Tuesday, September 20, 2011

Brightest gamma ray on Earth -- for a safer, healthier world

Physicists have discovered that ultra-short duration laser pulses can interact with ionised gas to give off beams that are so intense they can pass through 20 cm of lead and would take 1.5 m of concrete to be completely absorbed.

The ray could have several uses, such as in medical imaging, radiotherapy and radioisotope production for PET (positron emission tomography) scanning. The source could also be useful in monitoring the integrity of stored nuclear waste.

In addition, the laser pulses are short enough- lasting a quadrillionth of a second- to capture the response of a nucleus to stimuli, making the rays ideal for use in lab-based study of the nucleus.

The device used in the research is smaller and less costly than more conventional sources of gamma rays, which are a form of X-rays.

The experiments were carried out on the Gemini laser in the Central Laser Facility at the Science and Technology Facilities Council's Rutherford Appleton Laboratory. Strathclyde was also joined in the research by University of Glasgow and Instituto Superior Técnico in Lisbon.

Professor Dino Jaroszynski of Strathclyde, who led the research, said: "This is a great breakthrough, which could make the probing of very dense matter easier and more extensive, and so allow us to monitor nuclear fusion capsules imploding.

"To prove this we have imaged very thin wires - 25 microns thick - with gamma rays and produced very clear images using a new method called phase-contrast imaging. This allows very weakly absorbing material to be clearly imaged. Matter illuminated by gamma rays only cast a very weak shadow and therefore are invisible. Phase-contrast imaging is the only way to render these transparent objects visible.

"It could also act as a powerful tool in medicine for cancer therapy and there is nothing else to match the duration of the gamma ray pulses, which is also why it is so bright.

"In nature, if you accelerate charged particles, such as electrons, they radiate. We trapped particles in a cavity of ions trailing an intense laser pulse and accelerated these to high energies. Electrons in this cavity also interact with the laser and pick up energy from it and oscillate wildly - much like a child being pushed on a swing. The large swinging motion and the high energy of the electrons allow a huge increase in the photon energy to produce gamma rays. This enabled the gamma ray photons to outshine any other earthbound source.

"The accelerator we use is a new type called a laser-plasma wakefield accelerator which uses high power lasers and ionised gas to accelerate charged particles to very high energies - thus shrinking a conventional accelerator, which is 100m long, to one which fits in the palm of your hand."

The peak brilliance of the gamma rays was measured to be greater than 1023 photons per second, per square milliradian, per square millimetre, per 0.1% bandwidth.

The research was supported by the Engineering and Physical Sciences Research Council, the Science and Technology Facilities Council, the Laserlab-Europe Consortium and the Extreme Light Infrastructure project. It is linked to SCAPA (Scottish Centre for the Application of Plasma-based Accelerators), which is based at Strathclyde and is run through the Scottish Universities Physics Alliance.

The research has been published in the journal Nature Physics.

Saturday, September 10, 2011

Researchers at NIF moving closer to fusion ignition point

(PhysOrg.com) -- Researchers at the U.S. National Ignition Facility (NIF) report that they are growing ever closer to reaching the ignition point with their laser generated nuclear fusion project. The facility, part of the Lawrence Livermore National Laboratory has been doing research to find out if very high powered lasers could be used to create nuclear fusion that could then be used to drive steam turbines to make electricity. In related news, officials for UK companies AWE and the Rutherford Appleton Laboratory have announced that they are joining forces with the research team working on the NIF project, adding years of expertise in both nuclear fusion and laser technology.

To achieve inertial confinement fusion, researchers at the NIF project shoot multiple (192) very high powered lasers at a single pellet comprised of the hydrogen isotope deuterium, which causes it to compress to a fraction of its original size and fuse into helium atoms -releasing neutrons. The neutrons could then, in theory, be used to heat water to drive steam turbines. The only problem is, thus far, the power consumed by the lasers (some shots use more power than the whole rest of the United States) exceeds the power produced by firing them at the pellet. But, that appears to be changing.

