Varying the collisionality for the NSTX discharges 141031 and 141040

In order keep the plasma equilibrium unchanged, the density and temperature profiles should be changed with a scaling factor C_\nu:

n_{e,i,z,f} \rightarrow C_\nu n_{e,i,z,f} , T_{e,i} \rightarrow T_{e,i} / C_\nu,

where the indices e,i,z, and f stand for electrons, ions, impurities and fast particles correspondingly. The plasma pressure, Z_{\rm eff}, normalized gradients and scale lengths as well as ratio of the electron to ion temperatures will remain unchanged.

The plasma collisionality for the NSTX discharge 141031 is varied from 0.2 to 20 from the original plasma collisionality. The density and temperature profiles in these scans are shown below:

The electron density profiles for different collisionalities

The electron density profiles for different collisionalities
The electron temperature profiles for different collisionalities

The electron temperature profiles for different collisionalities

It has been found that increased collisionality results in smaller values of anomalous diffusivities computed with the Weiland model in MMM8.1:

Electron thermal diffusivities

Electron thermal diffusivities
Ion thermal diffusivities

Ion thermal diffusivities

The diffusivity is monotonically decreases with the collisionality. The results for the NSTX discharge 141040 are similar:

Electron thermal diffusivity

Electron thermal diffusivity
Ion thermal diffusivity

Ion thermal diffusivity

When used standalone, the Weiland model in MMM8.1 can not explain the experimental trend \tau_E \propto 1/\nu_\star. However, when used in the whole device integrated modeling code, the MMM8.1 model can reproduce the experimental profiles and \nu_\star trend reasonably well. The main remaining question is what are the changes in other profiles (e.g., safety factor and magnetic shear) can change the \chi(\nu_\star) dependence.