Calculating the electrical current in a Crookes Tube

I am planning on building a Crookes tube with the machine shop in my university's Physics department. I have pretty much all the details worked out, but since the tube has to operate at a high voltage (>5000 VDC) he wants me to bring in a current figure to make sure that the device will operate at a safe(ish) current. If I know the distance between the cathode and anode, the voltage, and the pressure of the gas inside the tube, how can I calculate (or at least reasonably estimate) the current in the circuit?

• You need to limit the current provided by your high voltage power supply. Assume that almost anything can (and will) happen inside your tube. – Jon Custer Nov 2 '17 at 22:18
• Not quite sure what you mean by that? What are some examples of "anything?" I've only got basic electronics experience and am only familiar with a few different "anythings" (blowing capacitors, arcing from a wall outlet, etc.) – Ely Beau Eastman Nov 3 '17 at 5:50
• You can have anything ranging from no discharge (so max voltage with no current) to an arc welder (max current) inside the tube (for various good or bad reasons). The only way to keep it to a "safe(ish)" current is to limit the current that is possible in the tube externally. Note that this does not have to be complicated - an in-line resistor could be sufficient. Size it to drop the full supply voltage at the max "safe" current, i.e. assume a dead short in the tube. – Jon Custer Nov 3 '17 at 13:31