Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Spark Plug feedthrough insulator
Date: Feb 03, 2:51 pm

On Feb 03, 2:51 pm, wrote:

Another key issue is; how good is the insulator at very high voltages? Car plugs have had to contend with ever soaring ignition voltages as super high voltage CD ignition systems have moved into autmotive use. Thus, most car plugs will easily standoff 30kv. But....., Most car plugs are resistive.

I wouldn't worry about breakdown through the insulator (porcelain), but creepage along the surface of the insulator. On the air side, cleanliness would be a big factor, as would anything causing a field concentration at the connection end. The usual rule of thumb for an insulator in a uniform field (like, a block of insulator between two large flat electrodes much larger than the insulator) is that the creepage breakdown will occur at 1/3 the free air breakdown. Oddly, making the surface slightly conductive, while increasing the leakage current, raises the breakdown voltage, because charge can't accumulate on the surface.

Inside the vacuum side, it is a bit different, especially considering you are running at glow discharge kinds of pressures. In a really high vacuum, you'd worry about field emission triggering a breakdown.