2.3 New Idea to Remove Helium Ash from a DT Fusion Reactor

Fig. 2-5
(a) Concept of helium ash removal by ICRF waves
(b) Mechanism of helium ash removal
Singly-charged helium ions produced by charge-exchange reactions at the plasma periphery are selectively heated and accelerated perpendicular to the magnetic field by ICRF waves, and driven into the weak magnetic field region. The helium ions trapped there are spontaneously lost to a certain limited area of the first wall due to the motion caused by the spatial variation of the magnetic field intensity across the field (magnetic field gradient), and they are easily exhausted from the vacuum vessel through the pumping duct placed at the strike area. Helium ions not trapped in the above local area move around the magnetic lines of force and are confined in the vacuum vessel.

Fig. 2-6
 Time evaluation of fusion power output and helium ash density
In this simulation the helium ash removal by ICRF waves is switched on at 300 s. To that time the fusion output is reduced to 1.2 GW (1 GW=1,000 MW), as the fusion reaction rate is decreased due to the dilution of ions in the fuel plasma by the gradual accumulation of helium ash in the plasma core. As shown in the figure, the helium ash density is reduced by about 40% and the fusion output power is increased to about 1.5 GW by applying the ICRF ash removal scheme.

Effective removal of "helium ash" from the reactor core is a very important issue in sustaining stable fusion burning in a DT fusion reactor as described in 2-1. Helium ash is DT reaction-born, high energy helium ions slowed down to temperatures as low as those of the fuel D-T ions after having heated the core plasma in a fusion reactor. A method to use externally launched electromagnetic waves in the ion cyclotron range of frequencies (ICRF) has been proposed. The idea is to heat helium ash ions selectively at the plasma periphery by ICRF waves and drive the ions into a localized area on the first wall by making good use of the spatial variation of the toroidal magnetic field intensity (ripple of the toroidal field), and exhaust them through pumping from the vacuum vessel. Thus, helium ash accumulation in the reactor core can be avoided (Fig. 2-5 (a)). In a tokamak geometry, the ripple field (or periodic spatial variation of the toroidal field intensity near the plasma periphery the long way along the doughnut-shaped vacuum vessel), is formed because the toroidal magnetic field coils have to be arranged at intervals along the vacuum vessel as shown in Fig. 2-5 (b).
We have performed a comprehensive and detailed numerical simulation to study the feasibility of the above idea for the removal of helium ash from a DT fusion reactor, using an orbit-following Monte Carlo technique in a realistic tokamak geometry on an ITER-like scale.
Figure 2-6 shows an example of the simulation results indicating the effectiveness of this method. It is found that the suggested mechanism for the removal of helium ash works well; the helium ash density is reduced significantly by applying the proposed scheme with a moderate input power (about 10 MW) of ICRF waves, and consequently the fusion output recovers by about 300 MW as shown in the figure. Because of the specific mechanism of this method using ripple magnetic fields, helium ash is exhausted from the main plasma in a specific direction to a spatially localized area on the first wall of the vessel with a size of about 1.3 m by 0.7 m the long way and the short way along the doughnut-shaped vacuum vessel, respectively. Since the issue of helium ash exhaust is a crucial one in the R&D of a DT fusion reactor, further extended studies are needed.

Reference
K. Hamamatsu et al., Numerical Analysis of Helium Ash Removal by Using ICRF-Driven Ripple Transport, Plasma Phys. Control. Fusion, 40, 255 (1998).

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