The electrical capacitance of the globe, as is known from the course of physics, is approximately 700 microfarads. An ordinary capacitor of such a capacity can be compared in weight and volume with a brick. But there are capacitors with the electrical capacity of the globe, equal in size to a grain of sand - supercapacitors.
Such devices appeared relatively recently, about twenty years ago. They are called differently: ionistors, ionixes or simply supercapacitors.
Do not think that they are available only to some high-flying aerospace firms. Today you can buy in the store a coin-sized ionistor with a capacity of one farad, which is 1500 times the capacity of the globe and close to the capacity of the largest planet in the solar system - Jupiter.
Any capacitor stores energy. To understand how large or small the energy stored in the ionistor is, it is important to compare it with something. Here is a somewhat unusual, but visual way.
The energy of an ordinary capacitor is enough for it to jump about a meter and a half. A tiny ionistor of type 58-9V, having a mass of 0.5 g, charged with a voltage of 1 V, could jump to a height of 293 m!
Sometimes it is thought that ionistors can replace any battery. Journalists depicted the world of the future with silent electric vehicles powered by supercapacitors. But so far this is far from it. An ionistor weighing one kg is capable of accumulating 3000 J of energy, and the worst lead battery - 86,400 J - 28 times more. However, when delivering high power in a short time, the battery quickly deteriorates, and it is only half discharged. The ionistor, on the other hand, repeatedly and without any harm to itself gives off any power, if only the connecting wires could withstand them. In addition, the ionistor can be charged in seconds, and the battery usually takes hours to do this.
This determines the scope of the ionistor. It is good as a power source for devices that consume high power for a short time, but quite often: electronic equipment, flashlights, car starters, electric jackhammers. The ionistor can also have military applications as a power source for electromagnetic weapons. And in combination with a small power plant, the ionistor allows you to create cars with electric wheels and fuel consumption of 1-2 liters per 100 km.
Ionistors for a wide variety of capacities and operating voltages are on sale, but they are expensive. So if you have time and interest, you can try to make an ionistor yourself. But before giving specific advice, a little theory.
From electrochemistry it is known: when a metal is immersed in water, a so-called double electric layer is formed on its surface, consisting of opposite electric charges - ions and electrons. Between them there are forces of mutual attraction, but the charges cannot approach each other. This is hindered by the attractive forces of water and metal molecules. At its core, the electrical double layer is nothing more than a capacitor. The charges concentrated on its surface act as plates. The distance between them is very small. And, as you know, the capacitance of a capacitor increases with a decrease in the distance between its plates. Therefore, for example, the capacitance of an ordinary steel spoke immersed in water reaches several mF.
In essence, an ionistor consists of two electrodes with a very large area immersed in the electrolyte, on the surface of which, under the action of an applied voltage, a double electric layer is formed. True, using ordinary flat plates, it would be possible to obtain a capacitance of only a few tens of mF. To obtain large capacitances inherent in ionistors, they use electrodes made of porous materials having a large pore surface with small external dimensions.
For this role, spongy metals from titanium to platinum were tried at one time. However, incomparably the best was ... ordinary activated carbon. This is charcoal, which after special treatment becomes porous. The surface area of the pores of 1 cm3 of such coal reaches thousands of square meters, and the capacitance of the electrical double layer on them is ten farads!
Self-made ionistor Figure 1 shows the design of the ionistor. It consists of two metal plates tightly pressed against the "stuffing" of activated carbon. Coal is stacked in two layers, between which a thin separating layer of a substance that does not conduct electrons is laid. All this is impregnated with electrolyte.
When the ionistor is charged in one half of it, a double electric layer is formed on the pores of the coal with electrons on the surface, in the other half - with positive ions. After charging, ions and electrons begin to flow towards each other. When they meet, neutral metal atoms are formed, and the accumulated charge decreases and may eventually disappear altogether.
To prevent this, a separating layer is introduced between the layers of activated carbon. It can be made up of various thin plastic films, paper, and even cotton.
In amateur ionistors, the electrolyte is a 25% sodium chloride solution or a 27% KOH solution. (At lower concentrations, a layer of negative ions will not form on the positive electrode.)
Copper plates with wires pre-soldered to them are used as electrodes. Their working surfaces should be cleaned of oxides. In this case, it is advisable to use a coarse-grained skin that leaves scratches. These scratches will improve the adhesion of the coal to the copper. For good adhesion, the plates must be degreased. The degreasing of the plates is carried out in two stages. First, they are washed with soap, and then rubbed with toothpowder and washed off with a stream of water. After that, you should not touch them with your fingers.
