Properties of Tungsten Ions in Fusion Plasmas

The proposed Coordinated Research Project (CRP) on the Properties of Tungsten Ions in Fusion Plasmas will address data needs in the area of the ionization balance and spectroscopic and collisional properties of tungsten at temperatures between 1 keV and 10 keV, which are currently subject to large uncertainties and disagreements between theory and experiment.

The three key areas of research within the project are:

1. Ionisation from metastable states of low ionisation stages of W

This process has been found to play a substantial role in ionisation equilibrium. Up until now, only ground state ionisation has been included in the collision calculations. One of the CRP objective is to perform collisional calculations of ionisation cross sections and rate coefficients from metastables of the first ionisation stages (e.g. W to W⁹⁺).

2. Neutral and proton collisions with low ionisation stages of W

Charge exchange plays an essential role in the plasma edge of fusion devices. Room temperature neutrals come from such fuelling methods as gas puffing or supersonic gas injection. There is also a population of 'hot' neutrals resulting from charge exchange between the cold neutrals and the outgoing H⁺/D⁺ flux from the plasma. The same goes for H⁺/D⁺: two populations must be included in the calculations. Those calculations must be accompanied with experimental measurements for validation.

It is expected that the results will modify the ionisation balance. This in turn can change the evaluation of the prompt redeposition of W, sputtering, plasma facing component lifetime and W transport in the scrape-off layer of fusion devices.

3. Spectroscopic analysis of low and medium ionisation stages of W

Many fusion devices use W as a plasma-facing material or inject it for spectroscopic investigations. The spectra measured in those devices are very complex. In several wavelength intervals, particularly in the EUV domain they show a superposition of individual spectral lines and so-called quasi-continuous features emitted by a range of W ionisation stages. The line assignment is extremely complex and time-consuming. It involves a detailed knowledge of the plasma conditions and scenario, including the recent experimental history and conditioning techniques of the device. Despite intensive work, lines from important ionisation stages (namely W IX to W XXVII) are still missing. As far as the quasi-continua are concerned, the interpretation by sophisticated atomic models need improvement before the measurements can be used to provide information about the behaviour of W. The objectives of this part of the CRP will be the following:

• To make (structure and collision) calculations to provide line wavelengths of W IX–XXVII in the EUV and visible ranges;
• To make a thorough line assignment for wavelength intervals of particular interest from existing measurements in various fusion devices and EBITs.