Publication
The largest operating stellarator, Wendelstein 7-X, is of the quasi-isodynamic type. For this design to scale up to a fusion reactor, several criteria must be met, one of them being good fast ion confinement. The latter still has to be tested experimentally, preferably before actually building a reactor. To this end fast ions have to be generated in present day machines. W7-X can use radio waves in the ICRF and neutral beams, the effectiveness of both is assessed in this work using the SCENIC package, combining the three different codes: VMEC (magnetic equilibrium), LEMan (full-wave) and VENUS-LEVIS (particle orbit following), used iteratively to find a self-consistent steady state. The propagation of radio waves in a plasma is described by Maxwell's equations, combined with the linearised Vlasov equation. Since these equations combine to form a linear time-invariant system, the natural approach is to solve the problem in the frequency domain. The main difficulty stems from the fact that the induced charges and currents are a non-local functional of the electric field. To address this problem directly, a new expression is derived for this constitutive relation in configuration space, and the resulting dielectric kernel is applied to mode conversion in simplified geometry. Such advanced wave physics is beyond the needs of heating schemes usually considered for tokamaks and stellarators, so it is reasonable that the full wave code LEMan has a number of approximations. Despite this LEMan is nevertheless capable of modelling minority, three-ion and synergetic RF-NBI heating schemes in 3D, now with the addition of hot plasma effects. Standard minority heating does produce fast ions in W7-X, but in limited quantities because the enhanced perpendicular velocity leads to a significant amount of trapped particles. These are lost far more rapidly in W7-X than in a tokamak. One way to remedy this is by instead accelerating ions that are born from the neutral beam. These particles are already fast, but under the right conditions their energy is increased considerably, which lends confidence in future fast ion generation experiments in W7-X.
António João Caeiro Heitor Coelho