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When light interacts with a mirror which is moving at a speed close to the speed of light towards it, its wavelength is shifted into the extreme ultraviolet region of the spectrum. This effect was first predicted by Albert Einstein. His theory was experimentally confirmed almost 100 years later, following the development of high-intensity laser light sources. Laser physicists at the Laboratory for Attosecond Physics (LAP) at Max Planck Institute for Quantum Optics in Garching (MPQ) and Munich’s Ludwig Maximilian University (LMU) have now characterized the phenomenon in detail under controlled conditions, and exploited it to generate high-intensity attosecond light flashes. Moreover, they show that these pulses can be shaped with unprecedented precision for use in attosecond research.

Pulses of light lasting for a few hundred attoeseconds (1 as equals a billionth of a billionth of a second, 10-18 sec) have become an indispensable tool for the investigation of ultrafast physical processes. For one thing, they afford insights into the motions of electrons in atoms and molecules. As a rule, these ultrashort pulses are created by allowing coherent laser light to interact with a sample of a noble gas, such as xenon. However, this method has one serious drawback – the resulting pulses suffer from low energies. An alternative approach to the generation of attosecond pulses makes use of relativistically oscillating mirrors. In this case, the light interacts not with a gas, but with a solid surface made of fused silica.

A small portion of the incident light serves to ionize the surface of the glass, thus creating a plasma – a dense cloud made up of free electrons and virtually immobile, positively charged atomic ions. This state of affairs can be compared to that found in normal metals, in which a fraction of the electrons can move freely through the material. In fact, this dense surface plasma behaves like a metal-coated mirror. The oscillating electric field associated with the light that impinges on this mirror causes the surface of the plasma to oscillate at peak velocities close to that of light itself. Off this oscillating surface, the light gets reflected. As a consequence of the Doppler effect, the frequency of the incoming light is shifted into the extreme ultraviolet (XUV) region of the spectrum – and the higher the peak velocities, the greater the frequency shift. Because the durations of mirror oscillations at maximum speed are extremely short, XUV light pulses lasting for a matter of attoseconds can be spectrally filtered out. Crucially, these flashes have a far greater intensity than those that can generated by the conventional interaction in the gaseous phase, being able to reach photon energies on the order of kiloelectron volts (keV), according to simulations.

In collaboration with scientists from the ELI (Extreme Light Infrastructure) in Szeged in Hungary, the Foundation for Research & Technology – Hellas (FORTH) in Heraklion (Greece) and Umeå University in Sweden, the physicists at the LAP have been able to gain new and valuable insights into the interaction of pulsed laser light with relativistically oscillating solid surfaces. They first analysed the intensity profile and energy distribution of the resulting attosecond pulses, and their dependence on the ‘carrier envelope phase’ of the driving input laser pulse in real time. “These observations permit us to define the conditions required for optimal generation of attosecond light pulses using the oscillating plasma mirror,” says Olga Jahn, the first author of the study. “We were able to demonstrate that isolated attosecond XUV light flashes can indeed be produced from optical pulses consisting of three oscillation cycles.”

The findings of the LAP team enable to simplify and standardize the procedure required to generate attosecond pulses by means of plasma mirrors. The comparatively high intensities achieved open up new opportunities for ultraviolet spectroscopy, and promise to unveil new aspects of molecular and atomic behavior.

Original publication:
Olga Jahn, Vyacheslav E. Leshchenko, Paraskevas Tzallas, Alexander Kessel, Mathias Krüger, Andreas Münzer, Sergei A. Trushin, George D. Tsakiris, Subhendu Kahaly, Dmitri Kormin, Laszlo Veisz, Vladimir Pervak, Ferenc Krausz, Zsuzsanna Major und Stefan Karsch
Towards intense isolated attosecond pulses from relativistic surface high harmonics
Optica, Vol. 6, Nummer3, März 2019
https://doi.org/10.1364/OPTICA.6.000280

Image: Attosecond flashes of light occur on glass surfaces through the process of being ionized by a strong laser, which then forms a dense mixture of free-moving electrons and nearly quiescent atomic hulls.  Every fragment on the glass surface marks the impact of a laser pulse. Photo: Thorsten Naeser 

A new publication from a team of laser physicists led by Prof. Stefan Karsch (Ludwig-Maximilian-Universität München and Max Planck Institute for Quantum Optics) brings research on plasma wakefield particle acceleration within the reach of university laboratories.

