After all, physics is the science of paradoxes. Its practical application quite often brings us to this very idea. Just imagine: a kilometer-sized object is being built in order to look at the arrangement of atoms in a molecule and the molecule itself in space at high magnification.
Suppose we want to capture some fast event happening with small particles. No, not even that - very fast, happening to very small ones. Perhaps a couple of molecules will work for us. Chemists, as a rule, know what was there as the starting substances and what was the result of the reaction, they do not know exactly how this result came about. Take hemoglobin as an example. In the textbooks, the pure truth is written: its molecule is designed in such a way that it captures molecular oxygen, and then gives it away for its intended purpose - for the respiration of cells. Instead of oxygen, it can mistakenly capture something unsuitable for breathing, like carbon monoxide, and then not give it away, turning into ballast useless for the body.
It is known that a rather large molecule, capturing its "target", vibrates or, if you like, makes movements. But what, in what sequence and how they are related to the final result - it is not known exactly, no one has yet been able to consider them in sufficient detail and without haste. The situation is similar with many other artifacts of the microworld - there are a lot of situations when you want to take a closer look at them in modern science. An ordinary optical microscope, unfortunately, has a fundamental limitation: everything that we look at in it must be much larger than the wavelength of the light wave, otherwise the picture will not work. Because of this, by the way, objects of molecular size and smaller do not even have color in our usual sense.
The way out is to switch to another range of electromagnetic radiation - to look at the studied object not in light, but in X-ray radiation. Its wavelengths are many orders of magnitude shorter, respectively, although a similar limitation works here, much smaller objects can be seen in its rays.
How it's done
Let's go back to the macrocosm. On the outskirts of Hamburg, the construction of a European free electron laser has come to an end. Its abbreviated name, XFEL, stands for European x-ray free electron laser, that is, the European X-ray free electron laser. On the surface of the earth, the facility consists of two sets of buildings spaced three kilometers apart. All the most interesting, as usual, is located below, underground. The accelerator route consists of a main tunnel with a length of about two kilometers and a diameter of 5.3 meters and a system of ventilation tunnels with a diameter of 4.6 meters.
For construction in 2009, a non-profit consortium European XFEL GmbH was created, the main shareholder of which was originally the German agency DESY. Financial costs are allocated. It is planned to spend 1.22 billion euros on construction and commissioning. 58% of this amount is provided by Germany, 27% - by Russia, the shares of the rest of the participants are in the range of 1-3%. In total, 11 countries are participating in the project.
Most of the tunnels are laid horizontally at depths of 6 to 38 meters, depending on the terrain profile and the location of the particular tunnel. The total length of the tunnels is 5777 meters. Two tunneling shields were involved in the creation of this complex system. An electron beam is formed by an injector.It is a miniature accelerator, only about a hundred meters in size, located underground at the same depth as the main complex. Escaping from there, the electron beam enters the main tunnel, where superconducting accelerators bring its speed to near-light speed. This in itself is fascinating, but this is where the work of the system only begins.
The accelerated electron flow enters the undulator. This device with the name of French origin (fr. Onduler - to worry, to oscillate) is a sequence of rather powerful electromagnets, the field of which deflects the passing particles in one direction or the other, forming something like a sinusoid. The meaning of what is happening is that when turning in the opposite direction, the electron moves with acceleration, which means it loses part of its energy, emitting a gamma quantum. Electrons moving nearby interact with the radiation of neighbors and "connect" to it.
What happens in physics is spontaneous amplification of spontaneous emission. It is very important that with a certain selection of the parameters of the undulator, this radiation is coherent, that is, the electrons emit phase-locked photons, as a laser should do. That is why the installation is called that. The length of the resulting wave generally depends on the angle of emission of gamma quanta with respect to the direction of motion of the electrons, on the period of the undulator, and on the beam velocity.
The first parameter in the laser is zero, the other two we can select at our discretion. Of course, they are calculated so that we get X-rays with the given parameters. This is where the life of the electron beam - short but bright - ends. He is taken aside and "drowned" in a special trap. And the laser beam flies further. There is no special technique for control and focusing - it is not needed for a coherent beam, and it is impossible due to the high power. The resulting X-ray beam is so "bright" that it will have time to melt through any lens. Of course, high-precision technology, which is responsible for the creation of particles and manipulation with them at insane speeds, works without the direct participation of a person who can only set the parameters of work.
Ancestors and relatives
The XFEL concept is nothing entirely new. It is easy to be convinced of this by a cursory glance at the special literature, in which the X-ray radiation generated in the system is called synchrotron radiation. The synchrotron is the fruit of the development of charged particle accelerators, which "matured" in the last quarter of the 20th century. It is a ring accelerator in which beams of electrons move in a circle, periodically passing through guide magnets and an accelerating electric field.
