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The Need for Steady-state Operation
　The steady-state operation for several months achieved by existing nuclear fission reactors at nuclear power plants is a prerequisite to realize a nuclear fusion reactor. However, current nuclear fusion experimental devices have succeeded only in pulsed operation for several seconds. The reason for this follows.
　In Tokamak-type fusion reactors for thermal energy production, plasma is generated in donut-shaped vacuum vessels and confined by a "cage" of magnetic lines generated by the added current is introduced via the transformer principal whereby the changing current produces an inductive electrical field and generates the current (Fig.1).
　To keep the plasma current, the primary coil current (like in transformers) must be boosted step by step endlessly.
But as there is a limit to increasing primary coil current, it must be shut off temperately. This is why pulsed operation must be applied. To realize steady-state operation of nuclear fusion reactors, a new method for generating plasma current that does not depend on the transformer principal must be developed.

Other Methods Do Not Use the Transformer Principal
　There are two options: to produce current by injecting neutral particle beams into plasma or to use the spontaneous current of plasma. Basically, both options enable realizing steady-state operation, but it has been pointed out that in the former method, energy consumed to drive the neural beam injecting device tends to increase hence the method is uneconomical. Thus, applying both methods and optimizing them to economize is under consideration. Judging from the calculated results, economy-wise it is best if operation can be effected by the following control: 20-30% of plasma current supplied by beam injection and the　remaining 70-80% by spontaneous current.
　In our experiment using JT-60, we successfully kept the plasma current for approximately 2.6seconds by a new operating method that does not depend on the transformer principal. The conditions of the experiment observing the above calculated results were 20-30% of plasma current supplied by beam injection and the remaining 70-80% by spontaneous current (Fig.2). The reason for the new plasma control method in reversed magnetic shear plasma. Thus a way to keep steady-state operation was experimentally demonstrated by introducing reversed　magnetic shear plasma.

What is Reversed Magnetic Shear Plasma?
　In reversed magnetic shear plasma, the current fed through plasma Is distributed in a concave profile (Fig.3, right). As shown, heat confinement performance in concave plasma outdoes that in the convex (Fig.3, left).
　To maintain nuclear fusion reaction, plasma must be held at high temperature, but heat easily escapes from the plasma. To offset heat loss, supplemental energy must be injected into the plasma continuously. But it is economically infeasible if a large amount of supply of energy is necessary to maintain reaction. It is vital that a mechanism to keep the heat in plasma by itself be established. This character is called the heat confinement performance of plasma.
　In the reversed magnetic shear plasma, there is a change in the mode of twisting in magnetic lines of force that confine plasma at the peak of current, and a heat insulation layer that blocks heat flow from center part of plasma forms. This produces an efficient confinement performance without significant loss of heat from within the thermal insulation layer. Then spontaneous current is concentrated in the insulation layera where a large slope of plasma pressure is formed.
　But the concave current profile tends to transform into the convex type. To stabilize and maintain the concave profile as efficient for the confinement of heat, the character of spontaneous current that tends to focus in the insulation layer was used. Presumably, the entire current can have a concave profile with its peak in the insulation a concave profile with its peak in the insulation layer if the spontaneous current is raised. So, if this easy-to-generate method of concave current distribution in the insulation layer is applied to maintain this type of current profile, it can become a most covenient way to control plasma (Fig.4).
　A highly important subject is how to increase the spontaneous current, which is proportionate to the ratio of plasma pressure and current. The spontaneous current can be boosted by highly pressurizing plasma. However, plasma will break down under excess pressure, like a balloon. In this experiment, the shape of plasma was changed from the slender to triangular (Fig.5), wherein plasma concentrates partially around the center of the donut-shape vacuum vessel (Fig.5, right). Magnetic fields confining the plasma tend to be stronger around the center, which means that if plasma is inclined in a more powerful magnetic field, the plasma becomes stronger than elsewhere. This is why the new shape of plasma can withstand pressure greater than when in ordinal slender shape. In other words, oblate-shaped plasma is stable.

What is the New Plasma Control Method?
　In the new method , plasma pressure is stably sustained by adjusting the strength of the neutral beam. As plasma pressure is proportionate to heat, its pressure will mount as the temperature rises. However, as mentioned, plasma will break down under excess pressure, which makes it highly important to adjust heating carefully.
　This year, a new control method was developed , one whereby plasma pressure is measured in real time and the beam is turned on and off periodically. In our experiment the technology described above was applied and admirably demonstrated steady-state operation that automatically retains the concave current profile and provides for good heat confinement.

Future Plan
　In this experiment steady-state operation was kept for 2.6 seconds. In future experiments a vital subject will be how to prolong operation time. Plasma current value was about 0.8 million amps in the test. We would like to boost this value to more than 1.5 million. In the future plasma performance will be improved by introducing new ideas to the operation.

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(Contact: Administrative Services Division, Naka Fusion Research Establishment; Phone: +81-29-270-7214)