March 21, 2017 at 00:14Electron microscope in the garage. Cathode and gun
If you missed previous releases, be sure to read .
Sufficient vacuum (
torr) has already been received, which means it is time to move on: install the cathode, deal with the high-voltage power supply and finally release the electrons!
This is how cathodes and an electron gun with a focusing electrode look like in reality. Under the cut is a simple explanation of how this works, as well as the insides of the electron-optical column in 4K quality.
In a scanning electron microscope, the sample under investigation is sequentially irradiated point-by-point with a thin electron beam, and it is very desirable that they also move at the same speed. An electron-optical column with a whole system of electrostatic and electromagnetic lenses also serves to create such an electron beam. And the first element in it is an electronic gun .
Peter W. Hawkes, a well-known scientist in the field of electron optics and electron microscopy, in his book Electron optics and electron microscopy (1972) gives such an electron gun scheme (see the figure on the left).
The direct source of free electrons, from which that thin beam is then formed, is the cathode .
Such electrons are obtained due to the phenomenon of thermionic emission . Generally speaking, there is also field emission , and it is used in modern microscopes, but its use is fraught with additional difficulties, so we will not consider it yet.
Thermoelectronic emission is very simple: the cathode is a tungsten wire, bent in the form of the Latin letter V, and heated by passing an electric current through it. To complete the picture, I will give another illustration from the aforementioned book, which shows various types of cathodes used in electron microscopes.
For understanding (and calculations) it is convenient when they say that the anodeIs the positive electrode, and the cathode is the negative, but in an electron microscope, the anode is the entire column. Applying a high voltage of tens of kilovolts to it is a bad enough idea. Therefore, they do it differently: the column (i.e. the anode) is grounded, and a negative high voltage is applied to the cathode and the glow current is mixed.
The resulting electron beam must be further prepared before directing it into electromagnetic lenses. Historically, the entire design of the thermionic electron gun has undergone few changes and consists of a cathode, a focusing electrode, called venelt and an anode.
The moment when it will be necessary to try the electronic gun and electromagnetic lenses in work is getting closer and closer, so I decided to conduct a total inspection of the entire column.
This became possible only after good people gave me a jar of high-vacuum grease (more on this below, in thanks). And, as it turned out, he was sorting the column for good reason. I found and fixed several problems there. And filmed everything in 4K quality. You can enjoy the internal structure of the microscope.
The first video is somewhat long, I did not mount it.
People who watched this video on my channel before publishing the article asked me to make the next part more eventful and less time consuming. So he did, cut out unnecessary moments, accelerated all kinds of screw loosening and tightening, and posted the second video about disassembling the column. There you can see all the beauty of the lower part of the electron microscope - deflecting coils, stigmatator, diaphragms.
What was not included in the video is the objective aperture and repair of the stage.
It stands at the pole end of an objective lens, and works in essence, like a diaphragm in a camera. If you want a large depth of field - you need to choose the smallest hole. If you want maximum beam intensity - select the largest hole.
As it can be said, everything is more or less in order. Well, see for yourself:
Crumpled, dirty, but all the wires are in place, the adjustments work. It's good! To wash traditional, straighten, grease a laying and forward.
In this microscope, three apertures are available on a single strip (presumably platinum foil).
Here is what the official instruction for the microscope says on this subject:
I have long wanted to get to the table, but there was no reason. And so he was found. It turned out that the wiring connecting the vacuum input for the sensor of the absorbed current and the table itself was cut off. Not that it broke, but in general there is no whole piece of wire. And at the same time, everything else is quite dirty. Look how it looked before:
But what it looks like after a little maintenance:
What to make a wire from was a big question. An ordinary wire is impossible, because the insulation will evaporate in a vacuum, and it will not lead to anything good. Even the material of the conductor itself matters, usually oxygen-free copper is used in such cases.
After much deliberation, an option was found with a special fiberglass tube, which is used as a heat shield for wires. It is unlikely to evaporate, but just in case, it was washed in isopropyl alcohol from pollution. And inside, I put in a copper core from a twisted pair, which is made of the very oxygen-free copper (correct if I'm mistaken).
Last time I wrote a list of what will help in this project, which will be useful for experiments. Many thanks to all who responded! I visited several universities and organizations, met very interesting people.
For the implementation of projects received very important components. For example, a small turbomolecular pump with a controller:
I think that after the final debugging of the vacuum system of the microscope, I will change the oil pump to this turbomolecular one.
