Plasma sheath

At the boundary of the plasma (such as between plasma and divertor target), an electrostatic sheath forms and impacts energy flux leaving the plasma as it filters most electrons while attracting ions. Heat flux $q_{se}$ at the sheath edge is related to the particle flux at the sheath edge $\Gamma _{se}$, electron temperature $T_{e}$, and the sheath heat transmission coefficient $\gamma$ by:

$q_{se}=\gamma T_{e}\Gamma _{se}$

Sheath theory gives

$\gamma =2.5{\frac {T_{i}}{T_{e}}}+{\frac {2}{1-\delta _{e}}}-0.5\ln \left[\left(2\pi {\frac {m_{e}}{m_{i}}}\right)\left(1+{\frac {T_{i}}{T_{e}}}\right){\frac {2}{\left(1-\delta _{e}\right)^{2}}}\right]$

where $T_{i}$ is the ion temperature and $\delta _{e}$ is the secondary electron emission coefficient, both of which are difficult or impossible to measure at the divertor plate. Setting $T_{i}=T_{e}$ and $\delta _{e}=0$ gives $\gamma =7$, which is often used due to the simplicity of the assumptions.