Laser – From the Cradle to ELI

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ELI Beamlines is a European research centre operating the most powerful laser system in the world. With its ultra-short laser pulses, lasting for only several femtoseconds, and the power of up to 10 PW, this research centre might provide new techniques and tools for basic research in areas such as medical imagining, diagnostics, radiotherapy or X-ray optics. On the occasion of the 60th anniversary of laser discovery, we spoke about the details of the inception of and the planned research at ELI Beamlines with Professor Jan Řídký, who conceived the project in cooperation with his colleagues.

CCF titulni strana 3/2020

This interview was released in the 3/2020 issue of the Czechoslovak Journal of Physics, published by the Institute of Physics of the Czech Academy of Sciences. This year the electronic subscription of the Journal has been made available on-line free of charge – have a look at the extract of this issue of the journal.

Prof. Jan Řídký was interviewed by Jana Žďárská.

Yes! It was Max Planck – a German genius, philosopher, Nobelist and an excellent physicist. The one who was standing directly at the cradle of the laser. When he extended the early concept of the laser in 1900 by assuming that light consists of tiny particles of energy – the so called quanta, he surely did not have any idea yet how big a discovery he laid the foundation to. But it was just the beginning of a long journey that led the mankind gradually to modern and powerful laser systems. There was still a lot of hard work to be done to put these ideas and visions into practice.

A unique part to this research was added by another famous physicist Albert Einstein, who postulated the very foundation of the laser – the so called stimulated radiation emission – as a phenomenon which is “symmetrical” to the absorption of the light quantum. For explaining another effect related to the existence of the light quanta – the photoelectric phenomenon – Albert Einstein received the Nobel Prize for Physics (1921).

After naming the energy quanta “photons” (in 1926, an American chemist Gilbert Newton Lewis), another period of intensive work and meticulous research followed. In the 1960’s, several scientists successively contributed to the creation of the laser. In 1964, the Nobel Prize for the invention of the laser and maser went to Charles Hard Townes, Ni­kolaj Genadievič Basov and Alexandr Michaj­lovič Prochorov. Interestingly, Arthur Leonard Schawlow, who was originally left out of the nomination, was awarded the Nobel Prize only 28 years later for a related research (spectroscopy), thus finally being granted the credit for his contribution to the design of the laser.

The hard research work paid off! On 16 May 1960, Theodore H. Maiman, an American physicist and engineer, built and launched the first operational (ruby) laser. There were many laboratories, not only in the US, working simultaneously to launch the laser but only one of them could be the first one, and this was Maiman’s! As early as a half a year later (1961), the laser was used to remove a tumour from the retina in a unique eye surgery performed at the Columbia-Presbyterian Hospital in Manhattan.

How about the situation in former Czechoslovakia? We can proudly say we did not lag behind the world too much as Czechoslovakia was the third country (after the US and the Soviet Union) to built its own operational laser. The first one was launched on 9 April 1963 at the Institute of Physics of the Czechoslovak Academy of Sciences. It was created by Karel Pátek, who used a neodymium glass as an active environment. Other scientists, Helena Jelínková from the Czech Technical University in Prague, and Jan Blabla from the Radioelectronic Institute of the Czechoslovak Academy of Sciences, subsequently built a ruby laser, which was presented to the public at the planetarium in Prague. A ruby laser was also launched by Dr. Pachman at the Research Institute of the Ministry of National Defence in Prague. The first semiconductor laser was put into operation by Tomislav Šimeček, also an excellent researcher, at the Institute of Solid Matter Physics of the Czechoslovak Academy of Sciences, and the first gas laser was launched by František Petrů and his team at the Institute of Instrumental Equipment of the Czechoslovak Academy of Sciences in Brno.

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ELI Beamlines is a European research centre operating the most intensive laser system in the world. Photo: Archive ELI Beamlines.

Regrettably, the unbiased realm of academic research was also impacted by the contemporary political situation, and since mid-1960’s the development of lasers in former Czechoslovakia was restrained at an intervention of the Soviet Union. No further research and development of lasers (specifically of iodine lasers) in Czechoslovakia could continue again until 1980’s when a powerful iodine laser was donated to the Institute of Physics of the Czechoslovak Academy of Sciences by the Lebeděv Institute in Moscow. In 1985, our scientists rebuilt the laser, modified its concept and put it into operation under a new name PERUN. Its more powerful version – PERUN II laser system – was subsequently launched in 1992. In 1997, the Institute of Physics of the Czech Academy of Sciences acquired an even more power ASTERIX IV terawatt laser from the Max Planck Institute of Plasma Physics in Garching near Munich, and a joint laser laboratory of the Institute of Physics and the Institute of Plasma Physics – PALS (Prague Asterix Laser System) was established in Prague.