Activated charcoal, bought at a pharmacy, is ground in a mortar and mixed with electrolyte until a thick paste is obtained, which is smeared with carefully defatted plates.
During the first test, the plates with a paper gasket are placed one on top of the other, after which we will try to charge it. But there is a subtlety here. At a voltage of more than 1 V, the release of gases H2, O2 begins. They destroy carbon electrodes and do not allow our device to work in the ionistor capacitor mode.
Therefore, we must charge it from a source with a voltage of no higher than 1 V. (This is the voltage for each pair of plates that is recommended for the operation of industrial ionistors.)
Details for the curious
At a voltage of more than 1.2 V, the ionistor turns into a gas battery. This is an interesting device, also consisting of activated carbon and two electrodes. But structurally, it is made differently (see Fig. 2). Usually, two carbon rods are taken from an old galvanic cell and gauze bags of activated carbon are tied around them. KOH solution is used as electrolyte. (Salt solution should not be used, as chlorine is released when it decomposes.)
The energy intensity of the gas accumulator reaches 36,000 J/kg, or 10 Wh/kg. This is 10 times more than that of an ionistor, but 2.5 times less than that of a conventional lead battery. However, a gas accumulator is not just a battery, but a very peculiar fuel cell. When it is charged, gases are released on the electrodes - oxygen and hydrogen. They "settle" on the surface of activated carbon. When a load current appears, they are connected to form water and electric current. This process, however, without a catalyst is very slow. And, as it turned out, only platinum can be a catalyst ... Therefore, unlike an ionistor, a gas accumulator cannot give high currents.
However, the Moscow inventor A.G. Presnyakov (http://chemfiles.narod.r u/hit/gas_akk.htm) successfully used a gas accumulator to start a truck engine. Its solid weight - almost three times more than usual - in this case turned out to be tolerable. But the low cost and the absence of such harmful materials as acid and lead seemed extremely attractive.
A gas accumulator of the simplest design turned out to be prone to complete self-discharge in 4-6 hours. This put an end to the experiments. Who needs a car that can't be started after a night of parking?
And yet, “big technology” has not forgotten about gas batteries. Powerful, light and reliable, they are on some satellites. The process in them takes place under a pressure of about 100 atm, and spongy nickel is used as a gas absorber, which under such conditions works as a catalyst. The entire device is housed in an ultra-light carbon fiber balloon. The result was batteries with an energy capacity of almost 4 times higher than that of lead batteries. An electric car could travel about 600 km on them. But, unfortunately, while they are very expensive.
If you are planning to build a laser, an accelerating tube, an electromagnetic interference generator, or something else of that kind, then sooner or later you will be faced with the need to use a low-inductance high-voltage capacitor capable of developing the Gigawatts of power you need.
In principle, you can try to get by using a purchased capacitor and something close to what you need is even commercially available. These are ceramic capacitors of the KVI-3, K15-4 type, a number of brands from Murata and TDK, and of course the beast Maxwell 37661 (the latter, however, is of an oil type)
The use of purchased capacitors, however, has its drawbacks.
- They are expensive.
- They are inaccessible (the Internet, of course, has connected people, but carrying parts from the other side of the globe is somewhat annoying)
- Well, and most importantly, of course: they still will not provide the record parameters you require. (When it comes to a discharge in tens and even a few nanoseconds to power a nitrogen laser or obtain a beam of runaway electrons from a non-evacuated accelerating tube, not a single Maxwell can help you)
According to this guide, we will learn how to make a homemade low-inductance high-voltage
capacitor on the example of a board intended for use as a driver
lamp dye laser. However, the principle is general and with its
using you will be able to build capacitors in particular (but not limited to)
even to power nitrogen lasers.
I. RESOURCES
II. ASSEMBLY
When designing a device that requires a low inductance power supply, one should think about the design as a whole, and not separately about capacitors, separately about (for example) a laser head, etc. Otherwise, current-carrying bars will negate all the advantages of a low-inductance capacitor design. Typically, capacitors are an integral part of such devices, and that is why the dye laser driver board will serve as an example.
Blessed is that do-it-yourselfer around whom sheets of fiberglass and plexiglass are lying around. I have to use store-bought kitchen cutting boards.
Take a piece of plastic and cut it to the size of the future circuit. 
The idea of the scheme is primitive. These are two capacitors, storage and sharpening, connected through a spark gap according to a circuit with resonant charging. We will not deal with the operation of the circuit in detail here, our task here is to focus on assembling capacitors. 
