What was the highest voltage achieved and was it produced by electrostatic means or just some transformers and multipliers? What are the limitations when it comes to producing voltage using electrostatic means?

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Hi Abdel, You mean - artificially? For lightning (Indeed, its electrostatic) - we don't need transformers, etc... :-) –  Waffle's Crazy Peanut Dec 11 '12 at 6:26
Yes i mean artificially –  Abdelrahman Esmat Dec 11 '12 at 13:41

Assuming you mean a macroscopic potential difference, the largest I know about was in the Nuclear Structure Facility accelerator at the Daresbury laboratory in the UK, and this was 30MV. The Wikipedia article on electrostatic particle accelerators claims this is about the highest possible in such devices.

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Well I've once heard that 321.5 megavolts was produced in 1979 by the National Electrostatics Corporation –  Abdelrahman Esmat Dec 11 '12 at 13:38
@AbdelrahmanEsmat - I can't find the reference, but I'm sure this was a typo. It was the Pelletron accelerator and the voltage was 32 $\pm$ 1.5 MV. It got transcribed to 321.5MV. –  John Rennie Dec 11 '12 at 15:11
What about the EMP in an atomic explosion? Everything I can look up in the time I have available is on the size of the electric field or the energy of the photons, so I don't know if voltages have been measured, but you would expect something relatively large. –  Jerry Schirmer Dec 11 '12 at 18:02
There must be lots of ways to develop transient very high field strengths. I bet the Z-pinch kit is the US (I forget the name) generates extraordinarily high electric field strengths. However I don't think this in the spirit of the question. –  John Rennie Dec 11 '12 at 18:17

Naturally, we already have Lightning which goes to some 120 MV.

A Van de Graaff Generator produces some $10^7$ volts provided a supply of only $10^4$ volts. It's the highest man-made voltage ever produced.