Is there a scientific name for this technique?
You can call $C(k_f)$ the "angular coherence", or "angular correlation coefficient" at frequency $k_f$.

Alright, I just made that up if you just need a name for it! If you need some interpretation, here is some detail:

A complex number can be represented by a vector in the polar system.  So we have $M$ vectors here. The numerator is a vector sum and the denominator is a normalizing factor. From the triangle inequality, the magnitude of $C$ is always in the interval $[0,1]$. $C$ is zero if the vectors are anti-symmetric pairs or if they completely cancel eachother, and is one if all are equal. So the remaining possibilities represent how close their composition are to either these two extremes. 

I suppose you asked for the name to find more resources. Alternatively, you can ask your possible related questions directly, and probably there are some people here to answer.