Role of Radial Electric Fields in Transport Barriers: Experimental Results

The importance of radial (i.e. perpendicular to the magnetic surface) electric fields was already recognised early in the research on controlled thermonuclear fusion. An initial description of electric field effects in toroidal confinement was given by Budker. Such a configuration with combined magnetic and electric confinement (“magnetoelectric confinement”, where the electric field provides a toroidal equilibrium configuration without rotational transform) was studied by Stix, who suggested that a reactor-grade plasma under magnetoelectric confinement (electric fields of order 1 MV/cm) may reach a quasi-steady-state with ambipolar loss of electrons and some suprathermal ions (e.g. 3.5 MeV α-particles). Experiments such as on the Electric Field Bumpy Torus EFBT provided quite favourable scaling for particle confinement. The possible importance of radial electric fields for transport was in the past repeatedly established. Since the early days the plasma potential has been measured in tokamaks such as ST, TM-4 and ISXB, but because no significant effects of the radial electric field Er on plasma transport were observed under the machine conditions at that time, no further research was conducted in tokamaks.