Electric Breakdown

This is a rather fascinating paper (at least to me) that studies the mechanism of an electric breakdown. While the occurrence of static electricity is common when you feel a spark between your hand and a doorknob, trying to get the exactly, step-by-step mechanism is not easy. This is especially true in trying to get a clear, direct experimental verification of all the different models of breakdown, because such an effect is very transient and occurs over such a short period of time. Even trying to sample the field with a probe would not work because the probe itself will alter the field that we are trying to measure.

This issue has quite a bit of relevance to the news story about the International Linear Collider. One of the main issues on any accelerator is the maximum electric field gradient that an accelerating structure can withstand. It is widely known that the regular copper cavity that is commonly used in many accelerating structures will start experience electric breakdown at fields above 40 MV/m. (This number actually has some variation depending on the RF frequency applied to the cavity and the pulse length). What this means is that to accelerate charged particles to a higher energy, you will need many of these structures, since each one can only have up to some maximum value. The ILC is going to use superconducting technology as its accelerating structure to get a higher gradient. Still, even in a superconducting structure, there is still a maximum gradient that can be applied before breakdown occurs. That's why the ILC is proposed to be 20 km long to achieve its targeted energy.

So understanding the physics of breakdown is crucial in predicting on when it will occur for a particular material and configuration. Knowing such a thing will allow us to investigate how to minimize such a thing, and maybe even try to engineer a material that can have a higher tolerance to high fields.

This is one of the projects that my group is going to do. We're hoping to investigate the onset of electric breakdown in an RF cavity by examining field emission current pattern from a surface before a breakdown. There are many models of breakdown that attribute the heating of protrusions due to a high field emission current as the main trigger of the creation of a plasma that precedes a breakdown. We are hoping to test this via correlating regions of high field emission current (or dark current) with the eventual breakdown. A dedicated facility to study this is being assembled and about 90% done. So hopefully, sometime within the next couple of months, we will start our initial testing.

Zz.