gyro-Bohm term

The Bohm-like expression so derived proved to be very successful in simulating JET discharges, but failed badly in smaller Tokamaks such as START[5].
For this reason a simple gyro-Bohm-like term, also based on dimensional analysis, was added:

$$\chi_{e,i}^{gB} = \alpha_{e,i}^gB \frac{cT_e}{eB} L_{Te}^{*-1} \rho^*$$

where $\rho^*$ is the normalized larmor radius:

$$\rho^* = \frac {M^{1/2}cT_e^{1/2}}{aZ_ieB_t}$$

This expression is what can be expected from small scale drift-wave turbulence. It is important to note that in large Tokamaks such as JET and TFTR the gyro-Bohm term is negligible, while in smaller machines, with larger values of $\rho^*$, the gyro-Bohm term can play a role especially near the plasma centre.