Depolarization of Magnetically Confined Plasmas

In a tokamak plasma, reaching the critical energy balance for ignition, for which the gain factor Q =energy output/energy input= ∞, or even the Q ∼ 40 − 50 value needed for reactor operation, is a very challenging task—for example, a plasma 4 temperature of the order of hundreds of million degrees is needed, a value much higher than the core temperature of the sun. The help that can come from a fusion cross section enhancement due to the appropriate polarization of the reacting nuclei could be instrumental in achieving the goal. Two works carried out in the 80s have provided insight on the effective ability of a spin-polarized D–T thermonuclear plasma to preserve the polarization status of the fuel nuclei (Kulsrud et al., Phys Rev Lett, 49:1248, 1982, [1], and Coppi et al., Phys Rev Lett 51:892, 1983, [2]). The conclusions are both encouraging and cautious. While Kulsrud’s work shows that many of the potential mechanisms for depolarization are weak, Coppi’s work points out that the presence of energetic alpha particles, products of the D–T fusion reaction, could generate collective modes able to depolarize the fuel. In the present contribution, we review the arguments and the main results of the two above-mentioned papers, while contextualizing them to present-day tokamak devices. In particular, we consider plasma regimes characteristic of ITER and IGNITOR, two tokamaks under construction and in advanced state of design, respectively, and which represent different approaches to magnetic fusion research. The depolarization rates estimated for these two devices indicate that polarization may not be maintained long enough for fusion reactions to occur, unless ion cyclotron resonances provide an effective damping mechanism for the excited modes. Only a targeted experimental campaign could provide a final answer on the feasibility of polarized fusion in tokamaks.