Fig. 2-3 Formation of hollow profile of plasma current, and plasma confinement A safety factor shown in Fig. 2-3(a) is deduced from the measurement of the plasma current density distribution. The current distribution is bell-shaped in standard tokamak operations as shown in Fig. 2-2, and the safety factor is increasing monotonically from the plasma center towards its periphery. On the contrary, the safety factor is decreasing from the center to a minimum at a certain position in the plasma in the hollow distribution of the plasma current, which leads to a formation of a negative shear region in the central part of the plasma as shown in the figure. The plasma confinement is remarkably improved under this particular condition of current distribution.

Fig. 2-4 Sustainment of hollow current distribution for 7.5 seconds by driving plasma current using microwaves of frequency in the lower hybrid range A hollow current distribution was formed and sustained stationary by driving non-inductive plasma current with externally applied microwaves at a frequency in the lower hybrid range. This is a proof-of-principle experiment demonstrating the formation and the control of an optimum distribution of plasma current for good confinement by external means.

To develop a "steady-state tokamak fusion reactor," the target of our research and development at JAERI, it is necessary to establish tokamak operation in which a good confinement of a high pressure plasma is maintained with a continuously flowing plasma current driven non-inductively using some mechanism other than the standard inductive method of the tokamak. Experiments on JT-60 have demonstrated that the "bootstrap current" driven by a particular plasma action occurring in the high temperature and high pressure plasmas can be an essential part of such a non-inductively driven plasma current in the future tokamak reactor. On the other hand, a theoretical study at JAERI has predicted that a stable confinement of a high pressure plasma can be realized by controlling a plasma current density distribution (profile) to form a hollow shape profile by means of the superposition of an externally driven non-inductive current on the bootstrap current due to the plasma action. A hollow profile is a particular distribution of current density that increases towards the plasma periphery with a minimum at the plasma center (formation of a "negative shear region") in contrast to a bell-shaped profile obtained by standard tokamak operations (see Fig.2-2). Recently JT-60 has experimentally confirmed the prediction. An excellent plasma performance was obtained with a hollow current density profile formed though transiently by a timing application of neutral beams for plasma heating during the current ramp-up phase. Figure 2-3 shows the experimental results. As clearly seen, a barrier is formed in the plasma and it prevents the escape of heat from the central plasma, leading to an excellent confinement of both the ions and electrons in the plasma, typically 2.6 times better than the standard confinement in JT-60. In particular, the definite observation of transport barriers for the electron density and temperature in the plasma is the first result of its kind in the world. Figure 2-4 also shows a world-first experimental demonstration for the quasi-stationary sustainment of a hollow current profile by externally applied microwave powers at a frequency in the lower hybrid range.

Reference H. Kimura and the JT-60 team, Recent Results from High Performance and Steady-state Researches in the JAERI Tokamak-60 Upgrade, Phys. Plasmas, 3(5), 1943 (1996).

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