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Phase contrast imaging measurements of reversed shear Alfvén eigenmodes during sawteeth in Alcator C-Moda)
a)Paper GI1 3, Bull. Am. Phys. Soc. 53, 79 (2008).
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View: Figures


Image of FIG. 1.
FIG. 1.

Experimental traces showing (a) total plasma current, (b) line-averaged electron density, (c) central electron temperature from the ECE diagnostic, (d) central deuterium ion temperature derived from the neutron flux, and (e) the injected ICRH power. The breaks in slope of the density trace, starting around 0.25 s, indicate transitions between mode and ELM-free mode. Ion temperature data are not available before 0.15 s and is represented as 0.8 times for this time range (dashed trace).

Image of FIG. 2.
FIG. 2.

(a) Spectrogram of RSAEs excited during a sawtooth cycle as measured by PCI, with mode numbers inferred from the best fit to Eq. (1) overlaid in dashed line. (b) Spectrogram for the same time period measured by a Mirnov coil. (c) Measured phases of the modes detected by the Mirnov coils as a function of toroidal angle, confirming the inference from the PCI analysis.

Image of FIG. 3.
FIG. 3.

The synthetic PCI analysis shows the greatest sensitivity to , which manifests as the peak spacing and width in the synthetic signal. (a) The input profile with , , and . (b) The two-dimensional density perturbation calculated by NOVA for an RSAE using the above profile and (c) the resulting comparison between the experimental and synthetic PCI signals. Panels (d)–(f) use an unoptimized value of to illustrate the differences in synthetic diagnostic signal.

Image of FIG. 4.
FIG. 4.

(a) Alfvén continuum plot from NOVA for an up-chirping RSAE. (b) The radial displacement eigenmode associated with the profile shown in (c). Plots (d)–(f) pertain to a down-chirping RSAE for a profile with a local maximum at .

Image of FIG. 5.
FIG. 5.

Plots of as a function of for the modes identified in Fig. 2 show a best fit solution near . No solutions at were found for the and 6 cases.

Image of FIG. 6.
FIG. 6.

PCI spectrogram of down-chirping RSAEs immediately following a sawtooth crash. Toroidal mode numbers of are identified by comparison with Eq. (1) which implies that following the crash.

Image of FIG. 7.
FIG. 7.

An example of the application of the Kadomtsev model. (a) The precrash profile (blue dashed) with , , and , and the postcrash profile (orange solid) with a local maximum of at . (b) The helical flux function before (blue dashed) and after (orange solid) the reconnection. The vertical dotted line indicates the precrash surface and the arrows indicate how the reconnection transforms the precrash state into the postcrash state.

Image of FIG. 8.
FIG. 8.

Evolution of profiles during a sawtooth cycle of 18 ms at equal time intervals of 3 ms. The initial conditions are provided by the Kadomtsev model results similar to those presented in Fig. 7. The two cases are (a) classical resistivity with flat profile and (b) neoclassical resistivity with peaked profile.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Phase contrast imaging measurements of reversed shear Alfvén eigenmodes during sawteeth in Alcator C-Moda)