"Marx generator kills dudes who want huge sparks and thus nobody should build it" :-)
Marx generator is generating very high voltage pulses with a huge currents. It was inventer in 1924 by Erwin Marx (not to be confused with Karl Marx). It consists of a large number of capacitors, which are charging in parallel and discharged in series. The the capacitors charges from high voltage power supply until flashover occurs at the lowest spark gap. This will increase the voltage accross next spark gaps, which also ingite and so on, until all the spark gaps are ignited. Now capacitors are connected in series and at the output the very high voltage pulse appears. Ignited spark gap has a very little resistance and the charging resistors have no effect at that moment. Lowest spark gap should have slightly smaller distance of electrodes, to set fire at the first. You have to experiment with the distance. Pulses have a frequency of about 1Hz. Resistors may have 100k to 2M2, capacitors 1nF or more. Resistors must withstand high voltages. My generator is supplied by voltage of 12kV and a has 20 floors, so it makes almost 240 kV voltage and discharge more than 20 centimeters long. Current is maybe one thousand amperes and pulse discharge are therefore very noisy. Lowest spark gap has gap of about 3 millimeters and the other 4mm. When operating the generator generates electromagnetic pulses and sometimes the electrical appliances switch on and off by itself occurs, CD player skips, digital clock resets, the computer restarts, radio changes stations, digital camera shoot by itself and fluorescent lamps also flash.