An original plasma process for carbon black and hydrogen synthesis is presented. A pilot scale (100 kW) reactor is developed to obtain new carbon black grades from hydrocarbon cracking. Even if three populations of particles are observed, the plasma carbon black is mainly characterised by spherical nanoparticles with a high degree of turbostratic organisation. A particle formation mechanism is proposed on the basis of a multiscale analysis of the plasma reactor accounting for: heat and mass balance, RTD measurement and internal flow modelling and particle characterisation by TEM. The mechanism is characterised by two steps: first, a low temperature particle growth and second, a high temperature annealing. As a result, particle structure is strongly related to internal mixing phenomena.