---
title: "Vacuum classification and breakdown mechanisms"
subject: "High Voltage Engineering"
chapter: 6
key: "semester_8/high_voltage_engineering:vacuum-breakdown-mechanisms"
source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/vacuum-breakdown-mechanisms
site: BCT Engineering
---

# Vacuum classification and breakdown mechanisms

High Voltage Engineering · Chapter 6: Dielectric Breakdown

- Asked in: 2069 Bhadra (old course); 2068 Bhadra (old course), 2+1+5 marks; 2067 Mangsir (old course), 6 marks; 2066 Magh (old course), 8 marks (asked 4 times)
- Video: [High Voltage Engineering: Lect-21: Breakdown Mechanism in Vacuum](https://www.youtube.com/watch?v=sK8uwcteteY) — Electrical Engineering Lectures-T. G. Arora
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/vacuum-breakdown-mechanisms
- Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-6

## Question

What is vacuum and how it is classified? Discuss the various categories of vacuum breakdown mechanism.

## Short answer

Vacuum is a region with pressure so low that electron mean free path exceeds the gap, classified as high (10⁻³–10⁻⁶ torr), very high, and ultra-high vacuum. Breakdown mechanisms are particle exchange, field emission (anode/cathode heating), and clump impact, all governed by electrode surface processes.

## Answer

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

| Category | Pressure range (torr) |
|---|---|
| High vacuum | $10^{-3}$ to $10^{-6}$ |
| Very high vacuum | $10^{-6}$ to $10^{-8}$ |
| Ultra-high vacuum | $10^{-9}$ and below |

(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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does Townsend gas breakdown not occur in vacuum?
   A. Electrodes are too hot
   B. Mean free path is much longer than the gap, so collision ionization cannot occur
   C. Voltage is too low
   D. Gas is too dense
   Answer: B. With almost no collisions, breakdown comes from electrode surface processes instead. (Key point: Definition of vacuum)

2. In the particle exchange mechanism, breakdown occurs when
   A. the gap is below 1 mm
   B. the product of secondary emission coefficients AB exceeds 1
   C. AB is less than 1
   D. the anode melts
   Answer: B. If AB > 1 the exchange of particles between electrodes becomes cumulative. (Key point: Particle exchange)

3. In the field emission mechanism electrons are pulled from
   A. gas molecules
   B. cathode micro-projections where the local field is about 10^6-10^7 V/cm
   C. the anode surface
   D. loose clumps
   Answer: B. Micro-points enhance the local field, giving Fowler-Nordheim emission, followed by anode or cathode heating. (Key point: Field emission)

4. The clump mechanism explains why Vb in vacuum rises roughly as
   A. d^2
   B. sqrt(d)
   C. 1/d
   D. constant
   Answer: B. A charged clump torn off and accelerated across the gap gives Vb = K sqrt(d) for large gaps. (Key point: Clump mechanism)

## Related in High Voltage Engineering

- [Breakdown mechanisms in commercial liquids](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Acommercial-liquid-breakdown) — High Voltage Engineering, chapter 6, asked 3×
- [Breakdown mechanisms in solids under working conditions](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-breakdown-actual-conditions) — High Voltage Engineering, chapter 6
- [Breakdown mechanisms in solids, explain two](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-breakdown-explain-two) — High Voltage Engineering, chapter 6, asked 4×
- [Breakdown mechanisms in solids, explain one](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-breakdown-explain-one) — High Voltage Engineering, chapter 6, asked 4×
- [Why vacuum and high-pressure gas are stronger](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Avacuum-high-pressure-strength) — High Voltage Engineering, chapter 6, asked 2×
- [Solid breakdown due to actual working conditions](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asolid-breakdown-working-conditions) — High Voltage Engineering, chapter 6

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