected into the trap for cooling purposes. Working with light ions along with heavier ones is termed evaporative cooling (3) and has been proven to be essential for the production and storage of very highly charged ions. A substantial prerequisite for the creation of sawtooth oscillations is that the commonly applied conditions for the cooling technique are changed: The rate at which the light element (coolant gas) is added to the trap is much larger than the influx of the heavy component. Operating in this way, highly charged ions can concentrate in the trap during extended accumulation periods until the cooling from decreased low-Z ions is no longer sufficient and a sudden collapse of the ion population occurs.