Having decided on the dimensions of future capacitors, cut pieces of an aluminum corner according to the dimensions of future contactors. Carefully process the corners in accordance with all the rules of high-voltage technology (round off all corners and blunt all points). 
Fix the leads of future capacitors on the resulting "printed circuit board". 
Mount those parts of the circuit that, if not assembled now, may later interfere with the assembly of the capacitors. In our case, these are connecting buses and a spark gap. 
note that the low inductance when installing the arrester is sacrificed for ease of adjustment. In this case, this is justified, since the intrinsic inductance of the (long and thin) lamp is noticeably greater than the inductance of the arrester circuit, and besides, the lamp, according to all the laws of a black body, will not shine faster than sigma * T ^ 4, no matter how fast the power circuit is. You can shorten only the front, but not the entire impulse. On the other hand, when designing, for example, a nitrogen laser, you will no longer mount a spark gap so freely.
The next step is to cut the foil and possibly the laminate packages (unless the size of the capacitor calls for a full package format, as is the case for the storage capacitor on the board in question.) 
Although the lamination is ideally airtight and edge flashing must be avoided, it is not recommended to make beads (dimension d in the figure) less than 5 mm for every 10 kV of operating voltage.
Edges of 15 mm in size for every 10 kV of voltage provide more or less stable operation even without sealing.
The size of the pins (size D in the figure) should be chosen equal to the expected thickness of the foot of the future capacitor with some margin. The corners of the foil, of course, should be rounded.
Let's start with the peak capacitor. This is how the blanks and the finished, laminated lining look like: 
For the peak capacitor, a 200 µm thick laminate was taken, since a voltage surge of 30 kV is expected here due to "resonant" charging. Laminate the required number of covers (in our case, 20 pcs.). Fold them in a pile (pins alternately in different directions). At the resulting stack, bend the leads (if necessary, cut off the excess foil), place the stack in the nest formed by the angle contactors on the board and press the top cover. 
Fetishists will fix the top cover with neat bolts, but you can simply tape it with tape. The peak capacitor is ready. 
The assembly of a storage capacitor is no fundamentally different.
Less scissor work as full A4 size is used. The laminate here is 100 µm thick because the plan is to use a charging voltage of 12 kV.
In the same way, we collect in a pile, bend the conclusions and press the lid: 
A kitchen board with a cut handle looks, of course, malicious, but does not violate functionality. I hope that you will have fewer problems with resources. And one more thing: if you decide to use pieces of wood as a base and cover, they will have to be seriously prepared. The first is to dry thoroughly (preferably at elevated temperature). And the second - hermetically lacquered. Urethane or vinyl varnish.
The point here is not electrical strength and not leaks. The fact is that when the humidity changes, the pieces of wood will bend. Firstly, this will disrupt the quality of the contact and lengthen the discharge time of the capacitors. Secondly, if, as here, a laser is supposed to be mounted on top of this board, it will also be bent with all the ensuing consequences.
When bending the leads, do not forget to lay an additional layer of insulation. And then in fact: the plates are separated from each other by two layers of dielectric, and the leads from the plates of opposite polarity - by only one.
Let's see what we got. Let's use a multimeter with a built-in capacitance meter.
Here is what the storage capacitor shows. 
And here is what the peak capacitor shows. 
That's all. Capacitors are ready, the topic of the guide is over.
However, I'm probably looking forward to trying them out. We complete the missing part of the circuit, install the lamp, connect it to the power source.
Here's what it looks like. 
Here is an oscillogram of the current, taken with a small ring of wire directly connected to the oscilloscope and located near the circuit that feeds the lamp. True, instead of a lamp, the circuit was loaded on a shunt. 
And here is an oscillogram of a lamp flash, taken with an FD-255 photodiode aimed at the nearest wall. Scattered light is enough. It's even more correct to say "more than." 
You can scold badly turned out capacitors for a long time and look for the reason why the discharge lasts more than 5 μs ... In fact, the flash lamp dumps a bunch of megawatts and even the light scattered from the walls drives the photodiode into deep saturation. Let's take the photodiode away. Here is an oscillogram taken from 5 meters, when the photodiode does not look exactly at the light bulb, but slightly away from it. 
The rise time is difficult to determine precisely due to interference, but it can be seen that it is on the order of 100 ns and is in good agreement with the duration of the current half-cycle.
