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by Vera PARAFONOVA, journalist
Superstrong field physics today is in the focus of attention of some of the most representative scientific conferences. One of them-"Laser physics. Interaction of laser emission and matter"-took place in the town of Sarov (Nizhni Novgorod region) at the Russian Federal Nuclear Center "The All-Russian Research Institute of Experimental Physics (ARRIEP)".
Sarov, former Arzamas-16, was selected as the venue of the international forum. It is there that scientists created not just nuclear and thermonuclear weapons but also Europe's largest laser installations. The technology is making rapid headway. Thus, Dr. Sc. (Phys. & Math.), Director of the Non-linear Dynamics and Optics Section of the RAS Institute of Applied Physics (IAP) Alexander Sergeyev devoted a major part of his presentation to the joint IAP-ARRIEP project of building in Sarov one of the world's most powerful installations for generation of multi-petawatt * emission.
The invention of lasers by the founders of quantum electronics, then future academicians Nikolai Basov and Alexander Prokhorov and the American scientist Charles H. Townson (1964 Nobel Prize) has brought about a significant breakthrough by way of reducing the emission pulse length. It has proven feasible to produce subnanosecond pulses-less than nsec (10 -9 sec), down to a picosecond (10 -12 sec) and even a femtosecond (10 -15 sec). Now we can even talk in terms of attoseconds (10 -18 sec). First femtosecond lasers, generating emission with the pulse length of less than 100 fsec, were designed in the late 1980s. And now researchers have mastered the range of 5 fsec.
The attainment of such values has been facilitated by the advent of new laser crystals. Besides, the mechanism of self-locking of modes (electromagnetic eigentones) within lasers has been discovered, by virtue of which the broadband emission with varying wavelengths generated by a crystal can be transformed into one flashing in super- short pulses. Finally, in mid-1990s optic mirrors were developed facilitating a controlled delay of reflected light waves depending on their frequency. The exploration of those three trends has brought us to the limit of pulse lengths attainable by laser generators.
So, what about the practical applications of supershort pulses? Today the dimensions of transistors, the backbone of modern computer technology, are of the order of tens of nanometers. Exploring processes going on in these semiconductor devices, constantly decreasing in
* Petawatt (peta=10 15 )-the power level following terawatt (tera=10 12 ). -Ed.
size, requires pulses of femtosecond lengths. Typical "events" significant for modern electronics last less than a picosecond: such miniscule fraction of time is required for an electron to cross a distance of a few tens of nanometers.
Telecommunications offer another example. To transmit maximum volume of information within a unit of time, the emission should be as broadband as possible. The transmission range up to 10 Gbit/sec is practically attained and not just in optic systems. But if we are to attain a rate over one terabit per second, then the fsec range is unavoidable.
Another sphere where pulse length sets the tone is the generation of superstrong electromagnetic fields. To obtain those, we must either build up energy or reduce time, or narrow the focal point of the emission. Energy buildup will require more and more condensers. Incidentally, the most unique structure in Eurasia, the ARRIEP Iskra-5 laser complex, occupies a whole building.
A more effective solution of the problem is to reduce pulse length, effectively increasing the intensity of the emission. Even now in some laser systems it reaches 10 20 - 10 21 W/cm 2 . Exposed to such radiation, matter will pass into a new state-plasma with amazing properties we still know very little of. It becomes a source of an X-ray emission with an enormous quantum energy, up to a y- range, and emits high-energy ions (up to tens of MeV). Such effects may be used in various areas-from elementary particle physics to medicine.
And those are far from all applications of femtosecond optics. Its capacities, both realizable practically and potential, rank this section of physics among those making the most rapid progress.
There is still another sphere of laser pulse applications which has no direct connection with either quick processes or strong fields. Back in 1960s scientists learned to locate submarine objects at the depths of a few dozens of meters with nanosecond laser emission. As for femtosecond emission, it allows to probe the heterogeneity of biological tissue with the resolution of the order of 10 mm. It is at such depth that many pathologies develop, including oncological diseases.
