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Edge energy transport barrier and turbulence in the I-mode regime on Alcator C-Moda)
a)Paper PI2 6, Bull. Am. Phys. Soc. 55, 239 (2010).
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color online) (a) Edge profiles of electron density (top) and temperature (bottom) for different phases of a 1.27 MA, 5.6 T C-Mod discharge (1091016033). Curves have been fit using a modified tan function as described in Ref 28 . The I-mode phase (red solid line, 1.41 s) has a steep gradient, but a density profile close to that in L-mode (black dotted line, 0.78 s). In a brief ELM-free H-mode (blue dashed curve, 1.47 s) is similar to I-mode but is much steeper. (b) Time histories of input ICRF power (top) and plasma parameters for the same discharge. In fourth panel, solid black curve is stored energy, and dashed red curve is confinement normalized to IPB98y2 scaling, reaching 1 in I-mode (Ref. 18 ).

Image of FIG. 2.
FIG. 2.

(Color online) Typical shape of C-Mod last closed flux surface for I-mode experiments in unfavorable drift (upper x-point) and favorable drift (lower x-point, solid). Plasmas in more usual C-mod configurations (e.g., dashed gray curve) have not exhibited transitions to I-mode.

Image of FIG. 3.
FIG. 3.

(Color online) Steady I-mode achieved in favorable configuration (C-Mod discharge 1100827022, 0.8 MA, 5.4 T, lower single null). Density fluctuations measured by O-mode reflectometry (top panel), at  = 7 × 1019 m−3 ( / 0.98), show a decrease in mid-frequency turbulence and the appearance of a broad high f feature soon after the application of 0.75 MW ICRH (bottom). The I-mode phase, which has edge significantly above that in similar L-mode discharges (middle panel), lasts for the duration of the RF pulse, about 10 τ.

Image of FIG. 4.
FIG. 4.

(Color online) (a) Power thresholds for the L–I transition (red squares) and I–H transition (black diamonds), normalized to the Martin scaling (Ref. 17 ). The circled points are for favorable configuration and fit the scaling, while the remaining points, in unfavorable drift, are significantly higher. (b) Energy confinement time for I-mode discharges, normalized to the IPB98y2 scaling (Ref. 18 ). Average H is about 1, in both configurations, for discharges with  > 5 T (closed symbols) and about 0.8 for  < 3.5 T (open symbols).

Image of FIG. 5.
FIG. 5.

(Color online) Fluctuations and edge thermal transport in L-, I-, and H-mode for a typical q = 3.1 C-Mod discharge 109120320 (1.3 MA, 5.8 T, upper single null). (a) Contours of reflectometer fluctuations (88 GHz,  = 9.6 × 1019 m−3 ( / 0.95) (b) Edge fluxes and gradients; ∇ (0.95 <  < 1)(green curve) increases from 25 to 88 keV/m in I-mode, whereas net power (red) drops slightly due to increasing /. (c) Computed χ decreases from L-mode to I-mode, with a further reduction at the transition to an ELM-free H-mode. (d) Fluctuations integrated over the 60–150 kHz frequency band, exhibiting a decrease similar to that in χ. Global stored energy (dashed line, scaled) increases during the I-mode phase as edge fluctuations and transport decrease. (e) Fluctuation spectra, averaged over 20 ms, in L-mode (1.03 s), I-mode (1.15 s), and H-mode (1.24 s).

Image of FIG. 6.
FIG. 6.

(Color online) Evolution of fluctuations and edge for a q = 3.8 C-Mod discharge 1100817013 (1.1 MA, 5.6 T, upper single null). (a) Contours of reflectometer fluctuations (88 GHz,  = 9.6 × 1019 m−3 ( / 0.98). (b) at 0.91, showing a gradual increase after application of ICRF power (c). (d) Integrated reflectometry fluctuations in the band 60–120 kHz, which are anticorrelated with both during the L–I transition period and at sawtooth heat pulses in the steady I-mode phase. (e) Reflectometry spectra in the L-mode (black) and I-mode phases indicated by bars in (c), and during a later I-mode phase with slightly higher .

Image of FIG. 7.
FIG. 7.

(Color online) CXRS profiles (circles) for boron + 5, averaged over I-mode phases of the same discharge as in Fig. 6 . Note that (a) does not have the same profile as (black diamonds, scaled by 200 for comparison). Its temperature pedestal (b), however, is the same as measured by edge TS (black diamonds) within respective mapping uncertainties. (c) is flat and near zero, while (d) exhibits co-current rotation of up to 40 km/s.

Image of FIG. 8.
FIG. 8.

(Color online) (a) Radial electric field in a 1.1 MA I-mode discharge, derived from the CXRS profiles shown in Fig. 7 . The 30 keV/m well in total (black) is dominated by the diamagnetic term (lower curve), with a significant gradient from the term (upper curve). (b) For comparison, an profile in a typical EDA H-mode, with  = 800 kA, which has a stronger contribution from the term. Reprinted with permission from McDermott , Phys. Plasmas 16, 056103. © 2009, American Institute of Physics.

Image of FIG. 9.
FIG. 9.

(Color online) (a) Emission fluctuations measured by gas puff imaging at the horizontal midplane during an I-mode phase of C-Mod discharge 1100204022 (1.3 MA, 5.8 T, upper single null). A weakly coherent mode is visible, centered at about  = 220 kHz,  =  + 1.25 cm−1 (electron diamagnetic direction). (b) Relative amplitude of the WCM (increase over L-Mode turbulence, normalized to average intensity) vs. radius. The radial profile of the quasicoherent mode in an EDA H-mode is shown for comparison (black).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Edge energy transport barrier and turbulence in the I-mode regime on Alcator C-Moda)