Congratulation! The white light indicates that the interior of the bubbles have reached solar temperature. At this temperature there must be some level of hydrogen fusion. It would be interesting to look at the electrode surface with SEM at intervals leading to the 1 hour initiation.
Good warning to heed about the hazards related to ingesting thorium. Cerium salt in the welding electrodes was introduced because of much lower radioactivity. The spark ignition welders don't need ionizing radiation from the electrode to help initiate striking the arc.