For a conventional tungsten-filament "electron gun" used in electron microscopes they say the Wehnelt cap suppress electron emission from the filament at the sides and base, concentrating emission from the tip only.
Is this done by changing the work function of the tungsten surface in these areas or is it by deflecting electrons emitted from these regions away to earth potential such that they don't contribute to the beam?
Sources say the Wehnelt cap is kept at -200 to -300 V relative to the filament.
Dear Thomas,
imagine (as the simpliest imagination) "compressing / condensing" emitted electrons... this mechanism used to
i) form, ii) focus and iii) optimize the e--cloud around the filament tip = pre-e--beam cloud.
Brightness / illumination intensity of / e- - 'transmission' through specimen / ultrathin sections usually is controlled by deflection of the beam
cf. eg.: Article Elements of a Transmission Electron Microscope
=(direct LINK to article, which is available PPV at Springer: NB: delete the two underscore characters after (or even before) pasting the following URL into your browser!)
https__://link.springer.com/chapter/10.1007%2F978-0-387-40093-8_4
Article On the monochromatisation of high brightness electron source...
=dito:
https__://www.sciencedirect.com/science/article/abs/pii/S0304399199000340;
https://link.springer.com/chapter/10.1007/978-3-540-38967-5_2,
and - for sure - other special sources too...
The last but one resource:
https://en.wikipedia.org/wiki/Transmission_electron_microscopy: Section Electron Gun: (QUOTE:) "The electron gun is formed from several components: the filament, a biasing circuit, a Wehnelt cap, and an extraction anode. By connecting the filament to the negative component power supply, electrons can be "pumped" from the electron gun to the anode plate and the TEM column, thus completing the circuit. The gun is designed to create a beam of electrons exiting from the assembly at some given angle, known as the gun divergence semi-angle, α. By constructing the Wehnelt cylinder such that it has a higher negative charge than the filament itself, electrons that exit the filament in a diverging manner are, under proper operation, forced into a converging pattern the minimum size of which is the gun crossover diameter." (End of QUOTE)
https://en.wikipedia.org/wiki/Transmission_electron_microscopy: Section:
Electron Source: (QUOTE) " In the case of a thermionic source, the electron source is typically mounted in a Wehnelt cylinder to provide preliminary focus of the emitted electrons into a beam while also stabilizing the current using a passive feedback circuit. ..... The combination of the cathode and these first electrostatic lens elements is often collectively called the "electron gun." After it leaves the gun, the beam is typically accelerated by a series of electrostatic plates until it reaches its final voltage and enters the next part of the microscope: The condenser lens system. " (End of QUOTE). In this Wikipedia source eventually also search for "deflect" [CTRL*F, input deflect, return]
The "last" - but not the least - re-source:
https://en.wikipedia.org/wiki/Wehnelt_cylinder SECTION: (QUOTE:)
Operation
A Wehnelt acts as a control grid and it also serves as a convergent electrostatic lens. 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 filament or Lanthanum hexaboride (LaB6) hot cathode 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 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. (End of QUOTE).
Hopefully this helps anyway...
Regards, W.H.M.
Apologies: edited a minute after initial posting of my reply:
cf. also: Master Thesis 2014 by
Martin Stermitz, BSc , Graz University of Technology , AUSTRIA
Free Standing Quasi-1D Resonators for Sensing Applications
p. -17- including sketch:
https://diglib.tugraz.at/download.php?id=576a72cf0d0eb&location=browse
QUOTE: "The Wehenelt cylinder is used to condense the cloud of electrons produced by the filament. It consists of a negatively biased Wehnelt cap which produces a repulsive electrostatic field and an aperture. The electric field focuses the beam in a spot which is called crossover. The electrons that leave the Wehnelt originate from a small area of the tip located above the aperture(see Figure 3-2)." (End of QUOTE).
Dear Thomas,
imagine (as the simpliest imagination) "compressing / condensing" emitted electrons... this mechanism used to
i) form, ii) focus and iii) optimize the e--cloud around the filament tip = pre-e--beam cloud.