However, the truly crucial milestone for Czech lasers was reached in 2011 as the support for an ambitious ELI Beamlines project was successfully put through. Together with ELI, a project of a smaller laser centre, HiLASE, also received support – and two rising stars of Czech laser research were born. And because this is a really big project – we would like to bring you – our readers – some details about the largest laser system in the world – ELI Beamlines – the home of which is the Czech Republic. The interview with Professor Jan Řídký was conducted by Jana Žďárská.

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The operation costs of ELI Beamlines for the full operation have been set to 25 mil. EUR per year. This is some 700 million CZK. Foto: ELI Beamlines Archives

Jana Žďárská: Dear Professor, the construction of ELI (Extreme Light Infrastructure) was just in progress when you led the Institute of Physics of the Czech Academy of Sciences as the Director. What did this project mean to you?

Jan Řídký: First of all, it was a great responsibility. The responsibility for the operation of the largest Institute of the Czech Academy of Sciences, which involved the period of a global economic recession after 2008, the responsibility for building ELI, as well as the responsibility for ensuring that a construction project of this size would not bury the Institute as such.

JŽ: Let’s return to the beginning – to your childhood. Could you tell our readers where you come from and what moments from that period you like to recollect the most?

JŘ: I was born in Lysá nad Labem, Czechoslovakia, in an appealing countryside of the Elbe region, with mildly undulating landscape, pine trees and blind shoulders of the river. When I was a child, the nature was in a much better shape. You could go swimming all summer, the water was cleaner. My family had a large garden where you could do all sorts of things throughout the year.

JŽ: What were your interests then? Were you already considering or dreaming of a profession?

JŘ: Like most boys, I suppose – first I wanted to become a fire-fighter, later an astronaut or a pilot.

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One of the four lasers that are being put into operation in Dolní Břežany does indeed belong to the top power class with a planned power of 10 PW. Photo: ELI Beamlines Archives

JŽ: You studied a general secondary school, formerly a grammar school in Nymburk. Did you, at that time, already yearn for a career in research?

JŘ: Not really. I just wanted to fully grasp what I was interested in. For this reason I decided to study nuclear physics. In my final year of the university, I started working at the department of nuclear medicine in a hospital and at the Institute of Physics of the former Czechoslovak Academy of Sciences. But the hospital was just my backup plan. The position at the Institute of Physics was intended for an experimenter but it was so impressive that I decided to take it up.

JŽ: Why was the Czech Republic selected as the location for ELI?

JŘ: We were prepared. In 1998 we managed to successfully relocate a large ASTERIX laser from Germany to Prague. Our physicist not only put it into operation but also significantly innovated the laser which was newly renamed to PALS (Prague Asterix Laser System), and used it for a number of interesting experiments. This made it clear to everybody that we had our own qualified pool of experts and a home-grown laser community. Ultimately, the state government was willing to provide funding for such a big project, and in 2009-2010 – as the location of the project was under consideration by the international laser community – we received support from several other countries that were very interested in hosting the international infrastructure but, for various reasons, their laser community at home was not able to attract sufficient funds.

JŽ: Do we really have the biggest laser device in the world?

JŘ: Lasers have gradually become very useful devices with a large scope of application as evidenced by the variability of their parameters that are tuneable to the specific purpose of each laser. This is a bit like if you asked me the question “Who is the biggest athlete in the world?” – to be able to answer it, I would have to ask you first which athletic discipline you have in mind. But I assume that by the “size” of the laser, the public typically means the laser’s peak power. And in this light, I can say yes, one of the four lasers that are being put into operation in Dolní Břežany does indeed belong to the top power class with a planned power of 10 PW. Other lasers of this class are momentarily being built or designed around the world but they are fewer than ten, and none of them has reached the power limit of 10 PW yet, and we, of course, hope to be the first to reach this milestone.

JŽ: Who funded this challenging project and how?

JŘ: The construction and technology development were funded from the 2011-2015 OP VaVpI (Research and Development for Innovation) Operational Programmes and the 2016-2018 OP VVV (Research, Development and Education). In the framework of the OP VaVpI, 85 % were then co-funded by the European Union and 15% by the Czech Republic. The Czech Academy of Sciences covered the so called ineligible costs. In the case of OP VVV, the EU again co-funded 85 % of the costs, the Czech Republic 10 % and the Czech Academy of Sciences 5 % covered the coinsurance and ineligible costs.

