Hot filament ionization gauge
Encyclopedia
The hot-filament ionization gauge, sometimes called a hot-filament gauge or hot-cathode gauge, is the most widely used low-pressure (vacuum) measuring device for the region from 10-3 to 10-10 torr. It is a triode
, whereas the filament is the cathode
.
Note: Principles are mostly the same for hot-cathode ion sources in particle accelerator
s to create electrons
) is emitted from a heated filament. The electrons are attracted to the helical grid by a dc potential of about +150 volts. In their passage from the filament to the grid
, the electrons collide with gas molecules in the gauge envelope, causing a fraction of them to be ionized. The gas ions formed by the electron collisions are attracted to the central ion collector wire by the negative voltage on the collector (typically a minus 30 volts). Ion currents are on the order of 1 mA/Pa. This current is amplified and displayed by a high-gain-differential amplifier/ electrometer
.
This ion current
will differ for different gases at the same pressure; that is, a hot filament ionization gauge is composition-dependent. Over a wide range of molecular density, however, the ion current from a gas constant composition will be directly proportional to the molecular density of the gas in the gauge.
A hot-cathode ionization gauge is composed mainly of three electrodes all acting as a triode
, wherein the cathode
is the filament. The three electrodes are a collector or plate, a filament, and a grid
. The collector current is measured in pico
amps by an electrometer
. The filament voltage to ground is usually at a potential of 30 volts, while the grid voltage at 180–210 volts DC, unless there is an optional electron bombardment feature, by heating the grid, which may have a high potential of approximately 565 volts.
The most common ion gauge is the hot-cathode Bayard-Alpert gauge, with a small collector inside the grid. A glass envelope with an opening to the vacuum can surround the electrodes, but usually the Nude Gauge is inserted in the vacuum chamber directly, the pins being fed through a ceramic plate in the wall of the chamber. Hot cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot.
Electrons emitted from the filament move several times in back and forth movements around the grid before finally entering the grid. During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron (Electron ionization
). The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current. Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current.
The low-pressure sensitivity of hot-cathode gauges is limited by the photoelectric effect. Electrons hitting the grid produce X-rays that produce photoelectric noise in the ion collector. This limits the range of older hot-cathode gauges to 10-8 Torr and the Bayard-Alpert to about 10-10 Torr. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are not attracted by a wire but by an open cone. As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be passed on to a
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
, whereas the filament is the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
.
Note: Principles are mostly the same for hot-cathode ion sources in particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s to create electrons
Function
A regulated electron current (typically 10 mAAmpere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
) is emitted from a heated filament. The electrons are attracted to the helical grid by a dc potential of about +150 volts. In their passage from the filament to the grid
Control grid
The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
, the electrons collide with gas molecules in the gauge envelope, causing a fraction of them to be ionized. The gas ions formed by the electron collisions are attracted to the central ion collector wire by the negative voltage on the collector (typically a minus 30 volts). Ion currents are on the order of 1 mA/Pa. This current is amplified and displayed by a high-gain-differential amplifier/ electrometer
Electrometer
An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made mechanical instruments to high-precision electronic devices...
.
This ion current
Ion current
An ion current is the influx and/or efflux of ions through an ion channel....
will differ for different gases at the same pressure; that is, a hot filament ionization gauge is composition-dependent. Over a wide range of molecular density, however, the ion current from a gas constant composition will be directly proportional to the molecular density of the gas in the gauge.
Construction
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
, wherein the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
is the filament. The three electrodes are a collector or plate, a filament, and a grid
Control grid
The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
. The collector current is measured in pico
Pico
Pico- is a prefix in the metric system denoting a factor of 10−12 or .Derived from the Italian piccolo, meaning small, this was one of the original 12 prefixes defined in 1960 when the International System of Units was established....
amps by an electrometer
Electrometer
An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made mechanical instruments to high-precision electronic devices...
. The filament voltage to ground is usually at a potential of 30 volts, while the grid voltage at 180–210 volts DC, unless there is an optional electron bombardment feature, by heating the grid, which may have a high potential of approximately 565 volts.
The most common ion gauge is the hot-cathode Bayard-Alpert gauge, with a small collector inside the grid. A glass envelope with an opening to the vacuum can surround the electrodes, but usually the Nude Gauge is inserted in the vacuum chamber directly, the pins being fed through a ceramic plate in the wall of the chamber. Hot cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot.
Electrons emitted from the filament move several times in back and forth movements around the grid before finally entering the grid. During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron (Electron ionization
Electron ionization
Electron ionization is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions...
). The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current. Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current.
The low-pressure sensitivity of hot-cathode gauges is limited by the photoelectric effect. Electrons hitting the grid produce X-rays that produce photoelectric noise in the ion collector. This limits the range of older hot-cathode gauges to 10-8 Torr and the Bayard-Alpert to about 10-10 Torr. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are not attracted by a wire but by an open cone. As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be passed on to a
- Faraday cupFaraday cupA Faraday cup is a metal cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup...
- Microchannel plate detectorMicrochannel plate detectorA micro-channel plate is a planar component used for detection of particles and impinging radiation . It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission...
with Faraday cup - Quadrupole mass analyzerQuadrupole mass analyzerThe quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of 4 circular rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the component of the instrument responsible for filtering sample ions, based on...
with Faraday cup - Microchannel plate detectorMicrochannel plate detectorA micro-channel plate is a planar component used for detection of particles and impinging radiation . It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission...
with Faraday cup - Quadrupole mass analyzerQuadrupole mass analyzerThe quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. As the name implies, it consists of 4 circular rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the component of the instrument responsible for filtering sample ions, based on...
with Microchannel plate detector Faraday cup - Ion lens and accelartion voltage and directed at a target to form a sputter gunSputteringSputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
. In this case a valve lets gas into the grid-cage.
Types of hot-filament ionization gauges
- Bayard Alpert (uses sealed tube)
- Nude gauge (uses the vacuum chamber to make a complete seal).
External links
- Hot filament ionization gauge tubes
- U.S. Patent 4792763 - Hot cathode ionization pressure gauge
- U.S. Patent 5373240 - Hot-cathode ionization pressure gauge including a sequence of electrodes arranged at a distance from one another in sequence along an axis