Properties of Tungsten Ions in Fusion Plasmas

A viable nuclear fusion reactor utilizing magnetic confinement will be operated with hydrogen plasma as its fuel. The energy is gained by fusing hydrogen isotopes, deuterium (D) and tritium (T). These fusion processes take place in the core of the plasma at temperatures of 10⁸ K. The significantly cooler outer regions of the D-T plasma will be in interactions with the reactor inner walls. The wall materials, i.e. the first wall materials or also known as armour materials, must therefore withstand heat and particle loads from the plasma. Currently the most favorable material to withstand these incomparable conditions is tungsten (W), which has the highest melting point of all metals, it has very good durability against high heat and plasma particle loads and the retention rate for hydrogenic fusion fuel species (D, T) is very low in W-based materials. Therefore, W is planned to be used in the next step fusion device ITER and in several future DEMO-scale and pilot plant fusion reactors around the world. Regardless of these favorable properties of W, during high flux plasma particle bombardments situations may occur where varying amounts of W particles are being ejected from the first wall components through erosion and sputtering processes. W particles entering the fusion plasma and especially its core can be extremely detrimental for the stability and lifetime of the plasma fuel. Impurity species entering the plasma contaminate the plasma and emit radiation through radiative-collisional processes, such as bremsstrahlung and recombination radiation. These power losses depend in addition to the plasma electron temperature and density also on the proton number (Z) of the impurity species as Z². W being a heavy metal element with Z=74 can create severe plasma power losses in the form of disruptions or other plasma instabilities and thus its presence in the plasma must be avoided and its collisional processes with the plasma as well as the subsequent transport properties must be understood. It is important to know as much as possible about the behavior of W at different charge states in different regions of the fusion plasma.  Of particular concern are the ionization processes and collisional properties of W at electron temperatures between 1 keV and 10 keV, which are currently subject to large uncertainties and disagreements between theory and experiments. Collisional processes of neutral atoms and protons with low ionization stages of W play a critical role as well as the ionization processes of low-charge metastable states of W. The overall objective is to provide evaluated experimental and computational collisional data on W ion properties at fusion-relevant plasma conditions. The significance of these results will be to any fusion reactor operating with W-based armour materials.