JŽ: Who can propose research projects for ELI and which countries have access to ELI’s lasers?

JŘ: ELI will soon become, as we hope, an organisation called ERIC. This abbreviation means European Research Infra­structure Consortium. The consortium is established according to the Union law and in practice this means that the countries associated in the organisation take part in the funding of and the decision making on the operation of an important research infrastructure. The condition of the establishment is the participation of at least three EU member countries but non-EU countries may also become its members. This is the perspective of the organizational structure. Then there is the scientific perspective, and from this perspective, any scientist around the globe may propose an experiment at ELI. Whether or not it will be implemented is purely based on the scientific quality of the proposal. The proposals are attracted in the so called campaigns – calls – this means that ELI announces a call to attract proposals for experiments in selected disciplines to the global science community. The received proposals are then evaluated by a group of internationally recognized experts. Last year, for example, we announced the so called “Zero Call” to tune the whole system. And within this call, we performed measurements proposed by a researcher from New Zealand.

JŽ: Could you tell our readers more about the operation costs of ELI’s laser systems.

JŘ: The operation costs of ELI Beamlines for the full operation have been set to 25 mil. EUR per year. This is some 700 million CZK. A half of the costs accounts for payroll costs, other costs are divided between energy bills, technology maintenance, consumables etc.

JŽ: How many people are currently involved in the laser’s everyday operation and how extensive and demanding an activity this is?

JŘ: The operation of each of the four big laser systems requires a team of about ten specialists of different professions. Other employees are involved in the development of laser technologies. In total, the centre employs 310 employees. This amount specifically includes groups of workers ensuring the operation of the lasers and internal experimental teams but also specialists taking care of the cleanroom environment in laser halls, air-conditioning, cooling, operation safety, IT support, network distribution and their diagnostics – there is indeed a lot to be taken care of. A very hard nut to crack is the coordination of this all. In this context I would like to mention Ing. Hvězda, who successfully managed to do this already at the construction phase of the project and is also doing great at the moment during the transition-to-smooth-operation phase.

JŽ: How many lasers have already been in operation at ELI and how many lasers are envisaged for full operation?

JŘ: All four lasers have been in operation. Each in a slightly different mode but they are all gradually being put into operation. The biggest progress has been achieved with the L1 and L3 lasers. The L1-ALLEGRA laser, which was fully developed and built at the Institute of Physics, already provides beams for a number of physical apparatuses.

The situation with L2 laser is a little bit strange. It was originally planned as a 1 PW power laser and the laser shot frequency of 10 Hz based on the technology of ceramic active part and a cryogenic cooling. The L3 laser was designed with similar parameters but it was based on neodymium glasses and intended for room temperature. This was for precaution because around 2000, when the whole ELI was conceived, there was no such a powerful laser in operation which used laser diodes, this means – a fully semiconductor technology for supplying energy to amplifiers, and we quite naturally relied on two alternative technologies. It turned out though during the construction phase of ELI that both technologies operate well but also that the price for the completion of the L2 system would be higher and the time needed to build it would be longer than with the L3. As a result, we decided to use the first stage of the L2, which was already complete and which is in operation, and to finish the whole laser on our own in order to significantly develop the ELI Beamlines potential. A laser renamed to L2-DUHA will use an OPCPA non-linear optical technology to amplify ultra-short pulses and will be used primarily to accelerate electrons due to its high repetition rate of up to 50 Hz.

The L3-HAPLS laser with a proposed 1 PW power and the laser shot frequency of 10 Hz, which was built in cooperation with Lawrence Livermore National Laboratory, is based on the neodymium glass technology. It is the only PW laser in the world to provide the laser shot frequency above 1 Hz. It is operating already as well and it is gradually approaching its nominal parameters in routine operation. A number of interesting experiments focusing mostly on proton acceleration have been conducted on it since the beginning of 2019.

The most powerful laser – the L4-ATON – is also operational in the basic mode. This means it provides pulses with energy of 1,5 kJ, with a frequency of about one pulse per minute. After completing the gigantic pulse compressor that represents one of the biggest devices of its kind in the world, measuring 18 m in length and 4 m in height, we will be able to reduce the pulse length to 150 fs and to reach the top power of 10 PW.

