Like Nigel said, For current measurements and current limit, place a resistor in the bottom end (cold end) of the 15kv supply. The voltage across this resistor will go negative with current. It is a good idea to have current limit on 15kv supplies. Example: if this voltage gets to -5 volts shut down the power.
I have high voltage resistors. Some the diameter of a broom stick and 8 inches long. Some the diameter of a pencil and long. Because you don't have these you need to make a resistive divider. Many small resistors are only good for 200 to 400 volts each. Make a long string of resistors to divide down the voltage to some thing you can measure.