Millimeter-wave reflectometry diagnostics on the DIII-D tokamak
A multichannel reflectometer diagnostic has been used to investigate density profiles and density fluctuations in the DIII-D tokamak. Dramatic decreases in high frequency microturbulence are observed at the L to H-mode transition. The spatial resolution of the diagnostic has allowed, for the first time, determination of the radial extent of these modifications. Fluctuation changes are observed to occur in a region from {approx}1 cm outside to {approx}10 cm inside the magnetic separatrix. These changes occur on a very rapid timescale, typically {le} 50 {mu}s and they occur simultaneously with the decrease in H{sub {alpha}} emission observed at the L to H transition. The density fluctuation spectra during an ELM event are similar to those in L-mode. However, the fluctuation changes precede the rise in H{sub {alpha}} emission at the ELM, often by as much as 100 {mu}s. In H-mode, the width of the frequency spectra is observed to scale inversely with plasma current, but has little dependence on toroidal field. In L-mode the situation is reversed and the spectral width varies inversely with B{sub {phi}} and shows little variation with I{sub p}. Edge density gradient modifications are observed to occur within 5-10 ms of the L to H transition, whereas the average density continues to rise for more than 100 ms. This is consistent with predictions of an edge transport barrier formation in H-mode. In addition two distinct profile effects associated with ELMs have been identified. These are the density gradient collapse and the density pulse ELMS. Coherent density oscillations associated with low m/n number MHD activity have been observed. Using the multichannel reflectometer it is shown that these fluctuations persist in a region located within 1 to 2 cm of the magnetic separatrix. ELMs and sawteeth are seen to affect the coupling between the electromagnetic and electrostatic components of these oscillations.
- Research Organization:
- California Univ., Los Angeles, CA (USA)
- OSTI ID:
- 6048916
- Resource Relation:
- Other Information: Thesis (Ph. D.)
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
PLASMA DIAGNOSTICS
MEASURING INSTRUMENTS
TOKAMAK DEVICES
EDGE LOCALIZED MODES
H-MODE PLASMA CONFINEMENT
PLASMA DENSITY
SAWTOOTH OSCILLATIONS
SPATIAL RESOLUTION
TURBULENCE
VARIATIONS
CLOSED PLASMA DEVICES
CONFINEMENT
INSTABILITY
OSCILLATIONS
PLASMA CONFINEMENT
PLASMA INSTABILITY
PLASMA MACROINSTABILITIES
RESOLUTION
THERMONUCLEAR DEVICES
700102* - Fusion Energy- Plasma Research- Diagnostics