Why magnetized discharges often show Bohm-scaled cross-field diffusion
In plain words
Since the 1940s, many magnetized gas discharges have been found to leak particles across the field at a rate falling only as one over the field strength, far faster than collisions explain. Why this simple scaling appears in so many devices has no accepted explanation.
Precise statement
Cross-field diffusion in many magnetized low-temperature plasmas follows the Bohm form $D_{B} = c T_{e}/(16 e B)$ ($T_{e}$ in erg), scaling as $1/B$ rather than the classical $1/B^{2}$. Determine whether a common mechanism (E x B drift-wave turbulence, sheath-coupled instabilities, gradient-drift or Simon-Hoh modes) produces this scaling, the origin of the coefficient near 1/16, and the conditions where it fails. An answer is a theory predicting $D_{\mathrm{perp}}$ versus $B, T_{e}$ and geometry, validated across at least two device classes.
What would settle it
Kinetic simulations and measurements in two device classes (e.g. Penning discharges and Hall thrusters) agreeing on $D_{\mathrm{perp}}(B, T_{\mathrm{e}})$ and its departures from Bohm scaling.