Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 02:42 PM 4/9/2006, you wrote:
Original poster: Robert Clark <bobbygc2001@xxxxxxxxx> Actually I'm interested in the results at a few microns. I found this after a web search: "Under constant atmospheric conditions, it is found that the breakdown voltage of a uniform field gap may be expressed in the form: V = A*d + B*SQRT(d) where d is the gap spacing [i.e., in centimenters - Bob] For air under normal conditions: A = 24.4kV/cm B = 6.29kV/cm^1/2" Anyone know if this formula would apply or know a more accurate formula at the micron scale?
That's an empirical relation for gaps greater than, say, a cm or so.Paschen's law says that the product of gas density and gap distance is constant, for a given voltage.
Furthermore, for most gases, at large distances, the breakdown is roughly at a constant E field (volts/meter): around 30 kV/cm for air at normal density, and, the breakdown E field is roughly proportional to density (halve the density and the breakdown field is halved.. so at 18000 ft elevation, it's 15 kV/cm)
When you get down to very small distances, comparable to a mean free path, the breakdown mechanisms change, so the rules break down. Or, when you get to low pressures.
In air, you won't get breakdown at a voltage lower than about 350V, no matter what the gap is. So, at 10 microns (0.001 cm), the breakdown isn't 300 V, and at 1 micron, it's not 30 V.