Faster-than-Bohm Cross-$B$ Electron Transport in Strongly Pulsed Plasmas

We report the empirical discovery of an exceptionally high cross-$\mathbf{B}$ electron transport rate in magnetized plasmas, in which transverse currents are driven with abruptly applied high power. Experiments in three different magnetic geometries are analyzed, covering several orders of magnitude in plasma density, magnetic field strength, and ion mass. It is demonstrated that a suitable normalization parameter is the dimensionless product of the electron (angular) gyrofrequency and the effective electron-ion momentum transfer time, ${\omega }_{\mathrm{ge}}{\tau }_{\mathrm{EFF}}$, by which all of diffusion, cross-resistivity, cross-$\mathbf{B}$ current conduction, and magnetic field diffusion can be expressed. The experiments show a remarkable consistency and yield close to a factor of 5 greater than the Bohm-equivalent values of diffusion coefficient $D_{\perp }$, magnetic-diffusion coefficient $D_B$, Pedersen conductivity ${\sigma }_P$, and transverse resistivity ${\eta }_{\perp }$.

Figure 1

The effective momentum transfer time ${\tau }_{\mathrm{EFF}}$ as function of the magnetic field strength, obtained from three different types of pulsed plasma experiments. For reference, a dashed line at ${\omega }_{\mathrm{ge}}{\tau }_{\mathrm{EFF}}\approx 16$, corresponding to Bohm diffusion, is also drawn.

Figure 2

The normalized Pedersen conductivity ${\sigma }_P$ and the cross-$B$ electron diffusion coefficient $D_{\perp }$, as functions of ${\omega }_{\mathrm{ge}}{\tau }_{\mathrm{EFF}}$.

Figure 3

Diamagnetic currents and fast magnetic diffusion in the plasma penetration experiment (adapted from 9). The lower panels show the diamagnetic cavities that remain, $1\mu \mathrm{s}$ after entering the barrier, in three streams with different plasma densities.

Figure 4

A sputtering magnetron. Experiments from two different experiments in this geometry are analyzed, with pulsed power applied in the range 1.5–300 kW.

Figure 5

The geometry of the toroidal theta pinch experiment, and magnetic profiles for the antiparallel ($\uparrow \downarrow$) bias case, adapted from 12. The yellow shading marks the current sheaths that are used to evaluate ${\tau }_{\mathrm{EFF}}$ for Fig. 1.