Possibility of using ion-Bernstein waves for alpha power extraction in tokamaks

If the fusion α-particle power is directly channeled to do useful things rather than simply undergo collisional absorption, there is an opportunity for significantly improved tokamak reactor performance. In the absence of the channeling, the α-particle power results predominantly in electron heating, which is a poor use of this power. One possibility is to direct this power into waves that damp on electrons traveling in one direction to sustain the plasma current. Since the power required to drive the toroidal current is about 10% of the fusion power output, whereas the α-particle power is 20% of the fusion power output, using even a fraction of the α-particle power to amplify the waves used for current drive can result in very significant savings in the required power for the current drive. A second possibility is to direct this power into waves that damp on fuel ions to increase the plasma reactivity. Such a channeling of the α-particle power could make possible the operation of fusion reactors in regimes in which the fuel ion temperature can be much greater than the electron temperature. It may also be possible to sustain a hot, nonmaxwellian component of the fuel ions, further enhancing the reactivity. By these means, if 75% of the α-particle power is diverted, the plasma reactivity, at constant pressure, can be increased by about a factor of two. It may even be possible to achieve at once the increased reactivity and the current drive, if, for example the α-particle power is channeled into waves that damp on fast ions traveling in one toroidal direction. (author) 10 refs., 4 figs