Brightness / illumination intensity of / e- - 'transmission' through specimen / ultrathin sections usually is controlled by deflection of the beam
cf. eg.: Article Elements of a Transmission Electron Microscope
=(direct LINK to article, which is available PPV at Springer: NB: delete the two underscore characters after (or even before) pasting the following URL into your browser!)
https__://link.springer.com/chapter/10.1007%2F978-0-387-40093-8_4
Article On the monochromatisation of high brightness electron source...
=dito:
https__://www.sciencedirect.com/science/article/abs/pii/S0304399199000340;
https://link.springer.com/chapter/10.1007/978-3-540-38967-5_2,
and - for sure - other special sources too...
The last but one resource:
https://en.wikipedia.org/wiki/Transmission_electron_microscopy: Section Electron Gun: (QUOTE:) "The electron gun is formed from several components: the filament, a biasing circuit, a Wehnelt cap, and an extraction anode. By connecting the filament to the negative component power supply, electrons can be "pumped" from the electron gun to the anode plate and the TEM column, thus completing the circuit. The gun is designed to create a beam of electrons exiting from the assembly at some given angle, known as the gun divergence semi-angle, α. By constructing the Wehnelt cylinder such that it has a higher negative charge than the filament itself, electrons that exit the filament in a diverging manner are, under proper operation, forced into a converging pattern the minimum size of which is the gun crossover diameter." (End of QUOTE)
https://en.wikipedia.org/wiki/Transmission_electron_microscopy: Section:
Electron Source: (QUOTE) " In the case of a thermionic source, the electron source is typically mounted in a Wehnelt cylinder to provide preliminary focus of the emitted electrons into a beam while also stabilizing the current using a passive feedback circuit. ..... The combination of the cathode and these first electrostatic lens elements is often collectively called the "electron gun." After it leaves the gun, the beam is typically accelerated by a series of electrostatic plates until it reaches its final voltage and enters the next part of the microscope: The condenser lens system. " (End of QUOTE). In this Wikipedia source eventually also search for "deflect" [CTRL*F, input deflect, return]
The "last" - but not the least - re-source:
https://en.wikipedia.org/wiki/Wehnelt_cylinder SECTION: (QUOTE:)
Operation
A Wehnelt acts as a control grid and it also serves as a convergent electrostatic lens. 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 filament or Lanthanum hexaboride (LaB6) hot cathode 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 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. (End of QUOTE).
Hopefully this helps anyway...
Regards, W.H.M.
Apologies: edited a minute after initial posting of my reply:
cf. also: Master Thesis 2014 by
Martin Stermitz, BSc , Graz University of Technology , AUSTRIA
Free Standing Quasi-1D Resonators for Sensing Applications
p. -17- including sketch:
https://diglib.tugraz.at/download.php?id=576a72cf0d0eb&location=browse
QUOTE: "The Wehenelt cylinder is used to condense the cloud of electrons produced by the filament. It consists of a negatively biased Wehnelt cap which produces a repulsive electrostatic field and an aperture. The electric field focuses the beam in a spot which is called crossover. The electrons that leave the Wehnelt originate from a small area of the tip located above the aperture(see Figure 3-2)." (End of QUOTE).
Dear Thomas,
the Wehnelt cylinder has 2 tasks:
1. it regulates the emission current of the cathode.
2. it forms an electronic point source. Therefore, different forms of Wehnelt cylinders are usual (as an example plates or cones), which influence the emission. The electron optic of an microscope (SEM) projects a minimized focus on the sample. Of course, the focus can be influenced by the settings of the electron optic. But the essential point is: the minimum focus diameter on the sample is due to the diameter and quality of the focus, produced by the Wehnelt cylinder.
Therefore, the resolution of an electron scanning microscope is defined by the quality of the Wehnelt focus.
With Regards
R. Mitdank
Back to work function.
It has been very long time since the last time I could call myself a physicist. If I remember correctly (IF!), the work function depends on specimen itself - material, temperature, shape, surface conditions. Electric field of Wehnelt cylinder is an outside influence, so it is not a part of work function (hope I am right).
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