IAS has developed femtosecond optic tomographs for non-invasive optical biopsy. As distinct from conventionally accepted methods, this one allows to avoid cutting tissue fragments-its structure in the area between epithelium and subiculum is made clearly seen. Five years back Nizhni Novgorod physics and physicians used these instruments to produce world's first tomographic albums of practically any internal organ making a weighty contribution to a new scientific branch-optic tomography.
Harnessing thermonuclear synthesis means to learn to control the deuterium/tritium synthesis reaction in order to generate power. Heated up very quickly, e.g., with a laser pulse, their mixture will linger to disperse, having some inertia. This is the essence of the method of inertial plasma trapping.
Installations for laser thermonuclear synthesis (like Iskra-5) are provided with a plasmic chamber in the center of which a deuterium/tritium target is placed. Focused on it is the emission of several powerful laser beams with the pulse length as small as 10 -10 sec and the total power of the
order of 10 14 W. Expanding gases and light pressure should compress the deuterium/tritium mixture approximately 50 thousand times and heat it up to 10 keV (about 120 mln o C). The sheathing holding the mixture will evaporate, the pressure in the latter will reach 1 million atmospheres and the density 50 - 100 g/cm (such conditions persist only as long as the laser pulse is active). It is at this point that thermonuclear reaction may be triggered off producing neutrons and a large quantity of energy-17.6 MeV during each act of synthesis.
Therefore, in the thermonuclear area the main task now is to "ignite" the target. A "quick" scheme is proposed for doing that: one powerful laser pulse is compressing the target, while another, a very short femtosecond pulse, applied from aside, ignites the thermonuclear fuel. That can be achieved only with the help of femtosecond lasers.
Research carried out in the USA, Japan and Russia has led to the creation of mathematical models allowing to design thermonuclear targets and identify the laser emission characteristics (energy, wave length, pulse shape, etc.) required for ignition in laboratory conditions. The latter requires higher energies than those developed by presently existing world installations. Hence, new generations lasers are required. In USA and France they are already under construction and aim at the output energy of about 2 MJ. In Japan the KONGON installation is in development.
ARRIEP also has plans for the development of the present and creation of new powerful laser systems. Institute specialists have designed a next generation installation on glass on the basis of neodymium- Iskra-6. It will consist of 16 modules, of 8 laser channels each. Incidentally, the performance of neodymium lasers is higher than that of iodine ones used in Iskra-5 and the cost is less.
Now the first module of Iskra-6- Luch-is under construction. One laser channel is ready out of four with the total emission energy of 12 - 16 kJ. The key objective ofLuch is to verify the selection of Iskra-6 design, to test its key elements and, primarily, to create on its basis a unique femtosecond source of peta-watt power level to experimentally study the characteristic features of the "quick ignition" scheme. This is what Dr. Sergeyev
says: "There are two factors that are important for us: the presence of a powerful kJ- laser in the ARRIEP Luch system and that we at RAS IAP grow wide-aperture non- linear crystals which are the backbone of the multi-petawatt parametric amplifier project. Its functioning is based on the phenomenon when a quantum of light going through a non-linear optic crystal splits in two. To get the optimum effect, we have synthesized unique water-soluble crys-tals-KDP and DKDP.
There are just two or three places in the world where something like this is manufactured. But crystals of such wide aperture, up to 40 cm in diameter, and of such high quality are hardly to be found anywhere else. Also, our technology allows to grow crystals at a rate of 10 mm a day which is very good.
So, at RAS IAP we are planning to build two parametric amplification cascades-up to the level of approximately 100 TW-and to test all operational concepts. Next we will install the system at ARRIEP and use one of the Luch channels for pumping. What we will get will be a petawatt machine.
And I would like to conclude by saying that in our research we are not trailing in anybody's wake but are exploring a new vein of our own. And the fact that US, UK and Japanese specialists are looking in the same direction just confirms our expectations.
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