Novel analysis technique for measuring edge density fluctuation profiles with reflectometry in the Large Helical
A J Creely1, K Ida2, M Yoshinuma2
1MIT Plasma Science and Fusion Center, Cambridge, Massachusetts 02139, USA.
The Review of Scientific Instruments
|August 3, 2017
Summary
A novel method combines reflectometry and pellet-induced scans to measure plasma density fluctuations near the edge of the Large Helical Device (LHD). This technique accurately maps density profiles, revealing strong peaks in the edge plasma turbulence layer.
Area of Science:
- Plasma physics
- Fusion energy research
- Diagnostic techniques
Background:
- Accurate measurement of plasma edge properties is crucial for understanding confinement and stability in fusion devices.
- Existing diagnostic methods for density fluctuations near the plasma edge have limitations in spatial and temporal resolution.
Purpose of the Study:
- To develop and validate a new method for measuring density fluctuation profiles near the plasma edge in the Large Helical Device (LHD).
- To characterize the spatial extent and intensity of density fluctuations in the LHD edge plasma.
Main Methods:
- Utilized reflectometry combined with pellet-induced fast density scans.
- Employed ray-tracing code (LHD-GAUSS) for reflectometer cutoff location calculation, scaled with fast far infrared laser interferometer (FIR) data.
- Validated plasma velocity profiles against charge exchange spectroscopy (CXS) measurements.
Main Results:
- Developed a reliable method for measuring density fluctuation profiles near the LHD plasma edge.
- Observed strong peaks in density fluctuation profiles near the plasma edge, consistent with tokamak observations.
- The turbulence layer width was found to be narrow (1.5-3 cm), less than 5% of the normalized minor radius.
Conclusions:
- The new reflectometry-based method provides accurate measurements of density fluctuations in the LHD edge plasma.
- These measurements offer valuable data for refining plasma models and informing inversion techniques for other diagnostics like phase contrast imaging (PCI).
- The narrow turbulence layer suggests localized edge phenomena influencing plasma behavior.