The plasma wakefield acceleration (PWFA) technique is regarded as a highly promising route to the next generation of particle accelerators. In this approach, a pulse of high-energy electrons is injected into a preformed plasma, and creates a wake upon which other electrons can effectively surf. In this way, their energy can surpass that of the driver by a factor of 2-5. However, many technical and physical problems must be resolved before the technology becomes practical. Their solution is slowed down by the fact that currently only large-scale particle accelerators, such as those at DESY, CERN or SLAC, are currently capable of producing the driver pulses needed to generate the wakefield. A team led by Prof. Stefan Karsch at the Laboratory of Attosecond Physics (LAP) – a joint venture between LMU Munich and the Max Planck Institute for Quantum Optics (MPQ) – has now shown that PWFA can be implemented in university labs. The new findings will facilitate further investigation of the PWFA concept as a basis for the development of compact, next-generation particle accelerators.

For scientists interested in exploring the innermost workings of nature, a powerful particle accelerator is a very useful tool to have. A prime example of the truth of this maxim is the discovery of the Higgs boson. This breakthrough in particle physics was only possible thanks to the construction at CERN in Geneva of the Large Hadron Collider (LHC), the world’s most powerful particle accelerator. In order to progress beyond the present Standard Model of Particle Physics, even more powerful particle accelerators will be required. But this presents a severe challenge for conventional technology. For example, plans for the Future Circular Collider – a possible successor to the LHC – envisage the construction of an accelerator ring with a diameter of 100 km.

Therefore, novel approaches to particle acceleration are being sought with the aim of drastically reduce the physical size – and hence the costs – of future colliders. One of the most promising of these is plasma wakefield acceleration (PWFA), which employs plasma waves rather than radio waves to bring subatomic particles up to speed. The basic idea is that electrons are accelerated by surfing on such plasma waves. This concept can be realized by first generating a very short but densely populated pulse of high-energy electrons (the driver pulse). This driving pulse is then injected into a plasma (a cloud of charged particles produced by the ionization of a molecular gas). The negatively charged electrons in the plasma are repelled by the electrons in the driver pulse. This repulsion has the same effect as the displacement of water by the bow of a cruising speedboat: it creates a bow wave which gives rise to a trailing wake as the boat progresses. Other electrons in the plasma can surf on this wake (also termed wakefield) and reach energies in excess of those of the electrons in the driver pulse. Up until now, the driver pulses necessary to generate the wakefields could only be generated in conventional accelerators. As a result, this technique has so far been investigated in large-scale facilities such as SLAC or CERN.

Professor Stefan Karsch’s international team at the LMU now reports not one, but two breakthroughs in the implementation of PWFA. The researchers developed a miniature laser-driven version of PWFA in LMU’s Laboratory for Extreme Photonics. Their experimental set-up can be compared to a (much) smaller-scale version of the ripple tanks used to model the dynamics of water waves and ocean currents. Karsch and his colleagues used the high-power ATLAS laser to accelerate the electron bunches that served as driver pulses. These pulses enabled them to create a wakefield and generate plasma waves over a distance of a few millimetres. Not only that, this purely optical approach allowed them to carry out novel diagnostic tests, which provided new insights into the behaviour of the plasma. The Munich group was able to observe the long-term dynamics of the wakefield and for the first time study the ion motion in the plasma, whose dynamics is commonly omitted in typical simulations of the process. These observations may now be directly utilized to optimize the performance of PWFA experiments at large-scale facilities.

The new results thus establish a new basis for detailed elucidation of the PWFA mechanism in laser labs and universities around the globe. Karsch and his team hope that their findings will contribute to a breakthrough in the development of the next generation of high-energy particle accelerators, which give renewed impetus to the search for physics beyond the Standard Model.

