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WelcomeDepartment of Gas Lasers DGL was created in 1990 out of the former Department of Gas Discharges as a part of Section Optics in the Institute of Physics of Academy of Sciences of the Czech republic. The change of name was just a belated recognition of a long running programme of iodine laser research and application, which started in the early eighties by a delivery from the former Soviet Union of a large iodine laser system formally installed in the Lebedev Physical Institute in Moscow. The system had two power amplifiers pumped by an open discharge initiated on the amplifier axis by an exploding thin tungsten wire. Though the imported system had a promise of a fairly high energy output of several hundred joule in a sub nanosecond pulse there were numerous problems. These were connected with the necessity of opening the system and changing the central wire after each shot as well as environmentally harmful gas exhaust with the poisonous byproducts of the high current pumping discharge burning directly in the laser mixture (C3F7I + SF6). They finally led to a decision to sacrifice the high energy output and to built a smaller, but a more practical machine pumped by sealed Xe flash lamps. The new system was using most of the vital components which came with the imported device, however, its concept was not without principle hitches. To ensure a sufficient longevity the self developed sealed Xe flash lamps had to be filled with a fairly high Xe pressure, which in turn limited from above the admissible power density released in the Xe plasma. Consequently, a fairly long pulse of about 300 ms of a comparatively modest voltage 5 kV produced a pumping light pulse with an efficiency of about 5 % in the useful UV interval. Such a long pumping pulse meant that the converging sound waves released from the inner wall of the laser tubes had enough time to engulf the whole volume of the lasing gas and to spoil its optical homogeneity. As a counter measure a large portion of He (in addition to SF6 preventing a pyrolysis of C3F7I) was added as a buffer gas to the laser mixture, which not only broadened the fluorescent line of the laser transition and suppressed self oscillations, but also minimized the refraction index fluctuations incurred by the propagating sound waves. Besides the obvious advantage of working with the sealed flash lamps and comparatively low voltage this slow pumping was sufficiently soft so as not release a lethal shock wave from the laser tube inner surface, which would destroy the inversion, so that the whole cross-section of the laser tube was available for lasing. The homogeneity of the pumping process was aided by placing the flash lamps comparatively far from the quartz laser tube and still securing a sufficient pumping intensity by carefully shaped reflectors. The design proved to be successful, the laser chain later named PERUN, which was finally launched in 1986 gave in a routine operation a stable energy output of 40 J in 500 ps pulses, and its 10 cm beam could be focused in a focal spot of 100 mm with a power density on the target of better than 1014 Wcm-2, enough for meaningful laser plasma experiments. The shots could be repeated every 20 minutes. A vacuum interaction chamber was procured from IPPLM Warsaw and the system was immediately harnessed for laser plasma generation and its application as a point like and instantaneous soft x-ray source, or if left to expand as a source of highly charged ions. Especially fruitful the ions research proved to be in connection with the plans of the CERN particle physics laboratory in Geneva to built the LHC ring, which will be designed for an eventual heavy ion acceleration. A laser ion source turned out to be an alternative for the ion injection requiring high values of ion current to fill the ring. The PERUN experiments performed with besides the Hadron Injection Group of the PS Division of CERN also in a close cooperation with IPPML Warsaw and ITEP Moscow proved that the ion produced by a sub-nanosecond NIR pulse produced the ions not only in sufficient amounts, but also with a much higher charge number than the routinely used CO2 laser driver was capable of giving. The data accumulated by the PERUN experiments paved the way for a laser ion source of next generation, which is nowadays waiting for a nanosecond NIR laser driver of about PERUN caliber with a sufficiently high reprate and endurance available at a reasonable price. Beginning of nineties the PERUN team got in touch with a hitherto closed laboratory in the former Soviet Union VNIIEF Arzamas 16, later called the Russian Federal Nuclear Centre, which was running the largest ever iodine laser installations Iskra 4 and Iskra 5. The specialists of that institute provided new impetus and also hardware for a further PERUN upgrade. A frequency conversion was applied to the PERUN beam using DKDP crystals grown by the method of accelerated growth in the Institute of Applied Physics in Nizhnyi Novgorod. The conversion line added very much to the flexibility of the PERUN machine (since then called PERUN II), because it had a possibility of generating pulse and pre-pulse of different colour, amplitude ratio and time delay. The new option offered itself immediately for model experiments mimicking the direct drive in ICF. If a heating blue 3wpulse is not illuminating a target directly, but it is absorbed in thinner plasma layer prepared by a week preceding red 2wpre-pulse, the absorption region of the main pulse is considerably extended and the lateral electron heat conductivity may smooth all the inhomogeneities of the main pulse illumination and provide a homogeneous ablation pressure profile needed for a successful application of the direct drive. This notion of ablation pressure profile smoothing was indeed verified by the subsequent PERUN experiments. However, the double pulse experiments and also the later experiments with a linear focus directed at the x-ray laser research exposed the fundamental weakness of the otherwise versatile PERUN system, its limitations in the available pulse energy. When trying to maintain a sufficiently high power density in each of the focused converted pulses or even in the linear focus the system had to be pushed to its very limits. The solution came in the form of shear luck. For several years in succession the DGL organized for the restricted iodine laser community regular workshops which served as a platform of exchange of experience, which the more advanced groups were providing for the newcomers. The regular visitors of these meetings were physicist of the Laser Plasma Group of MPQ Garching near Munich in Germany, who were running a large and highly developed ASTERIX IV iodine photodissociation laser system, an object of admiration and envy of other groups. The ASTERIX was in energy more than by an order of magnitude superior to PERUN, had a near diffraction limited beam divergence thanks to a short pumping pulse of open Xe flash lamps and several spatial filters inserted in the laser chain, elaborate re-circulation system of the laser gas and several other advantages. One of them was also an ideal age for a large experimental device, since its last amplifier was launched in 1991 and by 1995 all the bugs seemed to be eliminated. It was at one of the workshops in 1995, which took place at the Třešť castle in the Czech Republic, where the fairly stunned Czech participants learned from their German colleagues that the ASTERIX laser would become available in not too a distant future to any group who would be competent enough and also willing to run it. A decision to bid for the device was made on the spot, later confirmed by a letter from the director of Institute of Physics Dr Dvořák to prof. Hänsch, at the time the executive director of MPQ. A frantic search for a suitable laser hall which could accommodate ASTERIX followed on the Czech side. The procedure was neither straightforward on the German side, since according to the rules first the potential operators had to be addressed in Germany, then in EU and only then the device could have been offered outside the EU. Thanks to the kind support and a lot of patience of our German colleagues of the Laser Plasma Group of MPQ a contract about the laser transfer was signed end of June in 1997 endorsed by Euratom shortly after the last shot in Garching in the Spring 1997. The first full energy shot of the newly installed system took place in the new PALS experimental hall in Prague in the Spring 2000. The story of forming a new joint reserch centre PALS (Prague Asterix Laser System), building a new experimental hall and of the laser transfer is better described on the PALS pages http://www.pals.cas.cz.
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