Vacuum classification and breakdown mechanisms

A vacuum is a space in which the gas pressure is so low that the mean free path of electrons is much longer than the electrode gap. Breakdown then cannot occur by collision ionization of the gas (Townsend process); it is caused by processes at the electrode surfaces.

Classification of vacuum

(1 torr = 1 mm Hg.) For electrical insulation, as in vacuum circuit breakers and vacuum capacitors, the pressure is usually below $10^{-4}$ torr.

Categories of vacuum breakdown mechanism

The mechanisms are broadly grouped into three:

1. Particle exchange mechanism

• A charged particle (electron or ion) is accelerated by the field, strikes an electrode and releases particles of the opposite sign.
• Example: an electron hits the anode and releases positive ions; these ions hit the cathode and release more electrons (secondary emission coefficients $A$ and $B$).
• If the product of the coefficients exceeds unity, the process becomes cumulative: $AB > 1$, and breakdown occurs.
• Photons and negative ions can take part in the exchange as well.

2. Field emission mechanism

Electrons are pulled out of tiny projections (micro-points) on the cathode where the local field is enhanced to about $10^{6}$–$10^{7}$ V/cm. The current density follows the Fowler–Nordheim equation. Breakdown then occurs in one of two ways:

• Anode heating: the emitted electron beam hits the anode, heats a small spot and releases gas atoms and vapour. These are ionized by the beam; the positive ions move to the cathode, increase emission, and the process runs away into an arc.
• Cathode heating: the large current density through the micro-point heats it resistively until it melts and explodes, releasing metal vapour that forms a conducting plasma (vacuum arc).

3. Clump mechanism

• A loosely bound particle (clump) on one electrode becomes charged and is torn off by the field.
• It is accelerated across the gap and strikes the other electrode with high energy.
• The impact vaporizes material or releases gas, and breakdown follows.
• This explains why the breakdown voltage in vacuum rises roughly as $\sqrt{d}$ for large gaps ($V_b = K\sqrt{d}$).

Factors affecting vacuum breakdown

• Electrode material, surface finish and cleanliness
• Gap length (strength per cm falls for longer gaps)
• Conditioning (repeated sparks smooth the surface and raise $V_b$)
• Temperature and absorbed gases on the electrodes

Vacuum gives very high strength for short gaps (about $10^5$–$10^6$ V/cm) and rapid recovery after arc interruption, which is why it is used in vacuum interrupters up to about 36 kV (and higher in newer designs).