When the high frequency, high voltage potential is applied across the anode and cathode, a conducting ion path is formed in the tube between the anode and cathode. The cone shaped piece of copper on the anticathode ensures that the anticathode is in the voltage path. The tube is very sensitive to internal pressure changes. Operating the tube tends to decrease the pressure in the tube and not enough positive ions are present to produce electrons to make x-rays. When this happens, the resistance in the tube goes up and the voltage across the cathode and regeneration electrode builds up, and there is a spark from the tip of the wire to the cathode.  This heats up the asbestos packed into the tube just under the metal feed-through supportng the wire. The asbestos is permeable and has picked up gas during the manufacturing process, which is released into the tube and the resistance increases to an acceptable level. The gap between the wire and the cathode would need to be adjusted for proper operation.

The copper cone used to shape the electrical field and thus guide  the discharge between the anode and cathode, hence the lack of an electrical connection to the anticathode.