Fusion reactors may get some help from meandering electrons

Tokamak fusion reactors, like the one under construction in France for the ITER project, confine a hot plasma that initiates and maintains a thermonuclear reaction. Part of that confinement is provided by electrical currents in the plasma itself, which generate magnetic fields and prevent the plasma from expanding to the reactor walls. A tokamak the size of ITER needs a current of around 15 million amps to corral the plasma into shape.

Research published in Physics of Plasmas shows that at least some of this confining current may arise from the dynamics of the plasma itself. The work predicts that a centrally fueled tokamak — one in which frozen fuel pellets are injected along the plasma’s central axis — could yield a million-amp current, potentially easing the path toward fusion energy generation.

A team of physicists in France derived an analytical model describing the coupling between an outward electron flux and an axial current in an ideal cylindrical tokamak. A picture of a random walk emerged: Electrons beginning on the central axis are continually buffeted by turbulence in the plasma, eventually ending up at the edge. During the walk, they also gain momentum along the plasma axis, leading to an axial current.

The work extends pioneering numerical simulations from 1994 that first reported on the axial current, albeit in a regime that ignored collisions. “We discovered that dissipation does not destroy the coupling between the radial and axial fluxes,” said Jean-Marcel Rax, an author of the paper. “Indeed, accounting for collisions is mandatory to describe a steady-state burning tokamak.”

Source: “Kinetic theory of transport driven current in centrally fuelled plasmas,” by J. M. Rax, J. Robiche, R. Gueroult, and C. Ehrlacher, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5030536 .