2.1 The New Divertor of JT-60 Opens Up Prospects for Helium Ash Removal

Fig. 2-1 Cross section of the JT-60 new divertor and the concept of the helium ash exhaust experiment
To simulate the behavior of helium ions in a DT fusion reactor, a high energy beam of helium atoms with beam energy 60 keV and beam power of 1.4 MW is injected into the main plasma of JT-60. The behavior of the injected helium is investigated with and without the helium pumping of the divertor by controlling its fast movable shutter valves.

Fig. 2-2 Time evaluation of helium density in the JT-60 plasma
When helium pumping operates, the helium density reaches a saturation level at 1.2 s after the start of helium beam injection and is kept constant for 4 s. The ratio of the helium density to the electron density in the plasma, or the helium ash concentration is 4%. In this experiment the ratio of the helium ash exhaust time to the energy confinement time was estimated to be about 4, which is well below the ITER design requirement (less than 10) for its successful operation, demonstrating a controlled removal of helium ash.

As part of the JT-60 program for the research and development of a steady-state tokamak fusion reactor, we successfully demonstrated (in JT-60) the effective removal of helium ash from a reactor core using a new type of divertor modified last year. While the high energy helium ions produced from deuterium and tritium fusion reactions are essential for heating the core plasma and maintaining the fusion burning in a DT fusion reactor, the slowed-down helium ions having temperatures as low as those of the fuel deuterium and tritium ions should be removed efficiently, as helium ash from the reactor core. This is indispensable to avoid fuel dilution by helium ash accumulation in the reactor core; otherwise the fusion reactivity or the fusion output will decrease.
The new divertor is a pumped divertor with cryopumps, having a W-shaped structure with inclined divertor plates and a dome and baffle plates to reduce the back flow of neutral particles towards the main plasma, and is located at the bottom of the vacuum vessel as shown in Fig. 2-1. The overall design was intended to meet the performance require-ments of the International Thermonuclear Experimental Reactor (ITER) divertor.
The behavior of helium ions in a DT fusion reactor was simulated by injecting a high energy beam of helium atoms continuously into the core of the JT-60 plasma that has slightly deteriorated confinement at the boundary, a condition preferable for steady-state operation. Figure 2-2 shows the time evolution of helium density in the plasma with and without helium pumping at the divertor. The result indicates that the helium concentration in the main plasma can be controlled well within the range of the ITER design requirements (less than 10%) considered to be necessary for the successful operation of a future fusion reactor.

Reference
A. Sakasai et al., High Performance and Steady-State Experiments on JT-60U, Proc. 17th Symp. on Fusion Engineering, Oct. 6-9, 1997, San Diego, California, 18 (1998).

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