I also got a jar of Apiezon high-vacuum non-silicone grease, which opened up the possibility for me to finally sort out the entire column and fix all the problems that I wrote about above.
Thanks Goron_Dekar ! I am very excited after our conversation. I’ll definitely come back to talk, and try to bring something useful.
I thank Victor from MIPT, a trip to Pushchino was a useful and fascinating journey. They have a slightly older model of a similar JEOL, working in microanalyzer mode.
ZavDimka - thanks for the interesting and useful things (although I haven't picked them up yet :)).
This person makes an excellent high voltage power supply. And much more. And since Since the microscope is already entering the stage of electronics, it is now very important.
jar_ohty - although I haven’t visited you yet, I’ll be happy to visit to meet you. Maybe we’ll use the secondary electron multiplier somehow :)
And many more people, whom I also thank very much. Still looking for detectors and cathodes for a microscope, but there is some hope.
In the next series - ignite the cathode :)
Sufficient vacuum (
torr) has already been received, which means it is time to move on: install the cathode, deal with the high-voltage power supply and finally release the electrons!This is how cathodes and an electron gun with a focusing electrode look like in reality. Under the cut is a simple explanation of how this works, as well as the insides of the electron-optical column in 4K quality.
1. Electron gun and cathode
In a scanning electron microscope, the sample under investigation is sequentially irradiated point-by-point with a thin electron beam, and it is very desirable that they also move at the same speed. An electron-optical column with a whole system of electrostatic and electromagnetic lenses also serves to create such an electron beam. And the first element in it is an electronic gun . Peter W. Hawkes, a well-known scientist in the field of electron optics and electron microscopy, in his book Electron optics and electron microscopy (1972) gives such an electron gun scheme (see the figure on the left).
The direct source of free electrons, from which that thin beam is then formed, is the cathode .
Such electrons are obtained due to the phenomenon of thermionic emission . Generally speaking, there is also field emission , and it is used in modern microscopes, but its use is fraught with additional difficulties, so we will not consider it yet. Thermoelectronic emission is very simple: the cathode is a tungsten wire, bent in the form of the Latin letter V, and heated by passing an electric current through it. To complete the picture, I will give another illustration from the aforementioned book, which shows various types of cathodes used in electron microscopes.
For understanding (and calculations) it is convenient when they say that the anodeIs the positive electrode, and the cathode is the negative, but in an electron microscope, the anode is the entire column. Applying a high voltage of tens of kilovolts to it is a bad enough idea. Therefore, they do it differently: the column (i.e. the anode) is grounded, and a negative high voltage is applied to the cathode and the glow current is mixed.
The resulting electron beam must be further prepared before directing it into electromagnetic lenses. Historically, the entire design of the thermionic electron gun has undergone few changes and consists of a cathode, a focusing electrode, called venelt and an anode.
2. Dismantling the entire column
The moment when it will be necessary to try the electronic gun and electromagnetic lenses in work is getting closer and closer, so I decided to conduct a total inspection of the entire column.
This became possible only after good people gave me a jar of high-vacuum grease (more on this below, in thanks). And, as it turned out, he was sorting the column for good reason. I found and fixed several problems there. And filmed everything in 4K quality. You can enjoy the internal structure of the microscope.
The first video is somewhat long, I did not mount it.
Summary of the investigation of the first part
He began to disassemble with an electron gun, removed the anode, the top of the column and saw a lot of debris (dust, fingerprints, grease). But this is not the worst, the saddest thing is that someone has already climbed there, and not with very skillful hands. Dismantled, dismantled, and traces of what they climbed there - everywhere.
As a result, it turned out that the heads of the two screws were completely ripped off, and others that were longer in length were put in their place, and they simply rested on the body. Therefore, the anode was crooked. To compensate for this (and they apparently did not notice that the anode was at an angle), the condenser lens was also shifted to the side.
Moreover, when they put the condenser lens back on, they did not fall into the corresponding adjustment grooves at all, so smooth adjustment, as conceived by the manufacturer, was very difficult to carry out.
By the way, in the video I say a capacitor lens for unusual. Saw in one old book so called. I read later and found that the words of the condenser and the condenser - synonyms, before even an ordinary capacitor called a condenser. But now the terminology has developed so that the capacitor is called a capacitor, and the collecting electromagnetic lens is called a condenser lens or simply a condenser.
