Electronics > Projects, Designs, and Technical Stuff
Electron Beam Field
ZeroResistance:
--- Quote from: dzseki on July 31, 2019, 01:52:51 pm ---b.) 34W only, yes, concentrated to a 65um spot, that translates to an insane power density...
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If we supply 34W of electric power, what kind of beam power are we looking at. Is an electron gun in a CRT 80 to 90% efficient?
T3sl4co1l:
Referring to 2nd anode current and voltage only, the efficiency is 100%. The beam may not be perfectly collimated (there may be some leakage to the sides of the drift area, or the 2nd anode inside the electron gun); although in this case with focus being especially prioritized, we can probably assume they did very well at keeping the electron current towards just the phosphor screen.
30W in such a small area, is comparable to the power density of a soldering iron. Maybe not glass-meltingly hot (as long as the raster keeps going), but that kind of heat could easily crack open the tube, even made with hard (borosilicate) glass. These tubes are water cooled, with a clear jacket over the face.
The phosphor efficiency is probably in the 30% range, so not all of that is deposited as heat. I don't know if they were able to push efficiency crazily high, but figures like that were typical of fluorescent lamps at least (given that fluorescents go plasma --> UV --> phosphor, more like 20% overall).
Tim
Ultron81:
When it comes to an electron optics, the biggest limitation on what you can do has to do with the aberrations in the lens system. The objective lens is in charge of focusing the beam to a spot. The scan coils are in charge of deflecting the beam (scanning in an SEM or positioning in the case of an e-beam). The field created by the Objective Lens is only so uniform, and if the beam travels outside of that uniform field, the beam will become defocused and you will see distortions in your image on an SEM, or your writing pattern on an e-beam system.
So beam deflection can only get you so far. The next solution is to move the stage under a stationary beam. In this case, the accuracy, step size, and reproducibility of the stage come into play. As was said above, e-beams use laser interferometers to keep track of stage positioning. This is only so accurate, and has a limit to step size. Also, it is important that the stage position can be reproducible - if you move to a different position and need to move back to a previous position, willful be in the exact same spot as before?
Also, as far as deflection is concerned, an e-beam’s electron beam is energized to 50kV or 100kV. An electron beam in a CRT, for examples, is usually less than 20kV. The deflection system in an e-beam needs to be strong enough to deflect a beam with such high energy. Also, the distance between the CRT grid and screen is larger than the distance from the objective lens to the sample in an e-beam. Therefore, the beam in a CRT can have a larger sweep area.
I repair/install SEMs and TEMs for work. Occasionally I work on an E-beam.
David Hess:
--- Quote from: dzseki on July 31, 2019, 09:53:20 am ---Another art were the professional CRT projectors, these could fit 2000 pixels on a ~13cm wide picture tube, while the beam current was approaching 1mA (!) (@ 34kV), without cooling the tube the beam could melt the glass of the tubeface in normal operation!
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That points to what limited oscilloscope CRT performance. Optimizing for vertical bandwidth meant using scan expansion after deflection but this also expanded the dot size. The last generation of oscilloscope CRTs without scan expansion were about 50 MHz and easily exceeded the resolution of NTSC video on a relatively tiny screen area. Later oscilloscopes were faster but had poorer resolution and image quality yet eventually still produced outstanding resolution although not as good as a CRT projector. If those early CRTs had been scaled up, they would have handily outperformed color CRTs but there was no need for such a thing in monochrome.
This is why Tektronix developed their color LCD shutter technology. It allowed using a monochrome high resolution CRT in basic color applications where shadow mask technology had insufficient resolution. HP did something similar in their early DSOs with monochrome raster CRTs that had doubled horizontal resolution. LCDs in test instruments have barely caught up.
coppercone2:
--- Quote from: Nominal Animal on July 30, 2019, 10:33:52 pm ---In a way, wider electron beams work more like a jet of water than a ray of light; their control really is an art.
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not sure if it qualifies but I think the distinction between heavy particle stream collider and ultra fast collider sees this distinction (the heavy collider is underappreciated engineering wise because its not as fast)
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