Associate Professor, Department of Physics and Astronomy, Clemson University, the United States

Speech Title: Highly charged ion tungsten plasma: material for fusion energy diagnostics
Abstract: Spectroscopy of highly charged tungsten ions has gathered significant interest due to its relevance in plasma diagnostics of fusion plasmas. Tungsten is the plasma-facing material in magnetic confinement fusion devices such as ITER or the ASDEX Upgrade tokamak reactor. The need for the investigation of highly charged tungsten has been extensively discussed and highlighted by multiple research groups for over a decade. Measurements supporting this research have been performed in the extreme-ultraviolet and x-ray region in laboratory plasmas.
Electron beam ion traps (EBIT) are especially useful for these studies because of their charge state and excitation selectivity due a mono-energetic electron beam employed in these devices. The charge state distribution in the ITER plasma is expected to range from Ag-like W 27+ to F-like W 65+ moving from the outer edges to the core of the plasma, while the hotter regions of the ASDEX Upgrade can create Kr-like W 38+ to Ni-like W 46+ ions. Prediction of the spectra in multiple wavelength regions can be achieved with the use of collisional-radiative models that require the detailed knowledge of the excitation and recombination mechanisms in these environments. Combination of measurements and calculations of tungsten spectra emitted by the EBIT plasma allow the exploration of diagnostic parameters such as plasma density and temperature in fusion environments.
In this paper we review the current experimental capabilities of the EBIT at the National Institute of Standards and Technology (NIST) to aid fusion diagnostics based on tungsten ions. Previous measurements in the EUV region provided line identification in charge states ranging from W 39+ to W 63+ and magnetic-dipole and electric-dipole transitions were explored for their plasma diagnostics capabilities. Beyond the EUV region we have recently explored the use of high-resolution crystal spectrometers on the NIST EBIT to investigate transitions of highly charged tungsten in the x-ray region of the spectrum. The combination of two perpendicularly installed crystal spectrometers could potentially serve information about the polarization and anisotropic emission of x-ray transitions. These measurements can provide information about magnetic sublevel specific excitation mechanisms in fusion devices that could allow direction sensitive diagnostic capabilities.