To solve this problem we will apply the concept of current defined as the electron charge flow by the number of electrons per second. That is,
I = q*N
Here q is Flow of electric charge in one second and N the number of electron flow per second.
A the same time the power is described as the applied voltage for the current.
P = VI
We know the charge of electron,
Coulombs, then the current is
And the power in the Beam is