The electron gun is actually very close to a curved line, so I'm at the top draw a curved line. 0:15 on this curved line, there is current. [Click sound is noise] These green stripe represents the electron, [click sound is noise] When a current is passed through this line, some electrons escape from the tip, like that ran out. Figuratively, this process is a bit like water down the river so. In the process of water down the river, each over a period of time, there is always some water molecules, the water molecules collide with the adjacent, enough energy can be obtained, so that the surface of the liquid out of it, and then evaporated to the atmosphere went. Analogously, in this electronic curved line (gun) flowing, 1:00 over a period of time, the total number of electrons per electron collisions with the adjacent. This is because the line current flowing therein, the temperature is high. Sometimes, part of the electrons gain enough energy in a collision, it is possible to escape from this surface line. One important property of metal, is called the work function. (Annotation: one is to make the electrons escape immediately from the solid surface, the minimum energy that must be provided.) If the energy of the electrons obtained by a granulation exceeds this value, it will fly from the surface of the metal, which in turn down into the vacuum environment of an electron microscope. When flying, with their different electron energy, flying angle is also different. 1:37 in the electron microscope, the potential of this line (gun) itself is located very low, for example, the California Institute of Technology that sets we used relatively large microscope, it is the potential negative electron gun 300,000 volts. In the bottom of the microscope, a surface is grounded in fact, most of the structure is in fact a microscope on the ground, 2:05 so we continue to write, here is 0 volts. Thus, between the tip and the bottom plane, there is a great electric field causes electrons in the microscope downward acceleration, which is the basic process. Accelerating electrons to travel down. Well, let us wipe them, the next draw other elements. Electron beams discharged from the electron gun, the focus needs to be 2: 38 There is an important structure called Wehnelt cylinder. It's like a hole in the bottom center of the cup. The cup where the potential is lower than the potential where a heating filament. So here I wrote: minus 300 kV minus delta electron gun itself is even lower than many volts. For this reason, the electric field lines around here, near the bottom of the hole of the cup, [click sound is noise] like this. [Click sound is noise] they began to electronic ejected from the electron gun in focus, focus the electrons on a central point. Therefore, Wehnelt cylinder is a first focusing element. Next, electron beam will encounter other elements, not one step to reach the ground from negative 300 kV electron microscope, some stacked accelerating elements (Annotation: accelerating tube is often referred to in Chinese) this acceleration portion, is a series of perforated plates, 3:53 here I would draw ten. So these, seen from above like a washer, and this cross-section from the perspective of painting, we draw into many intermediate broken line, nine, and then draw a tenth. Now, we can wipe this part. Between each two discs in the accelerating tube, is connected by a resistor, which are connected integrally to 4:23. This disc below 4:27 is the most grounded, just like the rest of the microscope. 4:38 So, this dish is grounded, 4:42 where the potential is 0 volts. Since the resistance between the disk and the disk has, the potential progressively decreasing. For example, the disk may be negative 30,000 volts, and this disc, negative 60,000 volts, this disc, negative 90 volts, minus 120 volts, minus 150, 180, 210, 240 may be the uppermost disc minus 270,000 volts. Since the potential is changed stepwise, so the electric field lines in this region is flat, almost entirely flat, and a vertical beam. [Click sound is noise] Of course, these field lines continue to be distributed among the disks, I'm not going to draw them all out. Therefore, when the electrons enter the zone, under the action of the accelerating tube will accelerate, until they come out, they have been they have been 300,000 volts, now, which have 300,000 electron volts, the rate reached 76% the speed of light . These electrons are moving close to the speed of light. 2 picometer wavelength thereof is 200 points one Angstrom. Obviously, the wavelength of the radiation of electron microscopes used, the resolution will not be a limiting factor. Wavelength short enough so that you can see 6:22 less than a single atomic details. 6:27 These are the basic design of the electron gun. Also some things I would like to mention. First, the whole system is in a high vacuum. This is because, when electronic material encountered strong scattering occurs. So if within the system have any gas, many electronic will be due to those gases and scattered, and we hope that they do not scatter from the electron gun to our sample, and then through the lens, do not scatter, so this internal system needs a high vacuum state under. Nevertheless, there is always the number of water vapor, and other gases into the barrel of an electron microscope. Sometimes, these pollutants will leave things like stalactites on these accelerating elements. And each a stalactite will thicken over time, resulting in a potential difference between them increasing, large enough to discharge. That will destroy the shape of the electric field, but also can damage electronic components on the instrument. 7:45 So, in order to get clean electron gun, the electron gun occasionally need to be adjusted. The so-called adjustment, in fact, means that the voltage of the entire system is increased, up to or even higher than the normal operating voltage. For example, if we want to run our microscope at 300,000 volts, for example, when we adjust the 315,000 volts. That way, all will discharge may be let go. Then we put down voltage, down to a stable operating voltage. Finally, I would like to mention that on the electron gun, in order to maintain the potential difference of 300,000 volts and above, there is a groove, to generate the high voltage, the high voltage tank, isolated from the outside and one gas, i.e. sulfur hexafluoride gas, it is a good insulating material, the high-voltage tank helpful. However, this is a bit dangerous, because sulfur hexafluoride gas is an odorless and heavier than oxygen. 