This is an immense amount of work, which was fully started in 2010, so we are actually celebrating our 10th anniversary this year. At this occasion I cannot forget to mention the two main “fathers” of the laser systems – Dr. Rus and Dr. Bakule from the Institute of Physics of the Czech Academy of Sciences.

Prof. Jan Řídký, DrSc. was born in Lysá nad Labem in 1951, graduated from theoretical nuclear physics at the Faculty of Mathematics and Physics, Charles University in Prague. Since completing his studies in 1975, he has worked at the Institute of Physics of the Czech Academy of Sciences (formerly ČSAV). His research career includes experimental physics of elementary particles and cosmic radiation. He defended his CSc dissertation thesis in 1983. Then he spent more than seven years abroad at the Joint Institute of Nuclear Researches in Dubna, Russia, and at CERN. Since 1997, he has been participating in the project of Pierre Auger international observatory. In 2006–2010 he was holding a post of the coordinator for fluorescence detector of the Auger observatory. In 2010 he defended the academic title DrSc.

From 1991 to 2005 he was the head of the Department of Experimental Physics of Elementary Particles; from 2005 to 2007, he was the head of the Optics Division, and in 2007-2017, he was the Director of the Institute of Physics. Moreover, he was also a member of the Research Council of the Institute of Physics of the Czech Academy of Sciences (1992–1998) and a member of the Subject Area Board of the Grant Agency of the Czech Republic (2000–2005). Between 2001 and 2007, he was a Delegate of the Czech Republic at the European Committee for Future Accelarators (RECFA). On behalf of the Institute – as its Director - he participated in a number of projects funded from Structural Funds (OP PK, OP VaVpI and OP VVV), out of which the ELI project stands out in particular as an enormously significant international project. Since 2017 he has been a member of the Academy Council and the Deputy Chairman of the Academy of Sciences of the Czech Republic for Sciences on Inanimate Nature.

As a lecturer, he delivers selected lectures at the Faculty of Natural Sciences of the Palacky University in Olomouc and at the Faculty of Mathematics and Physics of Charles University in Prague. At both universities, he is a member of the Subject Area Boards for doctoral programmes. He has been a trainer or a consultant for 11 defended doctoral theses. In 2008, he habilitated at the Faculty of Natural Sciences of Palacky University, in 2014 he was appointed as Professor. He has authored or co-authored more than 400 works in experimental physics, with about 14 000 citations.

He is married to Eva, they raised four sons. They live in Prague.

JŽ: Could you sum up what the lasers’ individual powers are?

JŘ: At the moment the L1 ALLEGRA laser is operated at 1 TW power and at a 1 kHz laser shot frequency. The L2 DUHA laser will generate up to 200 TW, with pulses at the 50 Hz frequency, L3 HAPLS 1,5 PW with the 10 Hz frequency, and the L4 ATON 10 PW with the shot frequency of one per minute.

JŽ: What is the lasers’ planned frequency of use, and are there any experiments going on at night as well?

JŘ: Not yet. Until the lasers are put into full routine operation, we don’t plan such an operation mode. This does not apply only to the use of the lasers: it also applies to all auxiliary systems that must be put into a full routine operation such as air-conditioning, various cooling circuits, power supply systems, vacuum systems, etc. Each of these service systems has been handed over by suppliers but now the services need to be put into full operation. And the operation of many of the systems is influenced by the operation of the other systems, there are feedbacks between them, and, of course, the functioning of the lasers depends on all of the systems as a whole – there is still a lot to learn.

JŽ: How is laser calibrated?

JŘ: High-power lasers are very expensive devices; their beams transfer high-power and any inaccuracy on the way – as the pulse travels through the device – might cause a major damage to some of the device’s constituent parts. For this reason, the so called beam diagnostics is crucial for their operation. In practice, this means a number of beam parameters are monitored at each critical nod of the laser. In particular, the length and the shape of the pulse in time, its overall intensity but also its homogeneity and the shape of the transversal distribution of radiation intensities in a pulse, its spectral composition, etc. These parameters are measured using a sophisticated optoelectronic set of elements, involving, for example, non-linear crystals, spectrometers and compact interferometers enabling the conversion into the structures detected subsequently by the CCD sensors, photodiodes, etc.

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In the medicine the imagining is being used for the research of proteins, viruses, cells or other organisms for biological research. Foto: ELI Beamlines Archives

JŽ: The ELI laser system is planned for research in a variety of areas from medicine to astrophysics. Is it clear yet what basic research in physics will be performed there?

