found from the corresponding kinetic equation, is shown in Fig. 4.5. We notice that
at small b ε i , we have f i b ε i
ð Þ / b ε i , so the integral in Eq. (4.5) does not diverge [14].
Since in the edge plasma of fusion devices, rather strong ion-neutral collisions in
the vicinity of the divertor target are ubiquitous, these results justify the widely used
boundary condition for the plasma flow velocity, V k % C s at the target, which,
otherwise, is ill-defined by the inequalities (4.4) and (4.11).
In our analysis of the Bohm-Chodura constraint of the plasma flow onto the
target, described so far, we assumed that the electric field there results only from the
sheath effects and is normal to the target. However, in the edge plasma of fusion
Fig. 4.4 Comparison of
numerical solutions of
Eq. (4.15) for different
parameters P with the
asymptotic analytic solution
corresponding to P ! 1
Fig. 4.5 Ion distribution
function at different spatial
locations, found from the
solution of one-dimensional
kinetic equation allowing
for the ion-neutral charge
exchange and assuming
Boltzmann electrons with
a constant temperature T e .
(Reproduced with
permission from [15],
© AIP Publishing 2006)
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4 Sheath Physics