Poynting Vector: Calculating E For Charged Particles in +ve X-Direction

[Gayle]

what would be the poyinting vector of beam of charged particles moving in +ve x-direction.
S=ExH
calculating H is fine but how to calculate the E

 

[mfb]

The poynting vector is a property of the electromagnetic field, it is not a property of particles.
If you have a continuous beam you get the same electric field as from a charged wire: radially outwards.