JŘ: Lasers are used as tools to help in a number of scientific fields, whether in basic or applied research. They find use in a variety of forms of spectroscopy, surface investigation or imagining sample structures. These samples may then become part of the research of new materials, chemical reactions or they can be proteins, viruses or other organisms for biological research. Another big area of laser application in research is based on their ability to accelerate particles and to trigger nuclear reactions. Both aspects may serve either in physical or biological research. Finally, lasers are capable of creating dense plasma of different properties. Specifically for physics, they allow the study of the problems of atomic and molecular physics, physics of solids as well as to emulate the condition of matter existing on various space objects, for example on gigantic planets, stellar objects, etc. Additionally, power lasers can also be used to study some nuclear reactions and problems of thermonuclear fusion. The credit for coordinating different research directions goes to Dr. Korn from the Max Planck Institute in Munich, who joined us at the very beginning of the ELI Beamlines project.

JŽ: What type of biomedical research will be performed at ELI, and what might be its application in interdisciplinary use.

JŘ: As I have mentioned above, we perform imagining of proteins, viruses, cells or even higher organisms. Chemical processes and properties of different molecules in relation to biology can be investigated at ELI. Further, we are preparing the irradiation of biological preparations using protons or even heavier ions. All this may lead to application in pharmacology or hadrontherapy.

JŽ: No doubt scientists have high hopes they will be able to investigate the problems of laboratory astrophysics. Do you think they will be able to simulate the environment that is similar to that of the inside of a star, for example? 

JŘ: The great potential for astrophysics lies in the creation and research of plasma properties, which appears both in the atmosphere and in the core of stars. We will be able to measure the parameters of matter under conditions corresponding to the pressures acting inside gigantic planets; to study shock waves corresponding to a supernova explosion, etc. It goes without saying that this plasma has various temperatures and density. The possibility to create such conditions at a lab is immensely attractive.

JŽ: What practical outputs can we expect for medicine, imagining methods, radiotherapy and optics?

JŘ: I have already mentioned pharmacy and hadrontherapy. Advanced methods for imagining biological samples at a cellular level may be helpful for studying immune response or tumour growth. In hadrontherapy, lasers can be used to investigate the effect of even heavier particles than protons. This is scarcely possible in accelerators and the variability of the ions applied in accelerators is then rather small. Finally, a study is being rolled out of the potential to use lasers as accelerators in classic proton therapy.

JŽ: Could you explain what the “ELI White Book” is?

JŘ: To understand this, we need to go back in time a little bit. As early as in the 1990, a cooperation of big European laser infrastructures was launched to enable access to lasers by all European researchers in the field. This led to the creation of the first project of the Cooperation Network type within the Framework Programme 5 (FP5) which was entitled LASERNET and which was funded by the European Union from 2011 to 2004. This association continues to operate, nowadays under the new name Laserlab Europe, with its funding ensured at least until 2023. This association of European laser experts came up with an idea to concentrate human and financial resources and to build a laser device of the new generation. This idea was concisely formulated and submitted to the European Strategic Forum for Research Infrastructures (ESFRI) by Gérard Mourou1. The whole concept was allegedly summed up on a single page. Since 2007, this collective effort gained financial support by the EU and assumed the organizational structure of the ELI PP project, where PP stands for Preparatory Phase. The final outcome of the project is the ELI White Book you mentioned above. This book was authored by more than 170 authors primarily from Europe, including the Czech Republic, but also from Japan and the US. The book has over 500 pages and it provides an extensive summary of the status quo in laser physics as well as an outline of the ELI project, subdivided into ELI Beamlines, ELI ALPS and ELI NP. It describes their structure, focus and the parameters of the planned lasers. It is some sort of a road map. It was completed in 2011, and we proceeded according to it already at that time.

JŽ: ELI is expected to provide new research findings in ultra-intense fields and ultrarelativistic regime. What do these terms denote?

JŘ: A laser pulse is, in fact, a package of electromagnetic radiation localized in space and time. High-power lasers allow us to insert a field with a very high intensity into this package and to investigate its properties and effects. ELI allows us to create conditions in the laboratory like never before.  Expressed in figures, we shift from the relativistic regime where the intensity of the field reaches values 1018–1020 W/cm2, which are by several orders of magnitude closer to the ultrarelativistic region with intensities above 1024 W/cm2, where the field intensity should lead to the creation of e+e- pairs in a vacuum.

