This picture.(49KB)

Fig. 2-5 Typical wave forms of a discharge in which normalized pressure of 2.3 was sustained for 22.3 s Top: After the plasma current ramp up, the positive-ion based neutral beam (P-NB) was injected at optimized timing. After 10 s, optimization was continued with negative-ion based NB (N-NB) injection. Middle: Normalized pressure 2.3 was maintained for 22.3 s (during the current flat-top). Bottom: Current profile index saturated in the later phase this indicated that the current profile reached the steady state.

This picture.(49KB)

Fig. 2-6 Progress of normalized pressure and its sustained duration Closed circles indicate data obtained previously, while open circles indicate data obtained in 30 s heating. The blue region (1.8-2) indicates the ITER standard operation, while the orange region (2.5-3) indicates the advanced operation. Sustained duration of high normalized pressure, which is closer to the advanced operation is extended to a much longer duration.

The output power of a fusion reactor is proportional to the square of the plasma pressure. Therefore, confinement of high pressure plasma is one of the most important issues in economical reactor development. For a tokamak reactor, the plasma pressure relative to the toroidal field and plasma current is used as a measure (normalized pressure). In ITER, both the standard operation, and the so called advanced operation, in which long sustainment of high pressure plasma is aimed towards future steady-state reactors, have been discussed. JT-60 has sustained successfully high normalized pressure beyond the value that expected in the standard scenario but rather closer to that expected in the steady state operations for the world longest duration, 22.3 s (Fig. 2-5). When the plasma pressure becomes higher, magnetohydrodynamic instabilities occur and cause an increase in energy leakage from the plasma (degradation of the energy confinement). Together with the normalized pressure, the pressure and current profile in the plasma can influence the occurrence of these instabilities. In JT-60, positive-ion based neutral beams that have various injection angle and a negative-ion based neutral beam, which will be used in ITER, are equipped. By utilizing these, studies to increase normalized pressure, while avoiding the instabilities by using a heating profile and timing have been performed and notable results have been obtained. Towards realization of the ITER advanced operations and a steady-state fusion reactor, it is necessary to prove that the high normalized pressure plasmas obtained can be sustained for a long duration. It takes some time for the current profile, which is a key to avoid the instabilities, to be equilibrated. It is necessary to wait that long to confirm the influence of the instabilities. The time constant can be 10 s or even longer in JT-60. A modification of the control system extended the heating period in JT-60 was extended from 10 to 30 s. The results obtained by utilizing this extended heating period and the instability avoidance technique, that was developed using the optimized heating timing and profile (Fig. 2-6), should lead to the ITER advanced experiment and ensure its fulfillment.

Reference S. Ide et al., Overview of JT-60U Progress towards Steady-State Advanced Tokamak, Proceedings of 20th IAEA Fusion Energy Conference, Nov. 1-6, 2004, Vilamoura, Portugal, IAEA-CN-116/OV/1-1 (2005) (submitted to Nucl. Fusion).

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