I need to look at the glass structure later today, but you may go in thru the fill system and just need to heat the tube to almost 100c to get any water vapor that has leaked in. I see some nitrogen and water in the plasma, but not much. Probably an ultra slow leak
At the window seals.

If you can go into the resivoir, the pumping time is long , it will need many flushes of gas, but odds are you will not need new electrodes.

Steve