A Wehnelt cylinder (also known as ''Wehnelt cap'', ''grid cap'' or simply ''Wehnelt'') is an

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...

in the

electron gun An electron gun (also called electron emitter) is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode-ray tubes (CRTs), used in nearly ...

assembly of some

thermionic Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...

devices, used for focusing and control of the electron beam. It is named after Arthur Rudolph Berthold Wehnelt, a German physicist, who invented it during the years 1902 and 1903. Wehnelt cylinders are found in the electron guns of

cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), pictu ...

s and

electron microscope An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...

s, and in other applications where a thin, well-focused electron beam is required.

Structure

A Wehnelt cap has the shape of a topless, hollow cylinder. The bottom side of the cylinder has an aperture (through hole) located at its center, with a diameter that typically ranges from 200 to 1200 μm. The bottom face of the cylinder is often made from platinum or tantalum foil.

Operation

A Wehnelt acts as a

control grid The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consist ...

and it also serves as a convergent

electrostatic lens An electrostatic lens is a device that assists in the transport of charged particles. For instance, it can guide electrons emitted from a sample to an electron analyzer, analogous to the way an optical lens assists in the transport of light in an o ...

. An electron emitter is positioned directly above the Wehnelt aperture, and an anode is located below the Wehnelt. The anode is biased to a high positive voltage (typically +1 to +30 kV) relative to the emitter so as to accelerate electrons from the emitter towards the anode, thus creating an electron beam that passes through the Wehnelt aperture. The Wehnelt is biased to a negative voltage (typically −200V to −300V) relative to the emitter, which is usually a

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

filament or Lanthanum hexaboride (LaB₆)

hot cathode In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating elemen ...

with a "V" shaped (or otherwise pointed) tip. This bias voltage creates a repulsive electrostatic field that suppresses emission of electrons from most areas of the cathode. The emitter tip is positioned near the Wehnelt aperture so that, when appropriate bias voltage is applied to the Wehnelt, a small region of the tip has a net electric field (due to both anode attraction and Wehnelt repulsion) that allows emission from only that area of the tip. The Wehnelt bias voltage determines the tip's emission area, which in turn determines both the beam current and effective size of the beam's electron source. As the Wehnelt's negative bias voltage increases, the tip's emitting area (and along with it, the beam diameter and beam current) will decrease until it becomes so small that the beam is "pinched" off. In normal operation, the bias is typically set slightly more positive than the pinch bias, and determined by a balance between desired beam quality and beam current. The Wehnelt bias controls beam focusing as well as the effective size of the electron source, which is essential for creating an electron beam that is to be focussed into a very small spot (for scanning electron microscopy) or a very parallel beam (for diffraction). Although a smaller source can be imaged to a smaller spot, or a more parallel beam, one obvious trade off is a smaller total beam current.

References

{{DEFAULTSORT:Wehnelt Cylinder Vacuum tubes Electrodes