Application of fast reflectometer density profile measurements to investigate plasma instabilities in DIII-D.

The high spatial (≥2 mm depending upon plasma conditions) and temporal (10 µs) resolution possible with the DIII-D solid state profile reflectometer system makes it possible to investigate profile modifications associated with plasma instabilities. In order to take full advantage of this fast sweep and high resolution capability, multiple issues had to be addressed, including fast data acquisition, large data acquisition memory depth, improved signal to noise, accurate profile start location, and finally fast accurate automatic data analysis. Improvements in all these areas make the profile reflectometer ready for plasma instability studies. In a demonstration of this capability, fast, high spatial resolution reflectometer measurements have been successfully used to investigate the edge harmonic oscillation (EHO) in the quiescent double barrier regime, as well as the edge localized modes (ELMs) on DIII-D. It is found that the scrape-off layer density profile is modulated at the EHO fundamental frequency. The edge density profile evolution is also tracked during ELMs, showing that the density profile expands to the vessel wall at the onset of Type 1 ELMs. [ABSTRACT FROM AUTHOR]