density of electrons in solids is not in equilibrium is one condition for diffusion to take place.
what is one condition for diffusion to take place?
density of electrons in solids is not in equilibrium
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in 2015, new exact explanation for the original bohm's experiment is reported, in which the cross-field diffusion measured at bohm's experiment and simon's experiment were explained by the combination of the ion gyro-center shift and the short circuit effect. the ion gyro-center shift occurs when an ion collides with a neutral to exchange the momentum; typical example is ion-neutral charge exchange reaction. the one directional shifts of gyro-centers take place when ions are in the perpendicular (to the magnetic field) drift motion such as diamagnetic drift. the electron gyro-center shift is relatively small since the electron gyro-radius is much smaller than ion's so it can be disregarded. once ions move across the magnetic field by the gyro-center shift, this movement generates spontaneous electric unbalance between in and out of the plasma. however this electric unbalance is immediately compensated by the electron flow through the parallel path and conducting end wall, when the plasma is contained in the cylindrical structure as in bohm's and simon's experiments. simon recognized this electron flow and named it as ‘short circuit’ effect in 1955. with the help of short circuit effect the ion flow induced by the diamagnetic drift now becomes whole plasma flux which is proportional to the density gradient since the diamagnetic drift includes pressure gradient. the diamagnetic drift can be described as (kt/eb)∇n/n, (here n is density) for approximately constant temperature over the diffusion region. when the particle flux is proportional to (kt/eb)∇n/n, the other part than ∇n/n is the diffusion coefficient. so naturally the diffusion is proportional to kt/eb. the other front coefficient of this diffusion is a function of the ratio between the charge exchange reaction rate and the gyro frequency. a careful analysis tells this front coefficient for bohm's experiment was in the range of 1/13 ~ 1/40. the gyro-center shift analysis also reported the turbulence induced diffusion coefficient which is responsible for the anomalous diffusion in many fusion devices; described as (2/π)(δn/n)(kt/eb). this means different two diffusion mechanisms (the arc discharge diffusion such as bohm's experiment and the turbulence induced diffusion such as in the tokamak) have been called by the same name of “bohm diffusion”.
bohm diffusion