The zonal and non-zonal components in the diagnostics are not being calculated correctly.

[HanneThienpondt]

The zonal mode is the component of a ky=0 mode which does not vary along z. Therefore, to calculate the zonal and non-zonal contributions we need to work with the field-line average of the ky=0 mode instead of just splitting the spectrum up in ky=0 and ky!=0 modes.

[mabarnes]

Agree that it would be nice to have a diagnostic that makes this distinction, but it would also be nice to keep a diagnostic that keeps ky=0 modes that have z-variation. E.g., have the ability to look at ky=0 vs ky /= 0 and a separate decomposition of the ky=0 mode into parts that are constant in z and those that are not.