Beamlet divergence definition in the NBI IDS

[pietrovincenzi]

With the ASCOT team, we are discussing the beam(let) divergence definition used in the IMAS NBI IDS.

From the data dictionary, IDS beam divergence is:
"... defined for Gaussian beams as the angle where the beam density is
reduced by a factor 1/e compared to the maximum density."

At the moment, the neutral beam ionization module of ASCOT (named BBNBI) is implemented in IMAS, with its version "4", and it reads the NBI IDS.
In the latest BBNBI version, not yet in IMAS, "BBNBI5", the divergence corresponds to 1 / e folding in power. It
is therefore the "half-divergence angle" in engineering language. We are comparing IMAS BBNBI4 vs BBNBI5 results.

We don't understand what is the beam "density" in the IDS definition: is it the intensity?

We have been suggested to redirect this question to @imbeauf

[imbeauf]

This definition of the beam divergence was proposed by @lappel2, maybe he can comment on this discussion ?