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After completing the gigantic pulse compressor that represents one of the biggest devices of its kind in the world, measuring 18 m in length and 4 m in height, the laser system L4 ATON will be able to reach the top power of 10 PW. Foto: ELI Beamlines Archives

JŽ: The ELI laser centre is expected to strengthen the position of Europe on a global scale in laser research. How could this be achieved?

JŘ: By building ELI, Europe will acquire an infrastructure that is not available anywhere else in the world. Some sort of CERN but for lasers. CERN, of course, has existed for almost seventy years, has a number of accomplishments and represents a huge concentration of human potential and know-how in the field of particle physics, nuclear physics, accelerator and detector technologies. Let’s keep our fingers crossed that ELI will get closer to this level. Anyway, international organisations have already taken note of ELI and, for example, the prestigious association of National Academies of the US in their report from 2017 – which is fully dedicated to lasers with intensive ultra-short pulses (with high power) points out ELI as an example to be followed by the US.

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By building ELI, Europe will acquire an infrastructure that is not available anywhere else in the world. Some sort of CERN but for lasers. Photo: ELI Beamlines Archives

JŽ: One of the goals of the research at ELI Beamlines is developing new methods of processing nuclear waste. Has this research started yet? And what direction should it take?

JŘ: As lasers are capable of accelerating different ions, they might be used to transmute spent nuclear waste to less dangerous elements with shorter half-lives. Such elements would be easier to store. At the moment, we are conducting initial studies and arranging the cooperation with the Nuclear Research Centre in Řež, which is a daughter organisation of ČEZ.

JŽ: How do PhD students and/or the new generation of scientists in general contribute to research at ELI?

JŘ: PhD students and postdoctoral researchers are immensely important to ELI. They account for 15 % of all researchers, and as the ELI project is, indeed, a long run, the role of the current generation will be central; it will be up to them to raise another generation of scientists.

JŽ: Is ELI open to visitors from the public? Can visitors pay a visit to some of the facilities or are there any plans to open an Information Centre such as the Centre at the Temelín Nuclear Plant?

JŘ: The time when visitors could freely access experimental and laser halls, where construction was underway is gone. Today all the premises are cleanroom environments and they can only be accessed by people in protective suits who observe strict safety measures. Laser halls may now be watched from a gallery for visitors when lasers are not operating. The visitors will not miss anything, though; the laser and experimental halls may be viewed anytime in virtual reality. Also, visitors may learn many interesting details about the research and its physical foundation in a number of panel discussions, and possibly also from the presentations by the guides. Last but not least, I can recommend a small but very interesting exposition created from an archaeological survey of the building plot of the centre.

JŽ: Shortly before we finish, I would like to ask you about a thing that worries me a little bit. What would happen if – purely hypothetically – a laser beam would escape from the lab at ELI? Where to and how far would it fly?

JŘ: This, indeed, is quite a hypothetical question – it depends on the stage at which it would happen. When a beam is aimed at the target, its intensity is immensely high, and it is focused onto a single point; then it starts to diverge and the intensity decreases. When the beam is navigated to an experimental device and does not diverge, it is out of focus in a transverse cross-section to a large plane to prevent the optical set from being destroyed by the beam’s intensity. But to play along: if we try to approach the “Star Wars” idea, then the beam would have to penetrate a 1,6 m thick concrete wall, and then it would continue to the surrounding ground because we perform laser experiments at the basement. The strongest one, the 10 PW laser, would very likely cause a smaller explosion as the moisture contained in the ground would turn into steam extremely quickly. But if a giant managed to tilt all ELI in such a way that the beam would point to the sky, then the beam would fly very far. In this context, I can mention that a laser was used to accurately measure the distance between the Earth and the Moon by letting the beam bounce off a mirror that was left on the Moon by American astronauts.

JŽ: You were there when ELI was born, and you keep collaborating on its operation. What would you, personally, wish ELI to achieve in the future?

JŘ: As many interesting discoveries as possible! Although research work may seem laborious and lengthy, science is an extremely exciting activity, and if your work is rewarded with an unexpected result, you couldn’t wish for more.

JŽ: Dear Professor, thank you for an interesting conversation, and, on behalf of the whole editor team, let me wish you many important discoveries that will certainly be made at ELI in the future.

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Prof. Jan Řídký: "Although research work may seem laborious and lengthy, science is an extremely exciting activity, and if your work is rewarded with an unexpected result, you couldn’t wish for more." Photo: ELI Beamlines Archives

1 Gérard Mourou and Donna Strickland won the Nobel Prize in 2016 for the discovery of the principle enabling the construction of the new generation lasers.