K." Tanaka

S. "Ohdachi

National Institute for Fusion Science (NIFS-Repository)

Core density collapse (CDC) is a large-scale relax-ation event observed in internal diﬀusion barrier (IDB)plasmas. It has been shown that the CDC appears wherethe ideal ballooning modes are unstable 1). Pre-cursoroscillations just before the events are reported here.Figure 1 shows the equilibrium using the pressureproﬁle measured before the CDC. The rotational trans-form proﬁle (Fig. 1(B) circles) measured by the MSEsystem agrees well with the calculated proﬁle (solid line).The growth rate of ideal high–n ballooning mode is cal-culated using Hn-bal code 2) ; the ballooning mode islocally unstable around ρ ∼ 0.8 (Fig. 1(D)). The mag-netic shear is negative in that region; this mode might behigh-n ballooning mode that is expected only in helicalsystem.(A)BeforeAfter103960   4.033s, 4.066s 0.00 1.00 2.00 3.00 4.00beta [%](B) 0.00 0.20 0.40 0.60 0.80 1.00iota(C) 0.00 0.05 0.10 0.15well depth [a.u.](D)beta x 1.20 0.2 0.4 0.6 0.8 1ρ 0.00 0.10 0.20 0.30growth rate [/ωA]Fig. 1: Equilibrium proﬁle just before the CDC is shown.The beta proﬁle (A), the rotational transform (B), the mag-netic well depth (C) and the growth rate of the ballooningmode (D) are shown together.In the IDB plasmas, most of the diagnostics for ﬂuc-tuations does not work due to the high electron density.However, we can compare the ﬂuctuations at inboardand outboard side with an 80ch-CO2 interferometer sys-tem 3). The location of the chord and the ﬂux surfaceare shown in Fig. 2(A). Coherent oscillations with a fre-quency of about 8kHz are observed at several hundredsof micro second before the events only in the outboardside of the plasma as is shown in Fig. 2(B). There are twopeaks; while a larger peak (ρ ∼ 0.8) agrees well with theprediction by the Hn-Bal code, other peak (ρ ∼ 0.3) doesnot. One explanation is the line integration of the inter-ferometer. The measuring chord around ρ ∼ 0.3 alsomeasures the edge region; ﬂuctuations observed theremight be caused by the edge oscillations. We need tostudy 3D mode structure to resolve this mystery.In summary, relation of the CDC event and thehigh–n ballooning modes becomes more concrete sincepre-cursor oscillations localized only in the outboard sideare observed.(A) 3.4  3.5  3.6  3.7  3.8  3.9  4.0  4.1  4.2R [m]-0.2-0.1 0.0 0.1 0.2Z [m](B1) 1.0  0.8  0.6  0.4  0.2  0.0 0.2 0.5  1.0ρ0.000.050.100.150.20∆ neL / neL(C)BeforeAfter051015202530neL [ 1019  m-2  ] 3.4  3.5  3.6  3.7  3.8  3.9  4.0  4.1  4.2R [m]γ [/ωA]∆neL/neL(B2) 0.0  0.2  0.4  0.6  0.8  1.0ρ0.000.050.100.150.20Fig. 2: The ﬂux surface and the location of the chord areshown in (A). The ﬂuctuation level as a function of the majorradius (B1) and of the averaged minor radius (B2) is shown.The line-integrated density proﬁle is shown in (C).1) S. Ohdachi, et. al., Contrib. Plasma Phys. 50 (2010)552.2) N. Nakajima, et. al., Fusion. Sci. Tech. 51 (2007) 79.3) K. Tanaka, et. al., Rev. Sci. Instrum. 75 (2004) 3429.11§6. Observation of the Pre-Cursors in  the Core Density Collapse EventOhdachi, S., Tanaka, K.

Observation of the Pre-Cursors in the Core Density Collapse Event

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