Energetic ions are attracted towards the central electrode, and when they hit that graphite stick that I put in the centre, you see a wonderful mini-explosion from the collision. This all took place in a vacuum chamber at high voltages (ZVS circuit is the driver). The best view of it is at roughly 50 seconds into the video.

https://blossom.primal.net/17d7a9840f08f819c54671d37884c4dd644374ec447b757ca9f2197a967d94e2.mp4