A very similar related device, called a grading ring, is also used on high-voltage equipment.
Grading rings are similar to corona rings, but they encircle insulators rather than conductors.
Although they may also serve to suppress corona, their main purpose is to reduce the potential gradient along the insulator, preventing premature electrical breakdown.
The potential gradient (electric field) across an insulator is not uniform, but is highest at the end next to the high voltage electrode. If subjected to a high enough voltage, the insulator will break down and become conductive at that end first.
Once a section of insulator at the end has electrically broken down and become conductive, the full voltage is applied across the remaining length, so the breakdown will quickly progress from the high voltage end to the other, and a flashover arc will start.
Therefore, insulators can stand significantly higher voltages if the potential gradient at the high voltage end is reduced.
The grading ring surrounds the end of the insulator next to the high voltage conductor.
It reduces the gradient at the end, resulting in a more even voltage gradient along the insulator, allowing a shorter, cheaper insulator to be used for a given voltage.
Grading rings also reduce aging and deterioration of the insulator that can occur at the high voltage end due to the high electric field there.
In very high voltage apparatus like Marx generators and particle accelerator tubes, insulating columns often have many metal grading rings spaced evenly along their length.
These are linked by a voltage divider chain of high-value resistors so there is an equal voltage drop from each ring to the next.
This divides the potential difference evenly along the length of the column so there are no high field spots, resulting in the least stress on the insulators.
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