Statistical description of collisionlessα-particle transport in cases of broken symmetry: from ITER to quasi-toroidally symmetric stellarators

Abstract

Nuclear fusion has the potential to provide humanity with a safe, clean, abundant, efficient and reliable energy source for the generations to come, but up to date finding a viable fusion reactor concept remains an ongoing area of research. One of the main difficulties to attain economically viable magnetically controlled thermonuclear fusion reactors is the confinement of α-particles. These α-particles are responsible of sustaining the extreme temperatures required for nuclear reactions, and their loss poses a serious threat to the reactor operational control and to its plasma-facing components.viii RESUMEN tre los exponentes de Hurst, estimados por las técnicas Lagrangiana y Euleriana, muestra que a medida que el nivel de simetría aumenta, el transporte se vuelve fuertemente subdifusivo. Aunque, la validez del modelo fraccionario en sí mismo se vuelve dudosa en los casos limítes de alta y baja simetría.El trabajo presentado en esta tesis puede extenderse naturalmente para estudiar la validez del modelo de transporte fraccionario en otros tipos de campos magnéticos confinantes y estudiar varios efectos relacionados con las partículas α, como colisiones, perfiles de nacimiento de las partículas α, etc.

Non-diffusive nature of collisionless α -particle transport: Dependence on toroidal symmetry in stellarator geometries

“…The orbit widths, circulating and bouncing times were obtained in Ref. 30 , while the slowing-down and scattering times correspond to typical reactor conditions, i.e. n ∼ 1 and 5 ×10 20 m −3 as T ∼ 25 and 15 keV for tokamaks and stellarators respectively.…”

“…The procedure also allows to estimate the width and center of banana orbits, see Ref. 30 Lastly, with the aim to evaluate particle transport, it is necessary to select the kind of particles and time scales of interest for a given configuration. With this in mind, neither passing particles nor particles belonging to the saturation region in Figure 2 contribute to transport and, therefore, will be ignored together with the prompt losses, who's established convective behavior would only mask the results.…”

“…On the other hand, it was found 29 that the α-particle transport for realistic ITG and TEM turbulent regimes is diffusive and becomes significant only at energies ∼ 100 keV. A recent work 30 shows clear indications of the non-diffusive nature of 3.5 MeV α-particle neoclassical transport; i.e. when transport originates from the averaged radial drifts due to the non-uniform threedimensional magnetic field and not from the collisions dynamics, which is low enough to be neglected.…”

“…The aim of this work is to examine the collisionless α-particles neoclassical transport 30 by means of fractional transport theory adapting the techniques used in characterizing the nondiffusive dynamics of turbulent transport 9 . Of particular interest is the effect of the level of quasi-toroidal symmetry on the fractional transport coefficients.…”

“…To this end, five configurations stepwise breaking the symmetry were considered: from a perfectly symmetric ITER tokamak model to four stellarator configurations with different levels of quasi-toroidal symmetry. Fractional transport coefficients were estimated applying Lagrangian and Eulerian techniques to a set of trapped α-particle trajectories corresponding to the largest fraction of losses obtained with the Monte Carlo orbit following code MOCA 30,[35][36][37] . The set comprises only the confined part of these particle trajectories, i.e.…”