9:07 So, at any one electron microscope chamber, you will probably see a can of 9: 13 liquefied sulfur hexafluoride gas. If the high voltage tank or liquefied gas tank that has a little bit of sulfur hexafluoride will leak into the room, and you can not see it, smell it too, and because it is heavier than oxygen, it will be deposited to the floor, if Unfortunately, someone accidentally hit his head, fell to the floor, or sleep on the floor, or for any other reason their head in the area full of sulfur hexafluoride, they may suffocate . So a lot of room microscope, you will see that there are vents on the floor, there may be equipped with sensors, oxygen sensors, when hypoxia goes out. To give you an idea about these things look like, here's a photo, which we used at Caltech Polara300 kV electron microscope. This is the time to replace the filament photographs. This is the case where the filament. This is part of the accelerating tube. You see there are a bunch of rings, a total of ten, this is the accelerating tube. This red light above, since the staff to heat it, so that they close it before all gases are removed, and the near vacuum. 10:42 This is what I mentioned before, the bottom of the microscope is grounded. Very interesting, it is really well connected to the ground. Stable power is important for microscopes, so when we prepared the microscope at Caltech, is like this ground. In fact, the hole depth of 12 inches (Annotation: about 3.7 m), a massive brass plug to the inside, which are attached to the grounding cable thereof, about a foot (Annotation: about 2.54 cm) thick, it and be firmly connected. These are the introduction of the electron gun. Now let's talk about the concept of coherence. 11:20 We need to consider two coherence. The first is the spatial coherence. 11:29 in the electron microscope, spatial coherence means: if all the electronics from the same direction? 11: 38 It is clear that, in the electron microscope, if some electrons are shot straight down, through the sample until the bottom of the camera, and the other electronics come from the other direction, passing through the sample of cases are not the same, it will reach the camera this area. Therefore, different electronic imaging samples at different locations, here in this electronic imaging, the electronic imaging possible in that position, all of which are superimposed imaging, you get blurred images, which is relatively poor spatial coherence conditions. Therefore, it is important for electron gun is that all electronic needs from the same direction completely tip. Another 12:23 Next coherence, temporal coherence is called. 12:28 in the electron microscope, it means: if all the velocity of the electron is exactly the same? This is important, because if there is a 300,000-volt electron, which has a wavelength, and because that particular wavelength, it will be the focus of the microscope lens to be focused to a specific location, this location is where we put the camera . However, imagine if there is a next electronic 301,000 volts, it will move faster, so that the degree of focusing lenses will be weak, and is focused onto a lower plane. So, if we use the imaging speed of electrons is not uniform, we have to put the camera into a certain position, the camera will be receiving from various electronic imaging, and these electrons are focused to varying degrees, and some focus on the top of it some focus on its bottom, so that the resulting image is blurred. So it is important in terms of the electron gun is that all electrons emitted from the tip comes with the same energy, which is called the temporal coherence. 13:45 Now, this leads us to understand the most common electron microscopy three kinds of filament. The first is the tungsten filament, in fact, a bent tungsten wire, tungsten filament inside it is very cheap, it does not replace the time-consuming, it is very common in many microscope. But it is not a good coherence. Another filament in an electron microscope you will find that lanthanum hexaboride filament, which is a lanthanum hexaboride crystal, where the electron gun itself is a small piece of crystal lanthanum hexaboride. Since the tip of the crystal is very sharp, so that electrons from the inside, almost exactly the same position, and when they escape from the lattice, but also having substantially the same energy. 14: 48 Finally, these are field emission electron gun. Field emission electron gun has a very sharp, polished tip. It is called field emission electron gun, because there is a tip underneath 15:06 electric field, the electrons in the tip pulled out, so they (electronic) will be pulled out. And because of this, coherence becomes higher when they come out, we can think of this volcano, volcanic eruption, lava spewing out a Tuotuo, spewing in all directions, the speed also varies. Some fly very far, some relatively recent fly, which is poor coherence, temporal coherence only difference (difference in injection velocity), poor spatial coherence (injection in all directions). This and similar tungsten filament. The field emission electron gun is more like children play SLIDE, SLIDE Imagine a top, there are a lot of balls, each ball is placed at the edge of SLIDE, if you slap the ball, it begins to slide along slide down the roll, and finally reach the bottom of sLIDE. You filmed every ball will roll down, ultimately to the same direction, and the speed is almost the same. This is after it rolled down SLIDE kinetic energy, potential energy from the conversion made. It's like field emission electron gun: sophisticated electronics, electronic escape less, and the electric field to pull them out. They appeared, it begins to accelerate in the accelerating tube, and out from the same position, the same direction, the same speed. So the field emission electron gun is coherence best. There are two types: a thermal field emission electron gun is a cold field emission electron gun. 17:00 The difference is that: in a cold field emission electron gun, the electron 17:06 completely pulled out by the electric field; 17:11 and the thermal field emission electron gun, a large current through the tip, this will make the filament current warming, make it easier for electronics have enough energy to escape and be pulled out. 17:27 However, the higher cost of field emission electron gun, and when they run out of life, it takes a few days to replace. Because this operation requires a filament to heat up, so over time they will wear out. The filament is important to tune in optimum temperature: 17: The hotter the filament 46, the more the ejected electrons, the more you get bright beam; however, at the same time, the metal filament will actually wear and tear, cutting-edge quality, cutting-edge sharpness will gradually wear and tear, and finally we have to be replaced new filament. So when you learn to use an electron microscope, it should filament heating enough, until you can form a sufficiently bright light, but do not continue to lighten up, otherwise it will shorten the life of the tip.