OT - Does anyone know the effect of ambient radiation on measurements of Paschen's Curve?

points out that Paschen's Law is all about what happens to electrons in a gas-filled space between two electrodes. If there aren't any electrons to start with, there aren't any ionising collisions to create more of them.

Cosmic rays are ubiquitous, and this sets the minimum current you are likely to see. If you set up a situation where there's an appreciable current flowing through the gas, you get enough electrons generated at the negative electrode to sustain a discharge once it has been established in the first place; a "glow discharge" relies on positive ion bombardment of the cathode to generate these electrons.

With enough current flowing, the cathode surface gets hot enough to melt. Under the influence of the electric field the molten surface develops very fine spikes which release electrons by field emission and you've got an arc with a very much lower voltage drop between the electrodes.

It's all fascinating stuff, but decidedly complicated.

I had fun - many years ago - getting a xenon arc lamp to start. I had to generate some 20kV to get current flowing in the first place, and it took a couple of microseconds for the surface of the electrode to get hot enough for the discharge to transition from a glow discharge to an arc.

There wasn't a lot of energy stored in my 20kV starter circuit, and the starter turned out to work a lot more reliably if I let the light from the spark gap that triggered the 20kV starting pulse shine on the arc lamp electrodes - the photo-electrons generated at the arc lamp electrodes got everything going very nicely.

If the arc lamp had to wait for a cosmic ray to arrive at the same time as my 20kV impulse, the starter had to be fired repeatedly before the arc would eventually strike.

-- Bill Sloman, Nijmegen

The referenced paper is an excellent one:

Ionizing radiation creates the initial seed electrons that are necessary for avalanche processes that can ultimately result in spark breakdown across the gap. Intentionally introducing free electrons into the gap, via radioactive electrodes, gases, or UV light, mostly reduces the delay time between voltage application and breakdown (called statistical delay time). However, the presence of ionizing radiation has relatively little impact on the actual breakdown voltage unless huge numbers of electrons are intentionally injected into the gap (such as via an electron beam or high-intensity UV laser).

Externally-applied UV radiation, or slightly radioactive electrodes, were used in laboratory precision spark gaps and in packaged spark gaps/protectors. The constant supply of free electrons made the spark gap break down more quickly, more consistently and within a smaller voltage range. However, spark breakdown always has a statistical component - the measured breakdown voltage of even precision spark gaps will naturally vary within 1-2% irrespective of whether you "salt" the gap with a few free electrons or not. Even larger variations will occur as debris and cratering form previous breakdown events alter local E-field at the surfaces of the electrodes.

Bert

On a sunny day (Mon, 13 Aug 2012 16:06:12 -0500) it happened Bert Hickman wrote in :

Not only there:

LOL