Fig. 2-6 Effect of negative magnetic shear on the TAE instability (a) In the region where negative shear is strong (region limited by normalized plasma radius 0.7, where the safety factor q decreases with the plasma radius), no TAE was observed. Distributions of the electron density, and of the pressure of high-energy ions (simulated alpha particles) normalized by the magnetic field pressure betah are also shown. (b) In the case of a weak negative shear (the safety factor q decreases gently compared to (a)), the TAE instability was observed at the plasma radius of about 0.5. Observed experimental conditions for stabilization (a) and for excitation (b) of the TAE were found to agree well with theoretical predictions. (c) Observed spectra of the TAE in the experimental condition of (b).

Stable confinement of a high performance, reactor-grade plasma has been demonstrated by negative magnetic shear mode, plasma operation in JT-60. This operation is generally accepted as one of the promising candidates leading to a tokamak fusion reactor in the future, and has been investigated extensively in JT-60. One of the critical issues is to evaluate the effect of a plasma instability called the alpha particle-driven TAE modes (Toroidicity-induced Alfven Eigenmode) on high temperature plasma confinement in a fusion reactor. To investigate this problem in JT-60, we have conducted a simulation experiment on negative magnetic shear-confined plasmas using a high-energy neutral beam injection simulating fusion-produced alpha particles. Figure 2-6 summarizes the experimental results indicating the effect of the negative shear mode on the TAE instability. In a sufficiently strong negative shear region, no TAE instability was observed, namely the TAE is stabilized even the plasma condition of volume-averaged pressure of high energy ions is critical to excite the instability (Fig. 2-6 (a)). While in the case of the weak negative shear (Fig. 2-6 (b)), stabilization due to negative shear seems to be insufficient and the TAE was observed as shown in Fig. 2-6 (c). Simulation calculation has shown that the stabilization conditions obtained in the experiment agree well with theoretical predictions. In the experiments conducted so far in JT-60, we have not observed any deterioration of high temperature plasma confinement.

Reference Y. Kusama et al., Characteristics of NNB-Driven Alfven Eigenmodes, Burst and Chipping Modes in the Alfven Frequency Range in JT-60U, Nucl. Fusion, 39, 1837 (1999).

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