Effect of edge magnetic structure on H-mode pedestal width in JT-60U
T. Hatae, Y. Kamada, Y. Miura, T. Takizuka, T. Fukuda , T. Suzuki, T. Fujita
Japan Atomic Energy Research Institute, Naka Fusion Research Establishment Naka-machi, Naka-gun, Ibaraki-ken 311-0193 Japan
Abstract.
Recently it is recognized that the magnetic structure is a key parameter to determining the pedestal characteristics and consequently determines the core confinement. The width (i.e. pedestal width) and height of edge transport barrier represent the H-mode pedestal structure for the temperature and density. Key H-mode physics is contained in the width and height. From the point of view, to study the dependence of pedestal parameters on edge magnetic shear is important. Some H-mode pedestal width scalings have been proposed empirically so far [1-3]. In JT-60U, the pedestal width (Δ) is almost proportional to the poloidal gyro-radius of thermal ions (ρpi). On the other hand, a model for the pedestal width have been proposed based on turbulence suppression by the combined effects of magnetic and ExB shear[4]. The resultant scaling of the pedestal width includes the toroidal gyro-radius (ρtor) and the magnetic shear (S): Δ∝ρtorS2.
This paper mainly focuses on relationship between pedestal width and edge magnetic structure in ELMy H-mode. To verify the pedestal scaling based on the model Δ∝ρtorS2, current ramp up/down experiments were carried out. In the case of current ramp up, plasma current was increased from 0.7 MA to 1.2 MA. In the case of current ramp down, plasma current was decreased from 1.7 MA to 1.2 MA. The ramping rate was 0.7 MA/s in both cases, internal inductance values after current ramp up/down were 0.6 and 1.2, respectively. We concentrate on whether the edge pedestal structure is changed or not by the modification of the edge magnetic shear using the current ramp up/down. It is observed that the ion temperature pedestal width after current ramp up is comparable to that after current ramp down. The value of Δ/ρpi for current ramp up is almost the same as that for current ramp down. Since local magnetic shear at pedestal shoulder evaluated by MSE diagnostics shows difference of factor ~2 in this experiment, it seems that the pedestal width is not simply proportional to value of S2. Role of other edge parameter (e.g. plasma shape, safety factor) will be also studied in this paper.
References
[1] T. Osborne et al., Plasma Phys. Control. Fusion 40 (1998) 845.
[2] T. Hatae et al., Plasma Phys. Control. Fusion 40 (1998) 1073.
[3] Y. Kamada et al., Plasma Phys. Control. Fusion 41 (1999) 1371.
[4] M. Sugihara et al., Nucl. Fusion 40 (2000) 1743.