Original publication:

M. F. Gilljohann, H. Ding, A. Döpp, J. Götzfried, S. Schindler, G. Schilling, S. Corde, A. Debus, T. Heinemann, B. Hidding, S. M. Hooker, A. Irman, O. Kononenko, T. Kurz, A. Martinez de la Ossa, U. Schramm, and S. Karsch
Direct Observation of Plasma Waves and Dynamics Induced by Laser-Accelerated Electron Beams Phys. Rev. X 9, 011046 – Published 12 March 2019, DOI:https://doi.org/10.1103/PhysRevX.9.011046

Image:

Laser physicists led by Prof. Stefan Karsch at the Centre for Advanced Laser Applications at LMU have brought the research on plasma wakefield acceleration technique a step closer to implementation in university research labs. Photo: Thorsten Naeser

Researchers at the Laboratory for Attosecond Physics in Garching have built the first-ever laser-driven particle accelerator that can generate pairs of electron beams with different energies.

Particle accelerator-based radiation sources are an indispensable tool in modern physics and medicine. Some of the larger specimens, such as the LHC in Geneva or the European XFEL in Hamburg, are among the most complex (and costly) scientific instruments ever constructed. Now, laser physicists at the Laboratory for Attosecond Physics (LAP), which is run jointly by Munich‘s Ludwig-Maximilian University (LMU) and the Max Planck Institute for Quantum Optics (MPQ), have developed a laser-driven particle accelerator that is not only capable of producing paired electron beams with different energies, but is also much more compact and economical than conventional designs.

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At the open house day researchers and research institutes on the campus in Garching show with what they deal with and how it looks like in their laboratories. Thus, anyone who always wanted to know how a „super-laser“ looks like should definitely on

Saturday, 13. October 2018, 11 - 18 o’clock

come by to Garching-Forschungszentrum and visit CALA!

Researchers offer guided tours every hour, starting at 11.30. The last tour will be at 16.30. Tours last for approximately 45 minutes, there is no registration required.

Here you can learn how to explore new horizons in basic research and medicine through tiny, light-accelerated particles. Meeting point is CALA's control room, here we also offer some more information about our researchers work around light and some experiments for our visitors.

Prof. Jörg Schreiber's team is hard at work on the LION Cave in CALA, where the ATLAS laser beam is focused onto its target, an extremely thin carbon foil. Each encounter will accelerate ionized carbon atoms to energies of up to 400 MeV/nucleon (Laser-driven ION Acceleration). Leonard Doyle and Jens Hartmann are setting the next building block on the new mobile beam dump, which will effectively screen the workspace from the resulting radiation. The brightly lit vessel in the picture will later be filled with water, which acts as a neutron moderator to slow down the fast neutrons produced in experiments. The bulk(!) of the facility is made of blocks (of up to 900 kg) of a special type of concrete and, including the water tank, the whole thing weighs around 12 tons.

The Chair of Medical Physics was set up 5 years ago by LMU Munich as part of the MAP Excellence Cluster MAP. Its mission is to uncover new physical insights and develop novel technologies for the diagnosis and treatment of cancer. The establishment in Garching houses around 60 researchers and students. In the following interview, Prof. Katia Parodi looks back on her first 5 years as its Director. Parodi is also one of MAP’s coordinators and President of the German Society for Medical Physics.

Professor Parodi, your institute is now 5 years old, congratulations! Can you tell us a little about its beginnings?

Parodi: The instiute was set up in 2012 as an integral component of MAP. My predecessor, Professor Habs, held a Chair in Nuclear Physics, which was repurposed as a Chair of Medical Physics. So I was confronted with the exciting challenge of developing a new specialty at LMU. My first big task was to design a curriculum in Medical Physics for the Master’s degree course in Physics. That curriculum is now up and running and has been very well received by the students, as witnessed by the large numbers of Master’s students we have. Our graduates receive a Master‘s in Physics with a focus on Medical Physics, which opens up a wide range of career perspectives. In addition to being qualified for the role of medical physicist in a hospital, our graduates have excellent job prospects in both industry and academia. My second major task was to draft and implement a program of research.

What are your research goals?

