PLASMA HEATING AND CURRENT DRIVE TECHNOLOGY

This chapter gives an overview of the main heating and current drive methods. While it concentrates on those heating methods used in magnetically confined plasmas, the general principles are also applicable to other confinement concepts. To achieve fusion, ions (whose positive charges repel each other) are heated, allowing them to acquire enough energy to overcome the coulomb potential barrier. As the nuclei come close enough together, the strong nuclear force overcomes the electrostatic repulsion. Fortunately, quantum mechanical effects allow ions to 'tunnel' through, such that fusion can already take place at energies lower than the maximum value of the potential energy. The probability that two ions fuse expressed in terms of the cross-section is given in Fig. 2.1 as a function of the energy of a deuterium ion impinging on a fixed target. Since the time between coulomb collisions is much shorter than either the confinement time1 or the time to undergo fusion, the ions usually have a Maxwellian distribution, with a temperature Ti. The reaction rates as a function of this temperature (eV) are given in Fig. 2.2. The lower temperature at which the reaction rate peaks in Fig. 2.2, as compared to the energy of the D ion for which the reaction rate peaks in Fig. 2.1, is due to two effects. First, in Fig. 2.2 it is more effective to have the energy in both colliding ions rather than in ions colliding with a fixed target (Fig. 2.1); since in the second case non-useful energy is in the centre of mass. Second, ions in the tail of the distribution contribute (with an energy that is higher than the corresponding temperature) significantly to the fusion reaction rates. Non-Maxwellian ion distributions can be created by certain heating methods, which can further enhance the reaction rates at low temperatures. Note that in cases where the coulomb potential of the ion is shielded by a heavy negative particle (e.g. muonic fusion), significant fusion rates can be achieved at much lower temperatures and heating would thus barely be necessary. Such a scheme is not currently usable since the energy produced by the number of reactions a muon can catalyse is smaller than the energy needed to produce a muon. In the more standard case of non-muonic fusion, external heating is needed to heat the plasma to the high temperature required to improve the fusion reaction rates. If the fusion conditions are fulfilled, meaning high enough temperatures to overcome the coulomb potential, high enough density to have sufficient reactions and high enough confinement time for the power from fusion to compensate for the losses (i.e. high enough ntT), then heating by fusion products, such as the alpha fusion products, can take over and completely replace the external heating. Since the heating system will represent a significant fraction of the investment needed to construct a power plant and since, in a (quasi-)steady state reactor, its heating role may only be needed for a short time, there is a strong incentive to favour a system that can also be used for other purposes (e.g. driving a current, where needed, or controlling the plasma). These aspects will also be discussed for each heating system. However, the methods under consideration are still referred to as heating systems. This text will not emphasize the physical aspects of heating systems, for which the reader is referred to the corresponding books (e.g. Fusion Physics [2.1]).