The remaining tail in the light pulse is the glow of a slowly cooling plasma. The total duration is under 1 µs.
Will this be enough for a laser on a karasitel? This is a separate issue. In general, such an impulse is usually more than enough, but it all depends on the dye (how pure and good it is), on the cuvette, illuminator, resonator, etc. If I manage to get generation on one of the commercially available fluorescent markers, then there will be a separate guide on a homemade dye laser.
(PS) I had to add another 30 nF to the main storage capacitor and it really was enough. The pipe, the photo of which can be found right there in the "Photos" section, worked even better than from the two-maxwell GIN.
In general, a discharge time of 100 ns is by no means the limit for the described technology for creating capacitors. Here is a photo of a capacitor with which an air pumping nitrogen laser works stably in the superradiance mode: 
Its discharge time is already beyond the capabilities of my oscilloscope, however, the fact that the nitrogen tank with this capacitor effectively generates already at 100 mm Hg. allows the discharge time to be estimated at 20 ns or less.
III. INSTEAD OF CONCLUSION. SECURITY
To say that such a capacitor is dangerous is to say nothing. An electric shock from such a container is as deadly as a KAMAZ flying at you at a speed of 160 km/h. Treat this capacitor with the same respect as a weapon or explosives. When working with such capacitors, use all possible safety measures and, in particular, remote switching on and off.
It is simply impossible to predict all dangerous situations and give recommendations on how not to get into them. Be careful and think with your head. Do you know when a sapper's career ends? When he stops being afraid. It is at the very moment when he becomes "on you" with explosives that he blows his head off.
On the other hand, millions of people drive on the roads with KAMAZ vehicles and thousands of sappers go to work and stay alive. As long as you are careful and think with your head, everything will be all right.
Tank capacitor
This type of capacitor got its name from the similarity of the shape of the plates with the T-shirt package.
The inductance of this capacitor is greater than that of the conder described above or the candy one, but it is quite suitable for use in a CO2 or GIN. With difficulty it starts the dye and is not suitable for nitrogen.
The materials you will need are the same as in the guide above: mylar film (or lamination bags), aluminum foil and adhesive tape / electrical tape.
The diagram below shows the dimensions of the main gaps.
L - dielectric length
D - dielectric width
R is the outer radius of the capacitor
The gaps from the edges of the dielectric are 15mm. On the side where the contact strips of the plates come out, there is an indent of 50 mm. These offsets are made as small as possible for the maximum capacitance for a given L and D of the dielectric. Please note that these clearances are selected for 10kV. (I doubt it makes sense to make this type of capacitor for higher voltages, so I won't write formulas here to recalculate offsets and gaps for other voltages)
The distance between the leads of the plates is 30mm. This gap is also taken as the minimum possible for 10 kV. Increasing this gap will make the leads too narrow - increasing the inductance of the capacitor.
Manufacturing
The tank condenser is ready. You can install it with your laser, GIN or other high-voltage device.
Fans of different high-voltage experiments often face the problem when it is necessary to use high-voltage capacitors. As a rule, such capacitors are very difficult to find, and if you succeed, you will have to pay a lot of money for them, which not everyone can afford. In addition, the policy of our site simply will not allow you to spend money on buying something that you can make yourself without leaving your home.
As you may have guessed, we decided to devote this material to the assembly of a high-voltage capacitor, which is also devoted to the author's video, which we invite you to watch before starting work.
What do we need:
- knife;
- what we will use as a dielectric;
- food foil;
- a device for measuring capacitance.
We note right away that the author of a home-made capacitor uses the most common self-adhesive wallpaper as a dielectric. As for the capacitance measuring device, its use is not necessary, since this device is intended only so that at the end you can find out what happened in the end. Everything is clear with the materials, you can start assembling a homemade capacitor.
First of all, we cut off two pieces from self-adhesive wallpaper. You need about half a meter, but it is desirable that one strip is slightly longer than the other.
The resulting sheet of foil mode is exactly two parts in length.
The next thing we put on a flat surface is one piece of wallpaper, on which we carefully place one piece of food foil. The foil must be laid so that a gap of about a centimeter is obtained along three edges. On the fourth side, the foil will stick out, which is quite normal at this stage.
Put the second sheet of wallpaper on top.
We put a second sheet of foil on it. Only this time we make it so that the foil protrudes from the side opposite to the previous step. That is, if the author has the first piece protruding from below, then this time it should protrude from above. Separately, it should be noted that the sheets of foil should not touch each other.
Now we remove the substrate from one edge and glue our capacitor.