An ion vacuum gauge measures extremely low pressures (high vacuum) by ionizing gas molecules within the gauge and detecting the resulting ions. This type of gauge is commonly used in high and ultra-high vacuum systems, capable of measuring pressures down to 10⁻¹⁰ Torr or lower.

Working Principle

1. Electron Emission: The ion gauge contains a hot filament, typically made of tungsten or another refractory metal, which emits electrons when heated. These electrons are accelerated into the gauge by an applied voltage.

2. Ionization of Gas Molecules: As the emitted electrons move through the gauge, they collide with gas molecules present in the vacuum chamber, ionizing them. This process creates positively charged ions from neutral gas molecules.

3. Ion Collection: The positive ions are attracted to a collector electrode (a wire or a plate) maintained at a negative potential. When these ions strike the collector, they generate a small current.

4. Current Measurement: The ion current is directly proportional to the density of gas molecules in the vacuum, and hence to the pressure. By measuring the ion current, the gauge provides a reading of the pressure in the chamber.

Types of Ion Gauges

The most common types of ion vacuum gauges are:

• Hot Cathode Ionization Gauge: Uses a heated filament to emit electrons, which are then accelerated to ionize gas molecules. This is one of the most widely used types for ultra-high vacuum (UHV) applications.
• Cold Cathode Ionization Gauge: (Penning Gauge): Instead of a heated filament, this gauge uses a high-voltage field to create electrons. Cold cathodes are more durable in some cases because they don’t rely on a fragile hot filament, but they may require higher initial pressure to initiate discharge.

Key Components

• Filament (for Hot Cathode): Emits electrons when heated, essential for the ionization process.
• Collector Electrode: Captures the positive ions created in the gauge.
• Control Circuitry: Converts the ion current into a pressure reading.

Applications and Limitations

• Applications: Ion gauges are commonly used in scientific research, semiconductor manufacturing, surface science, and other applications requiring ultra-high vacuum.
• Limitations:
  • These gauges cannot function well at pressures above approximately 10⁻³ Torr, as ionization becomes unreliable at higher pressures.
  • Ion gauges are sensitive to contamination, especially by hydrocarbons, which can coat the filament or other components, reducing accuracy.
  • Exposure to atmospheric pressure can damage the filament in hot cathode gauges, so they need to be carefully isolated or turned off before venting.

Advantages

• Ion vacuum gauges are capable of measuring extremely low pressures, making them ideal for ultra-high vacuum (UHV) environments where other gauges would not work effectively.
• They provide a continuous and precise measurement of pressure in high vacuum regions, essential for many critical processes.

In summary, ion vacuum gauges operate by ionizing gas molecules in the vacuum and measuring the resulting ion current, providing precise pressure readings in high and ultra-high vacuum ranges.