The mixed model is derived using the dimensional analysis approach,
whereby the diffusivity in a Tokamak plasma can be written as:

where is some basic transport coefficient and F is a function of the plasma dimensionless parameters . We choose for the Bohm diffusivity:

The expression of the dimensionless function F is chosen according to the following criteria:

- The diffusivity must be bowl-shaped, increasing towards the plasma boundary
- The functional dependencies of F must be in agreement with scaling relationships of the global confinement time, reflecting trends such as power degradation and linear dependence on plasma current
- The diffusivity must provide the right degree of resilience of the temperature profile

It easily shown that a very simple expression of F that satisfies
the above requirements is:

where q is the safety factor and
, being a the
plasma minor radius. The resulting expression of the diffusivity
can be written as:

where is the plasma diamagnetic velocity, and , so that it is clear that this model represents transport due to long-wavelength turbulence.

The evidence coming up from the simulation of non-stationary JET experiments [4](such as ELMs, cold pulses, sawteeth,

where x is the normalized toroidal flux coordinate. The final expression of the Bohm-like model is:

where is a parameter to be determined empirically, both for ions and electrons.