What is thermionic electron gun?

An electron gun that utilizes thermionic emission. A tungsten (W) filament is widely used as the cathode. A single crystal of lanthanum hexaboride (LaB6) is also used as the cathode.

How does electron gun work?

In an electron gun, the metal plate is heated by a small filament wire connected to a low voltage. Some electrons (the conduction electrons) are free to move in the metal – they are not bound to ions in the lattice. As the lattice is heated, the electrons gain kinetic energy.

What is thermionic electron source?

We use two kinds of electron sources in TEMs: the first kind is called a thermionic source, which, as the name suggests, produces electrons when heated, and the second type is a field-emission source, which produces electrons when a large electric potential is applied between it and an anode.

Which material is used for thermionic emission?

Tungsten was previously used as the materials for the thermionic emitter. It has high work function but still, it was used as the cathode because of its high melting point and the material is mechanically very strong.

Which material the electron gun is made of?

An electron gun is placed on top of this column. This gun usually consists of a thermionic cathode made of tungsten or LaB6 in a triode configuration, i.e., an additional electrode (Wehnelt) between the filament and anode.

Where do the electrons come from in an electron gun?

The electrons are emitted from the filament surface to vacuum, and then travel towards the anode under the acceleration voltage (Vacc) between the cathode and the anode. This type of electron gun is structurally simple and does not require ultra-high vacuum, and, consequently, is inexpensive.

How is an electron beam produced in an electron gun?

Electrons are produced in an electron gun. A hot cathode emits electrons, which are accelerated towards an anode, passing through an aperture to reach the accelerating waveguide. A negatively charged focussing electrode narrows the electrons into a fine beam which then passes through the aperture in the anode.

What is electron gun in cathode ray tube?

The function of an electron gun It is a device used in a cathode ray tube for displaying the image on the phosphorous screen of CRT. The electron gun emits electrons and forms them into a beam with the help of a heater, cathode, grid, pre-accelerating, accelerating and focusing anode.

What are the types of electron gun?

There are three types of electron gun according to the method of emission: – field emission type (FE) electron gun, – Schottky electron gun and, – thermal electron gun.

Why tungsten is preferred as a source of electron in a thermionic electron gun?

Tungsten is a suitable metal for Thermionic emission, has a high melting point of 3655 K but its work function is high at about 4.52 eV(electron volt-energy unit). It starts emission at 2500 K. If thoriated tungsten (coated with carbon and thorium) is used work function reduces to 2.6 eV and emission begins at 2000 K.

What particles are emitted in thermionic emission?

thermionic emission, discharge of electrons from heated materials, widely used as a source of electrons in conventional electron tubes (e.g., television picture tubes) in the fields of electronics and communications.

Is tungsten poisonous?

Tungsten is a naturally occurring element. Exposure to very low levels of tungsten may occur by breathing air, eating food, or drinking water that contains tungsten. No specific health effects have been associated with exposure to tungsten in humans. Exposure to high levels of tungsten is unlikely.

Where does thermionic emission of electrons occur?

The heating of the filament leads to the release of electrons in a process called thermionic emission. The released electrons form in an electron cloud at the filament surface, and repulsion forces prevent the ejection of electrons from this negatively charged cloud.