Mahmood ,
typically the accelerators I work with produce a pulse of 7 Megawatt peak.The modulator designs generally use a pulse forming network using a network of capacitors and inductors rather that a resistive element.The impedance of the network is matched to the load such that the energy transferred is all dissipated in the load.
You can check out this link:
http://www.hvpowersupplies.com/pfn.htm for some idea of which I speak.
At the voltages and currents you are talking about you better make sure your thoughts are very clear before you start screwing around with this stuff.