For my first experiments I use a 7kV china high voltage supply.  (the black box on the right top). To get a impression of the capabilities of the HV Supply. I build a 200Meg Ohm voltage divider to measure the the voltage. I putted the resistor to connect the multi meter in the middle of the divider, just in case of a short circuit. The voltage divider has a ratio of 2001  =  (200M+0.1M) / 0.1M. Each voltage on the multi meter multiplied with 2001 is the voltage over the voltage divider .

The image above shows the test setup. The variable resistor RL I realized severl 10Meg Ohm resistors connected parallel.

The table above shows the measured values.

The diagram above show the voltage and the current of the HV supply.  The calculation of the internal resistor Ri is shown below.