History

The Department of Gas Lasers, DGL, was established in 1990 out of the former Department of Gas Discharges, as a part of the Section of 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, which had been then pursued for more about a decade.

Our activities in this field started in the early eighties by a delivery of a large iodine laser system, formally installed at the Lebedev Physical Institute in Moscow. The system had two power amplifiers pumped by a discharge initiated by an exploding tungsten wire placed on the amplifier axis. Though this system had a promise of an output of several hundred joules in a sub-nanosecond pulse, there were numerous problems. These were connected with the necessity to open the system and replace the central wire after each shot, as well as environmentally harmful gas exhaust with the poisonous by-products of the pumping discharge burning directly in the laser mixture (C₃F₇I + SF₆). These facts finally led to a decision to sacrifice a multi-100 J output and to build a smaller, more practical device pumped by sealed Xe flashlamps. The new system was using a number of the components of the imported Russian device, however, its design had to address several new issues. To ensure a sufficient lifetime, the in-house developed Xe flashlamps had to be filled with a high pressure, which in turn limited the admissible power density released in the Xe discharge. Consequently, a fairly long pulse of about 300 µs of a modest voltage 5 kV produced a pumping light pulse with an efficiency of about 0.5 % in the useful UV spectrum. 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 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 sealed flashlamps and comparatively low voltage, the slow pumping was sufficiently “soft” so as not release a shock wave from the laser tube inner surface, which would destroy the inversion near the tube walls. The homogeneity of the pumping was aided by placing the flashlamps relatively far from the quartz laser tube and still securing a sufficient pumping intensity by carefully shaped reflectors. The design being successful, the laser chain named PERUN, which was launched in 1986, provided in a routine operation an energy output of 40 J in 500 ps pulses. Its 10-cm-diam beam could be focused to a focal spot of 100 µm, with a power density on the target of 10¹⁴ Wcm^-2. The shots could be repeated every 20 minutes. The system was immediately employed for laser plasma generation to produce a point soft X-ray flash source, or if left to expand, to generate highly charged ions. Studies of the latter topic were especially fruitful in collaboration with the CERN laboratory. In its planned Large Hadron Collider (LHC) ring, designed for acceleration of heavy ions, a laser ion source turned out to be an alternative when requiring high values of ion current injected to the ring. The PERUN experiments demonstrated that a plasma produced by a sub-nanosecond near-IR pulse yields highly-stripped ions not only in sufficient amounts, but also with a much higher charge number than achievable by the routinely used CO₂ laser. 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 near-IR laser driver of about PERUN calibre with a sufficiently high repn rate and endurance available at a reasonable cost.

In the beginning of nineties, the PERUN team got in touch with a hitherto classified laboratory in the former Soviet Union VNIIEF Arzamas 16, later called the Russian Federal Nuclear Centre, which was operating the world’s largest iodine laser systems Iskra 4 and Iskra 5. The Russian researchers were willing to provide us, for a rather moderate cost, frequency conversion crystals and other beam hardware, thus making it possible to upgrade the PERUN system with the financial resources available to us at that time. A frequency conversion was applied to the output beam using DKDP crystals grown by the method of accelerated growth developed at the Institute of Applied Physics in Nizhnyi Novgorod. The conversion line added very much to the flexibility of PERUN (since then called PERUN II), because it had a possibility of generating pulses and pre-pulses of different colours, amplitude ratio and time delay. The upgraded system was immediately used for model experiments mimicking the ICF direct drive, and for experiments addressing basic plasma physics of prepulse-pumped soft X-ray lasers.

However, the double pulse experiments and also the later experiments with a linear focus directed at the X-ray lasers exposed the fundamental weakness of the otherwise versatile PERUN system, that is its limited 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.

For several years DGL organised for the iodine laser community international workshops. The regular visitors of these meetings were physicists of the Laser Plasma Group of MPQ Garching near Munich in Germany, who were running an advanced, kilojoule iodine photodissociation laser system ASTERIX IV, an object of admiration of other groups. The ASTERIX IV 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 flashlamps, 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 completed in 1991 and by 1995 all the bugs seemed to be eliminated. It was at one of the workshops in September 1995, which took place at the Trest castle in the Czech Republic, where the stunned international community learned from their German colleagues that ASTERIX IV will have been available to any group who would be capable and willing to run it. A decision of our group to bid for the device was made on the spot, later confirmed by a letter from the former director of Institute of Physics Dr. Dvorak to Prof. Hänsch, at the time the MPQ executive director. A search for a suitable hall adaptable to host ASTERIX IV followed, but since no appropriate solution was found, a project to build a new, dedicated laboratory was made. This project was subsequently approved by the scientific councils of the Institute of Physics and Institute of Plasma Physics, and by the Academic Council of the Academy of Sciences. Finally, it was supported by the Czech government in the autumn 1997, which allocated the necessary financial resources for the new laboratory into the budget for 1998. The transaction took also several steps 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 outside the EU. Thanks to the support and patience of our German colleagues of the Laser Plasma Group of MPQ a Euratom-endorsed contract (see its text in English) about the laser transfer was ratified in June 1997, shortly after the last shot in Garching was fired.

The construction of new building, located in the academic campus in north Prague, began in January 1998, and was completed in spring 1999. Re-assembly of the laser, the components of which were in the meantime transported from Garching to Prague, followed. The works involved building an entirely new interaction facilities, including a tandem of automated vacuum interaction chambers of an advanced conception. These chambers were designed in collaboration with the Université Paris-Sud. Their components and electronic control systems were build by several new small- and medium-scale Czech enterprises involved in high-tech production, along with the in-house workshops of the Institute of Physics.

The first full energy shot on the re-installed laser system took place in the new experimental hall in late spring 2000, shortly before the PALS laboratory was ceremonially inaugurated on June 8, 2000. The story of forming the research centre PALS (Prague Asterix Laser System), building a new experimental hall and of the laser transfer and its re-installation is described in more detail on the PALS pages (http://www.pals.cas.cz).