Fluctuations and Fast Wave measurements by the Phase Contrast Imaging on Alcator C-Mod
Abstract
The Phase Contrast Imaging (PCI) diagnostic is being used to measure low-frequency (2-500 kHz) and RF induced density fluctuations in Alcator C-Mod. A quasi-coherent mode ( ~100 kHz) was detected in enhanced-D_α H-mode plasmas. It originates in the plasma pedestal and propagates poloidally in the electron diamagnetic drift direction (ω/k ~1.5 km/sec). The dispersion relationship corresponds to ω^*e and theoretical comparisons are being made with resistive instability theories(B.N. Rogers and J.F. Drake, Physics of Plasmas 6), 2797(1999). A recent upgrade now allows measurements of density fluctuations associated with propagating RF waves. An optical heterodyning technique is used: the intensity of the laser beam is modulated near the RF frequency (80 MHz). A signal is then recorded at the beat frequency of a few hundred kHz. A fast wave was observed for some peaked density profile shots. The measured wave number of 1-2 cm-1 is in good agreement with the fast wave dispersion relation. Comparison of the experimental results with theory and computer simulations will be presented.
- Publication:
-
APS Division of Plasma Physics Meeting Abstracts
- Pub Date:
- October 2000
- Bibcode:
- 2000APS..DPPUP1105M