As a result, it turned out that the heads of the two screws were completely ripped off, and others that were longer in length were put in their place, and they simply rested on the body. Therefore, the anode was crooked. To compensate for this (and they apparently did not notice that the anode was at an angle), the condenser lens was also shifted to the side.
Moreover, when they put the condenser lens back on, they did not fall into the corresponding adjustment grooves at all, so smooth adjustment, as conceived by the manufacturer, was very difficult to carry out.
By the way, in the video I say a capacitor lens for unusual. Saw in one old book so called. I read later and found that the words of the condenser and the condenser - synonyms, before even an ordinary capacitor called a condenser. But now the terminology has developed so that the capacitor is called a capacitor, and the collecting electromagnetic lens is called a condenser lens or simply a condenser.
People who watched this video on my channel before publishing the article asked me to make the next part more eventful and less time consuming. So he did, cut out unnecessary moments, accelerated all kinds of screw loosening and tightening, and posted the second video about disassembling the column. There you can see all the beauty of the lower part of the electron microscope - deflecting coils, stigmatator, diaphragms.
Summary of the investigation of the second part
I decided to make out everything "to the ground, and then ...". I saw that the previous owners also dismantled everything to the ground. We missed all-all gaskets with vacuum grease, even those that were not to be smeared at all. They also slightly damaged the stigmatator's windings when they inserted it back, but this was not critical. Spread out, put them back on, should work. Oriented the deflection coils correctly, sort of.
What was not included in the video is the objective aperture and repair of the stage.
3. Aperture of the objective lens
It stands at the pole end of an objective lens, and works in essence, like a diaphragm in a camera. If you want a large depth of field - you need to choose the smallest hole. If you want maximum beam intensity - select the largest hole.
As it can be said, everything is more or less in order. Well, see for yourself:
Crumpled, dirty, but all the wires are in place, the adjustments work. It's good! To wash traditional, straighten, grease a laying and forward.
In this microscope, three apertures are available on a single strip (presumably platinum foil).
Here is what the official instruction for the microscope says on this subject:
- 100 microns - for observations that require a large depth of field or low beam intensity
- 200 microns - for ordinary applications
- 600 microns - for x-ray analyzer or real-time observations
4. The subject table
I have long wanted to get to the table, but there was no reason. And so he was found. It turned out that the wiring connecting the vacuum input for the sensor of the absorbed current and the table itself was cut off. Not that it broke, but in general there is no whole piece of wire. And at the same time, everything else is quite dirty. Look how it looked before:
But what it looks like after a little maintenance:
What to make a wire from was a big question. An ordinary wire is impossible, because the insulation will evaporate in a vacuum, and it will not lead to anything good. Even the material of the conductor itself matters, usually oxygen-free copper is used in such cases.
After much deliberation, an option was found with a special fiberglass tube, which is used as a heat shield for wires. It is unlikely to evaporate, but just in case, it was washed in isopropyl alcohol from pollution. And inside, I put in a copper core from a twisted pair, which is made of the very oxygen-free copper (correct if I'm mistaken).
Thanks!
Last time I wrote a list of what will help in this project, which will be useful for experiments. Many thanks to all who responded! I visited several universities and organizations, met very interesting people.
For the implementation of projects received very important components. For example, a small turbomolecular pump with a controller:
I think that after the final debugging of the vacuum system of the microscope, I will change the oil pump to this turbomolecular one.
I also got a jar of Apiezon high-vacuum non-silicone grease, which opened up the possibility for me to finally sort out the entire column and fix all the problems that I wrote about above.
Thanks Goron_Dekar ! I am very excited after our conversation. I’ll definitely come back to talk, and try to bring something useful.
I thank Victor from MIPT, a trip to Pushchino was a useful and fascinating journey. They have a slightly older model of a similar JEOL, working in microanalyzer mode.
ZavDimka - thanks for the interesting and useful things (although I haven't picked them up yet :)).
This person makes an excellent high voltage power supply. And much more. And since Since the microscope is already entering the stage of electronics, it is now very important.
jar_ohty - although I haven’t visited you yet, I’ll be happy to visit to meet you. Maybe we’ll use the secondary electron multiplier somehow :)
And many more people, whom I also thank very much. Still looking for detectors and cathodes for a microscope, but there is some hope.
In the next series - ignite the cathode :)