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I know how an air gap behaves: for lower voltages the resistance is extremely high, until spark voltage is achieved, at which point resistance rapidly drops, creating electric arc. Now, how does that behave if we remove air?

Let's assume a part of electronic circuit that is a vacuum pipe with two electrodes at its ends. The electrodes are otherwise separated by an extremely (infinitely) highly resistive material. What would the I:U graph of voltage across the void gap versus amperage of current flow look like for element like this? What would it depend on, besides voltage (and possibly distance between the electrodes)?

As I understand some charge flows through vacuum between oppositely charged electrodes but it's nowhere near as straightforward as in case of normal conductors. How does it look like? What are the caveats? Is there a plain equivalent in common circuitry that behaves just the same as a void gap for common electrical characteristics (voltage-resistance curve etc) ?

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Does a vacuum tube still count as "common circuitry"? Because, you know... –  Ilmari Karonen Sep 16 '13 at 11:17
    
@IlmariKaronen: More or less yes, but with currents of heating the cathode varying between "nominal" and "zero" for answering my question more completely. In particular, what are the characteristics with cold cathode? –  SF. Sep 16 '13 at 11:27
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A high vacuum will also break down under a sufficiently high DC field. Under an increasing DC voltage, small projections on the negative electrode (called "asperities") begin to experience a localized electric field that is sufficient to initiate field emission, often accompanied by intense localized heating, thermoelectric emission, or even explosive emission of electrons and ejection of cathode material into the gap. Field emission is a function of the cathode material's Work Function - i.e., how easy electrons can be stripped from the cathode. Each of these events generates a microplasma at the site of the asperity, and electrically, each event shows up as a small spike of current through the gap. Under somewhat higher voltages, one of these events may culminate in a spark that completely bridges the gap. If driven from a low impedance source, the spark may evolve into a sustained vacuum arc. Vacuum arcs often cause significant heating and surface melting of the anode as well as the cathode.

A high voltage vacuum gap (as in a transmitting vacuum tube cathode-anode, vacuum capacitor, or vacuum switch) can be "conditioned" to operate at higher voltages by using an adjustable HV supply, current limiting resistor, and a small capacitor. Any asperities are melted or vaporized by the controlled energy in the HV capacitor, but damaging arcing is prevented by the resistor. The high voltage is slowly increased until the gap can operate at maximum voltage without further breakdown. This process is also called "spot knocking".

Although the mechanisms are different, both air gaps and vacuum gaps may show pre-breakdown current pulses. And, once spark breakdown or arcing occurs, the negative resistance VI characteristics are also similar for gaseous and vacuum sparks and arcs. However, when current flow is temporarily stopped (as during a current zero-crossing in an AC circuit), a vacuum arc will usually recover its dielectric strength more quickly than an arc within a gas.

A good discussion can be seen here: http://cartan.e-moka.net/content/download/409/2311/file/ptv.pdf

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