Strong Turbulent Pinch of Plasma Confined by a Levitated Dipole Magnet

The rearrangement of plasma due to turbulence is among the most important processes that occur in planetary magnetospheres and in experiments used for fusion energy research. Remarkably , fluctuations that occur in active magnetospheres drive particles inward and create centrally peaked profiles. Until now, the strong peaking seen in space has been undetectable in the laboratory because the loss of particles along the magnetic field is faster than the net driven flow across the magnetic field. Here, we report the first laboratory measurements in which a strong superconducting magnet is levitated and used to confine high temperature plasma in a configuration that resembles planetary magnetospheres. Levitation eliminates field-aligned particle loss, and the central plasma density increases dramatically. The build-up of density characterizes a turbulent pinch and is found equal to the rate predicted from measured electric field fluctuations. Our observations show that dynamic principles describing magnetospheric plasma are relevant to plasma confined by a levitated dipole. 1 Since the discovery of the Earth's radiation belts more than fifty years ago, observations of energetic particles trapped in the Earth's dipole magnetic field have illustrated a remarkable and non-intuitive process: random, low-frequency fluctuations caused by solar activity creates diffusion that drives particles inward towards the Earth and increases particle density 1–4. Instead of flattening density gradients, diffusion causes particles trapped in a magnetic dipole to become peaked. The central peaking of particle density–occurring in opposition to the usual direction of diffusion–characterizes a " turbulent pinch. " In strongly magnetized plasma, charged particles have gyro-radii very much smaller than the radius of the Earth 5 , and plasma motion along the magnetic field is fundamentally different from motion across the field 6. Low frequency fluctuations cause the random radial motion of entire populations of particles contained within field-aligned tubes of magnetic flux, and this motion links the geometry of the magnetic field to the particle density profile. For the Earth's dipolar magnetic field, the volume enclosed by tubes of a given flux decreases rapidly as the plasma moves inward. During active periods of the magnetosphere, radial diffusion equalizes the number of particles within volumes of equal magnetic flux (and not within equivalent volumes of space), and this causes the density of inward diffusing energetic particles to increase dramatically 3, 8–10. While laboratory experiments have observed space-related plasma phenomena before 7 , the study of the cross-field transport of plasma trapped in a dipole …