Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>One impovement to this design would be using a toroid on the bottom of the coil as well as on the top. Just using one on top shapes the field but the smaller dia copper tubing in the base area does not provide effective electrostatic field control in this area. Using a toroid on the bottom as well as on the top will help provide better control of the fields and reduces the insulation required. As you noted just using more insulation does not give good uniform field control. You need to shape the fields with proper electrostatic shielding techniques.
These principles are used in very high potential 60 Hz xmfrs used for research work. A nice toroid is placed in the case entry area and many times actually inside the case near both the bushing tip and also near the area when the bushing departs the case.
Dr. Resonance
Original poster: Ed Phillips <evp@xxxxxxxxxxx>People who work with tightly coupled transformers for high voltage know it already, but I have only recently found out: I cannot insulate my way out of a corona problem. The more dielectric I stuff into the gap, the more severe the corona gets in the remaining air.Ain't that the truth..."Of course; "it has to be that way". The more (higher dielectric constant than air) material you put in the gap the greater the voltage drop across the remaining air and hence more corona. IF you completely fill the gap and IF you exclude all residual air the corona will be suppressed. If a little air remains somewhere it will break down due to corona and may very well result in breakdown of the associated dielectric material. Major failure mechanism for capacitors operated at high AC voltages.Ed