Meeting Report

This meeting was held to evaluate and recommend fundamental data concerning tungsten, hydrogen, their ions and molecules in the edge plasma region of experimental nuclear fusion devices with a view to quantifying and reducing the uncertainties in the modelling of its collisional, radiative and plasma-material interaction properties.

The meeting was held virtually, from 29 March – 1 April 2021. More details, including abstract and presentation materials upload, are available on the IAEA conferences page (login required). The Scientific Secretary is Kalle HEINOLA.

####Organizing Committee

• Ursel FANTZ (University of Augsburg, Germany)
• Kalle HEINOLA (IAEA)
• Christian HILL (IAEA)
• Sebastiján BREZINSEK (Forschungszentrum Jülich, Germany)

####Sessions

#####A. Experiments: Tokamaks, Linear Plasmas, Stellarators

Experimental devices: JET (attached/detached), AUG, DIII-D, MAGNUM, PSI-2, ITER , JT60-SA, T10

• Metallic vs. graphite surfaces
• Isotope experiments
• Reflection and recycling
• Surface conditions and processes
• Spectroscopy and other methods

#####B. Modelling: Tokamaks, Linear Plasmas, Stellarators

Codes: SOLPS-ITER, EMC3-EIRENE, SONIC NEUT2D, EDGE2D-EIRENE, TOKAM3-X, SOLEDGE-EIRENE

• Issues with benchmarks
• Attached and detached conditions
• Isotope effects
• Spectroscopy
• Neutral pressure benchmarks

#####C. Experiments: Atomic and Molecular Data, Cross Sections, Processes

• Isotope effects
• Rotationally and vibrationally-resolved spectroscopy
• Highly-excited species
• Mixed molecules
• Other hydrogen-containing molecules
• Spectroscopy tools

#####D. Modelling: Atomic and Molecular Data calculations

• Convergent-close coupling methods
• High-temperature molecular spectroscopic line lists

#####E. Collisional Radiative Models

Codes: YACORA, EIRENE, EUMONIA, NEUT2D

• Outcome of EU-ADAS for molecules
• Vibrationally resolved CR
• Time dependent methods
• Application to detached conditions

#####F. Needs and Conclusions

• Publications
• Potential CRP

####Topics

#####Recombination rate coefficients for W^q+: experiments and theory Existing experimental recombination rate coefficients and cross sections are known for electron energies up to several hundred eV (i.e. for W^{18+ – 21+} ). However, at low energies the total recombination cross sections are orders of magnitude above those for radiative recombination. EBITS can be used for obtaining recombination cross sections, but energies below about 20 eV are not accessible experimentally, and this is the region with largest theoretical uncertainty.

The role of external electromagnetic fields on dielectronic recombination: high fields may have a large effect on plasma rate coefficients. For example, in Fe¹⁵⁺ the dielectronic recombination can change by a factor of 3 at temperatures of 10⁵ to 10⁶ K and at an electric field strength of kV/cm.

#####Electron-impact ionization of W^q+ The effect of long-lived states: experimental electron-impact ionization cross sections for W^{1+ – 19+} with energies up to 1 keV exist. However, in the experiments even if the multiply-charged ions are stored for some time, the effect of long-lived excited states (in the parent ion beam) will be present in the cross section measurements. Fine energy scans and good statistics can reveal these metastable ions. Theoretical modelling of the resulting cross sections can provide information on the long-lived beam components. Fusion plasma will contain such species in long-lived excited states whose cross sections are needed.

Resonant processes in high charge states and their contribution to net ionization: it is necessary to explore which charge states of W might have resonant contributions and to assess the related cross sections (modelling with, for example, R-matrix methods may be necessary as experiments are challenging).

#####Electron-impact excitation of W^q+ There is a lack of experimental data for electron-impact excitations in W.

#####Charge-transfer collisions of W^q+ with plasma species Particular species of interest are: H/D/T, He and He+. The cross sections are well known in general, but at very low energies their uncertainties are unquantified. As with electron-collision experiments involving multiply charged W, the role of metastable states is largely unknown.

#####Spectroscopic issues in the divertor region

Non-LTE modelling: near the divertor the electron density is higher and the temperature lower than in main plasma, but due to divertor conditions non-LTE modelling may be required: emission data are needed for W erosion assessments (maximum charge states anticipated are W^{6+ – 8+}; excluding transient events, are higher charge states expected?) No reliable NLTE modelling exists for W^{0 – 5+}.

Re-assessment of the role of MAR in tungsten machines: Molecular Assisted Recombination (MAR) can play an important role in assisting the volume recombination (by three-body and radiative recombination of atomic ions) in detached divertor plasma. Detailed investigations of MAR were done almost two decades ago, but on machines using carbon as a plasma-facing material. The properties of carbon and tungsten with respect to their interaction with hydrogen (i.e. the probability of H₂ formation at their surfaces) are very different; therefore, the role of MAR in modern fusion devices may differ from that determined for carbon machines.

Tungsten charge state distributions: plasma transport may shape the W charge state distribution (CSD): transport effects may change the averaged charge density <Z> and broaden the spatial range. The CSD deviates from transport-free, equilibrium values and offers information on plasma transport, provided that adequate ionization balance calculations are available. This is crucial for ITER: plasma transport measurements focus on the ionization balance of W. In ITER there will be no low-Z neutral beams employed, which are used in other fusion devices to monitor and observe plasma transport.

#####Atomic and molecular data: theory and modelling Collisional-Radiative (CR) modelling: the relation of CR modelling to plasma transport simulations; non-LTE code activities on testing CR codes for W; CR model implementations: configurational average (i.e. <Z>) vs. detailed models (up to thousands of charge states).

Molecular radiation of hydrogen and deuterium (OES and UV/VUV ranges): Detecting and interpreting molecular radiation gives access to molecular fluxes as well as vibrational and rotational populations: parameters that are of relevance to assessing the effectiveness of MAR (see above), The accuracy of modelling molecular radiation by collisional-radiative models is directly correlated with the accuracy and completeness of the set of input data used, including the cross sections for electron-impact excitation of molecular states. Up to now, two strongly-diverging sets of cross sections were available; the first steps towards establishing a more reliable and accurate alternative set are currently in progress.

#####The effect of opacity Self-absorption caused by optical thickness of the atomic Lyman emission lines can result in significant radiation trapping, influencing the energy balance and the ionization / recombination rates in the divertor plasma. If the radiation trapping is high, this may make the transition to a detached divertor more difficult. A second effect of opacity is that it strongly affects the intensity of emission lines and has to be taken into account in population models used for interpreting such emission. To do so, profiles of the opacity along the line-of-sight used are needed; this information is not presently available.

#####Middle-charge states of W (W^{10+ – 25+}) In ITER medium charge states of W are expected to be responsible for the majority of re-radiation from transient pulses. Key processes for which data is needed are: ion line data and ionization, recombination and excitation rates.

####Related Projects and Meetings

• CRP on Spectroscopic and Collisional Data for Tungsten from 1 eV to 20 keV, 2010 – 2015. Results from this project are reported in the Special Issue: "Atomic Data for Tungsten" of the journal Atoms: published in 2015.

• CRP on Atomic and Molecular Data for State-Resolved Modelling of Hydrogen and Helium and Their Isotopes in Fusion Plasma.