Parodi: From the beginning, we focused on image-guided radiation therapy for cancer patients, using both conventional and innovative radiation sources – in particular particle-based therapies with protons and heavy ions. But we are also working on issues relevant to photon-based therapy. The goal is to provide effective, high-precision therapies for the elimination of tumors, while reducing as far as possible the degree of damage inflicted on the surrounding tissue. This can be achieved in two ways. First, we aim to characterize the patient’s anatomy with greater precision and design the optimal radiation schedule prior to treatment. The second step then involves enhancing the accuracy of tumor targeting by taking advantage of physical processes that allow real-time, in-vivo visualization of the radiation dose actually delivered. One approach makes use of what is called ionoacoustics, in which we measure the minuscule soundwaves that a particle beam generates at the end of its trajectory within the tumor.

What do you find special about the institute and about your work here?

Parodi: Our great strength lies in our personnel, who come from very diverse backgrounds. We have a team working on the physics of laser-generated plasmas, and a group of people with experience in nuclear physics is working on the development of new detectors. We have computer experts for numerical simulations, and specialists in clinical research. The whole field is very international. We recently recruited Marco Riboldi from Milan to a new professorship. His expertise in Bio-Engineering and Magnet Resonance Imaging will be very valuable in our collaboration with the University Medical Center in Großhadern, as well as for CALA. The ultimate purpose of our research is to improve outcomes for patients, so our contacts with partners in clinical medicine and healthcare technology are very important for us. The University Medical Center is one our most important partners, and we also cooperate with centers for particle therapy and with leading international manufacturers of radiation sources and systems that compute appropriate radiation doses and schedules.

What are your plans for the coming years?

Parodi: We will continue to pursue the route we have chosen and follow where it leads. In the new DFG-funded Graduate School in Advanced Medical Physics for Image-Guided Cancer Therapy, we intend to expand our interdisciplinary research and training programs over the next 4½ years. Furthermore, we recently received a 4-year grant from the ERC for a project called SIRMIO, whose goal is to develop a novel instrument with which to monitor the efficacy of proton therapy for cancers in an animal model. Collaborations with hospitals and manufacturers will be extended. In addition to clinically and pre-clinically oriented research with conventional radiation sources, we want to press ahead with our research on the ground-breaking laser-based accelerator concepts being developed by CALA. And we are working to develop a long-term perspective for the new research fields that emerge from MAP in the period after its funding ends.

And what is your personal perspective after 5 years here?

Parodi: It has been an arduous but very rewarding time. Working with so many gifted and motivated colleagues has been a great pleasure for me, and the research environment in Munich is excellent.  Although the institute has grown so much, we all pull together on many different projects, and the working atmosphere is very harmonious. Indeed, it is sometimes difficult to be sure which group a certain individual belongs to, because there are so many contacts and so much interaction between them. I hope this will continue, and will do everything I can to ensure that it does.

Interview: Karolina Schneider

Dear readers,

Who isn’t fascinated by the view of the night sky? And, at the same time, as a scientist, to be driven by the desire for a deeper understanding of the connections in the birth and death of countless cosmic objects? The fundamental question here is how, following its hydrogen beginnings, the universe has been enriched with heavier elements in the course of its development.

It is not for nothing that the question of the conditions and mechanisms of the formation of heavy elements in the universe ranks among the most pressing mysteries in physics today. We already have quite clear theoretical ideas about the underlying mechanisms of the cosmic nucleosynthesis of heavy elements by fast neutron capture with subsequent beta decay (‘r-process’).

This is constantly occurring under enormous force in the universe. But the way in which the elements are actually distributed is difficult to understand under Earth-bound conditions. The understanding of this requires experimental data on atomic nuclei which we cannot yet produce experimentally.

To bring the elementary formation in the universe to light, large-scale particle accelerators are in operation – or under construction – worldwide, with costs in the billions. Their goal is to get closer to the key nuclei important to cosmic element formation in the laboratory. Atoms with the ‘magic’ neutron number of N = 126 in the nucleus are the ultimate goal.

But the problem is: on Earth, the conditions are not extreme enough. Thus isotopes with 15 neutrons less than N = 126 currently provide the most extreme limits of nuclear physics accelerator experiments. These nuclei are produced in very small quantities and only for short fractional seconds in the laboratory in order to infer their properties. Considering that a typically 10-20-fold accelerated particle flow is required to achieve a nucleus more ‘exotic’ by just one neutron, it is quickly clear that the r-process nuclei of N = 126 will not be accessible with conventional accelerator technology for a long while.