The energy they accumulate is partially spent on undulators, like the one described above. The design suffers from fundamental limitations imposed by the need to maintain circular motion. On the one hand, electrons moving in a circle emit uselessly all the time, and the losses for this radiation are colossal. On the other hand, when generating a working beam, too much energy cannot be removed from them, otherwise they will not get to the next acceleration in time.
In sum, this leads to the fact that, although the achieved energies are very high (electrons can be accelerated almost to the speed of light), a very small part of them is spent on useful purposes, and today it is no longer possible to increase it to any appreciable extent. The results are low brightness and an incoherent beam. This, of course, does not mean that synchrotrons are useless. This means that this technical idea has exhausted itself. The appearance of "direct" lasers was only a matter of time.
Today their advantages over synchrotrons are as follows: 1. Significantly brighter radiation - about a billion times. 2.Generation of very short flashes - up to several femtoseconds, which opens up the possibility of making a "movie" from the life of molecules participating in very short scenes. 3. Coherent radiation, which gives researchers additional opportunities. Today there are only four scientific free electron lasers in the world - in the USA, Japan, Germany and Italy. Three more are under construction.
The European laser will be the most powerful among them, but above all the fastest. EXFEL gives 27 thousand flashes per second, while the Japanese installation SACLA generates 60, and the American LCLS - 120. This value should not be confused with the duration of the flash itself, for EXFEL we are talking about femtoseconds, that is, quadrillionth of a second.
The fate of the artists
The colossal brilliance of new technology, in addition to new opportunities, entails some problems. Practically any experimental sample in those unimaginably small fractions of a second that it is exposed to irradiation will have time to be left without "its" electrons. They will be carried away by a stream of oncoming photons, just as a gust of wind carries away the cap of a gaping passer-by. The difference is that a passer-by, finding himself without a headdress, rushes after and with a probability close to unity will catch him, and under the X-ray laser beam there will be a body in which there will be only positively charged particles.
They are heavier and therefore will stay in their places a little longer. In short, our sample will not live up to the next take - it will be torn apart by electrostatic repulsion. This process takes tens of femtoseconds and does not get into our frame - the flash is over. But, alas, it is inevitable. Continuing filming will require a new cast of participants. When they say that a new technique can work with one molecule - this is true, but not all. One molecule is enough for us for portrait photography, but for cinema we need a new cast of actors for each shot. By the way, the "portrait" captured by the detectors is not at all like the sample being shot. In the frame, you can see some kind of blot with outgrowths and burrs. To turn it into a picture corresponding to what it actually was, special processing procedures are needed, among which spectral analysis is not the most difficult thing.
Why is this needed?
We come to what is probably the most important question. Why is this all? Expensive installation, qualified (that is, also expensive) specialists, years of work? According to the experience of using synchrotrons, the most active users are invariably representatives of modern biology and medicine. X-ray radiation is actively involved in the study of the spatial structure of biomolecules, the structure and functioning of cells at the level of individual organelles. In 1980, the spatial structure of several dozen biomolecules was deciphered; by 2017, their number exceeded 93 thousand.
There are several obstacles that cannot be avoided with existing technology. Samples exposed to X-rays are damaged - we wrote about this above. It is assumed that EXFEL, due to short flashes, will circumvent this problem: damage to the sample will occur after it has been captured. Another difficulty is associated with the fact that in order to operate at the synchrotron, the protein must be properly prepared and exposed under appropriate conditions. The first, translated into normal language, means that the protein must grow in the form of a crystal.
It's almost always difficult, sometimes very difficult. It turns out that if the work itself takes minutes, then preparation for it sometimes requires several years of work. For EXFEL, all this is insignificant - very small portions of the substance are enough and they can be exposed under conditions close to natural: at room temperature, etc. Let us separately mention the processes occurring with protein molecules when they are folded into the structures required by the nature and during movement.
Both begs for "filming" - and now we have the right technique for it.Another interesting layer of problems is associated with photosynthesis, more precisely, with our attempts to reproduce it with human technologies. In nature, the splitting of water molecules and then hydrogen atoms into protons and electrons occurs in complex biomolecular complexes using only cheap, widespread metals: manganese and calcium. Nobody has yet seen them in action, so the mechanism of the process is still not clear to us. A separate, very interesting class of problems is in the field of catalytic chemistry.
More or less everyone who studied chemistry at school knows what the catalyst does. But how he does it - until no one knows, it has not yet been possible to look at him at the moment of the action. This leads to the fact that new catalysts are still selected at random, with a huge expenditure of labor and time. Both can be saved. This, of course, does not exhaust the list of tasks for a new laser. New times open up new perspectives for us.