X-ray Ross filter method for impurity transport studies on DIII-D (abstract)

Abstract

The injection of Ar into the region of the DIII-D divertor is a promising technique for energy dissipation (through radiation and collisions) and consequently for reduction of the heat load on the plates. An important problem related to this technique, is the inherent poisoning of the core plasma by migrating Ar. The Ar core contamination seems also to improve the thermal transport in an advanced operating mode of the tokamak. It is therefore of great importance to measure the evolution of the impurity concentration profile within the core plasma. This goal could be achieved by using the Ross filter method in conjunction with the existing x-ray diagnostics on DIII-D. A basic Ross filter system consists of two identical detectors placed behind two different x-ray absorbing foils looking at the same plasma volume. The foils are made of different elements or compounds with adjacent or nearly adjacent atomic numbers. Their accurate thickness causes the x-ray transmission curves of the two foils to be effectively identical over the entire energy range except within the narrow region between their absorption edges. Since the transmission characteristics of the foils above and below their absorption edges are the same, any difference in the two detected signals is proportional to the total x-ray power of the emission spectrum between these two edge energies. An x-ray Ross filter with its energy pass band centered on the Ar XVII Kα line at 3.14 keV has been designed. This allows for the discrimination of the Ar Kα line only, regardless of Ar ionization state, against any background radiation with energies outside the energy pass band. The Ross filter was installed in front of two of the fan shaped poloidal x-ray arrays on DIII-D. The first measurements showed very good discrimination against Ne, another injected impurity. Emissivity profile evolution of the Kα lines and Ar enhanced continuum within the energy pass band of the Ross filter can be determined from the x-ray brightness signals by inverting techniques and by using the Te, ne, and Ar16+ profiles as measured by other diagnostics. The transport code MIST1 can be used to calculate both the emissivity profiles of the Kα of all the ions and their concentration profiles when the measured Te, and ne are used as input. The Ar16+ profiles as measured by charge exchange spectroscopy can be used as a constraint for the MIST code to accurately calculate the Ar18+ profile and thus unfold all the Ar ions Kα emissivity profiles. From these one can determine the Ar concentration profile evolution and the particle diffusion coefficient. In conclusion, using the Ross filter method with the existing x-ray imaging systems results in a powerful and cost-effective diagnostic for impurity transport studies.