This is precisely where new possibilities with the use of laser-driven ion acceleration open up – those which will be available in the future at CALA. In addition to the focus of biomedical research, the ‘High-Field (HF) Beamline’ offers optimal conditions for entering new territory in the field of astrophysics.

The basis is a significant difference between laser-accelerated ion pulses and those of conventional accelerator systems: in laser ion acceleration, ion pulses with an unrivalled high particle density – ideally close to the density of solid bodies – can be generated, thus achieving a density many orders of magnitude higher than by conventional acceleration. Thus, a novel nuclear reaction mechanism is possible, where heavy, fissile atomic nuclei are first detached from a thin film and accelerated by means of laser light to energies above the fission barrier of approximately 7 MeV/nucleon. These nuclei then hit a second film. There, the projectile and the target nuclei alternately undergo fission. This results in light and heavy neutron-rich fission fragments, respectively. The decisive factor is that the high projectile particle density also means that the fission fragments have a high density, as a result of which a subsequent fusion, for example, between two light fission fragments, becomes possible. This reaction of two nuclei rich in neutrons now leads into the region of the extremely neutron-rich, astrophysically eminently important nuclei with 126 neutrons in their interior. There are still a number of development steps on the way to achieving this ambitious goal. But by exploiting a unique feature of laser-driven ion acceleration, we are confident that we will approach the goal of a better understanding of the formation of heavy elements in the universe.

In this issue of our newsletter, we would like to introduce you, dear readers, to many other exciting research approaches of the scientists working at CALA. We hope you enjoy reading.

Dr. Peter Thirolf

One of the world’s most advanced high-tech laser systems has been constructed at the new Center for Advanced Laser Applications (CALA). The ATLAS 3000 delivers around 25-femtosecond-long pulses whose peak power is equivalent to the power output of multiple nuclear power plants. These light pulses could provide the basis for new directions in medical technology. They could lead medical imaging and cancer therapies into a new era. The ATLAS 3000 is being developed by Thales Optronique and the working group led by Prof. Stefan Karsch, who introduces the new laser system in this interview.

Prof. Karsch, could you outline the basic data of the new laser?

The ATLAS 3000 will provide light pulses with a peak power of 2-3 petawatts (1015 W). More specifically, once per second it releases an energy of 60 joules within about 25-30 femtoseconds. 60 joules is approximately equal to the energy required to lift a full beer stein up to a height of three meters, and 30 femtoseconds is the time is takes light to travel a distance equivalent to 1/10 of the diameter of a human hair. Thus, quite a considerable amount of energy is released in an extremely short time to achieve this high peak power.

How can we classify the power of the light pulses – how “strong” are they?

2-3 petawatts corresponds roughly to the electrical output of two million nuclear power plants (or 2 billion wind turbines). If this output were to be released continuously, it would exceed the total average power of all power plants worldwide by about a factor of 1000. For this reason, such power can be generated only by the storage and subsequent release of energy within an extremely short time. ATLAS 3000 is in the top group among similar short-pulse laser systems world-wide. The current most powerful operational laser has produced a peak power of just under 2 petawatts, while another system has demonstrated 5.5 petawatts, but practical viability has not been achieved. Additionally, several projects with 10 petawatt peak performance are planned.

What is the role of ATLAS 3000 at the CALA research facility?

ATLAS 3000 provides the central infrastructure for the majority of CALA’s goals in medical and physics research. It will supply four of five experimental beamlines with laser light, which are used to generate, study and apply laser-generated secondary radiation – that is, to produce high-energy charged particles and X-ray radiation.

What are the possible applications of these light pulses in medicine?

The main application of the CALA laser is the acceleration of charged particles (electrons and ions) by the extremely strong electric fields generated by the irradiation of plasma with intense laser light. Compared to conventional particle accelerators these fields are, depending on the exact conditions, a factor of ten thousand to ten million times stronger, which means the acceleration distances can be shortened accordingly. Furthermore, the strong fields confine the particle bunches into a much smaller source volume, which in the case of electrons is used to emit high-brilliance X-ray radiation. In this way, very compact accelerators as required by modern radiation diagnosis and therapy are possible.
With ion beams generated in that way, in the medium term radiation damage to healthy and tumor tissue by ultrashort pulses will be investigated, so that these beams can be used in the long term for irradiating tumors. Similar approaches are also pursued by e.g. the Dresden/Jena joint initiative Oncooptics. In addition to pure irradiation studies, CALA aims at the integrated approach of simultaneously using the brilliant X-ray radiation generated by the same laser for diagnostic and therapy-monitoring purposes before, after and even during tumor irradiation.

