simulating a lightning generator
You have the below picture of an idealized transient you want to recreate: -



Comparing the current waveform with the voltage waveform tells you that this cannot be the current thru a capacitor (as shown by C2 in your schematic). This is because 

\$I_{C_2}\$ always must = \$C\cdot \dfrac{dv}{dt}\$

Clearly, in your idealized voltage waveform, there are "points" where the voltage change with time would result in an obvious "disturbance" in the current and that is just not seen in the picture. Also, the current returns to zero (apparently) whilst the voltage is still decaying. This cannot happen with a simple capacitor load like C2.

I believe you need to rethink what you are trying to achieve and what might help is trying to find the transient specification for the generator you are hoping to emulate. Most transient generators will have some equivalent circuit and I've definitely seen a couple on the web so maybe keep searching.