Resonant diffusion of passing alpha-particles in a Helias reactor

Passing alpha particles in a Helias reactor (major radius 22 m, plasma radius 1.8 m, magnetic field 5 T) are well confined if only Coulomb collisions perturb the particle orbits and neoclassical diffusion is the only transport process. In a turbulent plasma close to the beta limit particle orbits can become chaotic and enhanced losses of these particles may arise thus diminishing the alpha particle heating rate. The paper describes the guiding center orbits of passing particles in the magnetic field of the Helias reactor, where islands of the magnetic surfaces occur at the boundary. Drift surfaces also exhibit these islands, however, resonance surfaces with islands in drift surfaces can also exist inside the plasma. Numerically the rotational transform of the drift orbits has been computed verifying this feature. Using a mapping technique, the long term behaviour of the drift orbits is analysed taking into account the variation of energy due to plasma oscillations. This leads to two coupled area-preserving maps, which describe enhanced radial transport of particles satisfying the resonance condition. In phase space the anomalous transport occurs along the resonance lines of the Arnold web. The energy of the particles can vary in a chaotic manner if the amplitude of the oscillations is large enough. In model calculations it is shown that resonant alpha particles can be rapidly lost by Arnold diffusion If this loss of alpha particles only occurs at the beta limit, it may help to stabilize the operation point against thermal instability by reducing the heating power. If drift waves contribute to alpha particles losses way below the beta limit, the Arnold diffusion, this anomalous loss is threatening the ignition in the Helias reactor. (orig.)