How good are the chances that this technology will be successful?

The CALA project is not without risk, but if successful, it might pave a viable route towards making current large and costly ion therapy centers more efficient and to link them with high-resolution X-ray imaging, thus making treatment accessible to more patients. Until then, we have what is certainly a long way to go, but researchers involved in CALA have made significant pioneering work and promising progress over the past decade. The scientists at CALA representing both Munich universities possess a unique knowledge base: not only in the field of the underlying physical processes in the field of high-intensity laser-matter interaction, but also in the investigation of the interaction of radiation with cells and novel imaging methods.

We congratulate Professor Katia Parodi on her election to the office of President of the German Society for Medical Physics (Deutsche Gesellschaft für Medizinische Physik e.V. (DPMG)) for a 2-year term with effect from January 2017. Prof. Parodi holds the Chair of Experimental Physics (Medical Physics) at LMU.

One topic of particular concern to the new DPMG President will be to stimulate greater interest in and enthusiasm for the eminently interdisciplinary field of Medical Physics among young generations. In an interview for the DGMP, Prof. Parodi answers three questions related to her own professional career and her goals during her term in office: Link zum Interview

The DGMP’s declared purpose is the advancement of knowledge through the promotion of education and research in the field of Medical Physics, which encompasses Medical Technology and the application of physics-based methodologies to medicine. Membership is open to those who are actively engaged in the area of Medical Physics, as well as individuals who are interested in furthering the development of the subject. In addition, the DPMG provides a range of further education and training programs for young researchers, which is steadily expanding.

This 360-degree picture (keep the left button of the mouse pressed and pull) shows the experimental chamber of the team of Prof. Jörg Schreiber at the Laboratory for Extreme Photonics (Lex Photonics). Here Schreiber and his colleagues perform experiments regarding laser-driven ion acceleration. The developed prototype setups are going to move to the Centre for Advanced Laser Applications (CALA) in the coming months, we recorded them in detail before its move.

The time has come: interior work at the new Centre for Advanced Laser Applications (CALA) has begun! The first laser tables have arrived and been hoisted by crane into the hall. Even the smallest of the tables weighs around 800 kilograms; the largest about 1.2 tons. On these tables, the post-amplifier will be set up to convert the ATLAS 300 Laser into the ATLAS 3000 Laser. With a capacity of three petawatts, this will be the primary light source for the laser-driven experiments at CALA.

The Centre for Advanced Laser Applications (CALA) was given a new face: after a year and a half of construction time the facade has been finished. Now the front of the building reads "Centre for Advanced Laser Applications" in into each other nested letters.

Christian Zeilhofer from the company Sabatrust Fassadentechnik and his colleagues had to work very carefully when painting the letters: "The challenge is to attach all the ornaments at one level and to find the correct horizontal and vertical adjustment of the letters", said Zeilhofer. The point of reference was the central axis of the CALA building.

Overall nearly nine tons of materials have been applied to the building. At first the insulation, then an eight millimeter thick reinforcement and upmost three millimeters of plaster - with the actual design of the facade. For the application the facade was especially heated, since the plaster needs a temperature of fife degrees plus to dry off. For this it was already too cold at the beginning of December. "The application of the plaster must be performed quickly, we did it with eight people within two days." Zeilhofer said.

The letters have been colored as well - the facade specialists used almost 300 liters of paint for it. With this the CALA building is now finished from the outside. And everyone who passes by can try to decipher the single letters of "Centre for Advanced Laser Applications" at the facade.

Groundwork at the Garching Campus is now underway for the future site of CALA. The workers have only recently completed the ground plate of the new building, a process that started at 3am on 28th October, and lasted until 1pm that afternoon. We managed to reserve a spot and film the exciting process in time-lapse, showing the last two hours of concrete being laid in only 1.5 minutes.