At a meeting this past week, sponsored by London’s Royal Society, representatives from the US facility and its two new British partners met to announce the terms of agreement between them all. NIF Director Ed Moses told the group (according to the BBC) that one shot of the NIF recently produced, for just the tiniest fraction of a second, more power than all the rest of the world was consuming. And while that is certainly impressive, it’s still just a fraction of what is needed to achieve ignition; the point where a self-sustaining chain reaction occurs (required for energy gain). Moses added that he believes the group will achieve ignition within the next couple of years. Part of the reason for his optimism is the advances that have been made in high power laser diodes over the ten years since the NIF was first designed.

One possible issue the group will certainly need to discuss is the enormous number of hydrogen pellets that would be needed to feed a facility that was actually engaged in producing electricity; some estimates range to 10 pellets a second, or a million every day.

If the team is successful in its endeavor, the enormous amounts of tax dollars spent will be more than made up for in energy production. Just 1300 pounds of water, for example, could provide as much electricity as 2 million metric tons of coal.

White laser pulses with precisely tailored waveform enable control of electrons in microcosm

(PhysOrg.com) -- An expedition through the fast-paced microscopic world of atoms reveals electrons that spin around at enormous speeds and have gigantic forces are acting on them. Monitoring the ultrafast motion of these electrons requires ultrashort flashes of light. However, in order to control them, the structure of these light flashes, or light pulses, needs to be tamed as well.

This type of control over light pulses has now, for the first time, been achieved by a team of physicists lead by Eleftherios Goulielmakis and Ferenc Krausz of the Laboratory of Attosecond Physics at the Max Planck Institute of Quantum Optics (MPQ) and the Ludwig-Maximilians-University Munich in Garching, along with collaborators from the Center of Free-Electron Laser Science (DESY Hamburg) and the King Saud University (Saudi Arabia). Additionally, the researchers were able to make their pulses shorter than a complete light oscillation, thereby creating for the first time isolated sub-optical-cycle flashes of light. These novel tools allow for the precise control of electron motion in atoms and molecules.

The motion of electrons in the microcosm occurs on an attosecond time scale, where one attosecond is a billionth of a billionth of a second. On such a short scale, only light itself is able to keep up with the motion. Because of the fast oscillations of its electromagnetic field, light can act somewhat like a pair of tweezers on electrons, influencing their motions and interactions. The time it takes light, generated by modern laser sources, to complete one full oscillation amounts to around 2.6 femtoseconds, where one femtosecond is a thousand attoseconds, or one millionth of a billionth of a second.

This is the reason why light is a promising tool for controlling electron dynamics in the microcosm. Yet, before this can become reality, the light’s field oscillations have to be tamed, i.e. its field has to be precisely and completely controllable on a time scale which is shorter than one full oscillation cycle. In order to achieve this lofty goal, researchers first have to learn how to develop and perfect these extraordinary tweezers.

The international team at MPQ around Eleftherios Goulielmakis and Ferenc Krausz has now mastered a big step towards this ambitious aim, managing to sculpt wave forms of laser pulses with sub-cycle precision. Furthermore, the researchers were able to make their pulses shorter than a complete light oscillation, thereby creating for the first time isolated sub-optical-cycle flashes of light.

Extremely short and intense pulses of light with a new waveform

In order to control light pulses on a sub-cycle time scale, it is necessary to use white laser light, as it contains wavelengths (light colours) ranging from the near-ultraviolet through to the visible and all the way to the near infrared region of the electromagnetic spectrum. The physicists have created these light pulses and sent them into a newly developed “light field synthesizer”. The light field synthesizer is analogous to a sound synthesizer, as used by electronic musicians. Just as the sound synthesizer, which superimposes sound waves of different frequencies to create different sounds and beats, the light field synthesizer superimposes optical waves of different colours and phases to create various field shapes. The apparatus first splits the incident white laser light into red, yellow and blue colour channels. After manipulating the properties of the individual colours, they are recombined to form the synthesized wave form. Several components of this novel device, e.g. its mirrors and its elaborate beam splitters, were developed in the service centre of the Munich Centre for Advanced Photonics (MAP) located at the LMU.

Utilizing this technology, the scientists achieved the generation of completely new isolated waveforms. Furthermore, in doing so they managed to compose the shortest pulses ever measured in the visible spectral range, lasting only 2.1 femtoseconds. These pulses are more intense than the ones commonly afforded by current femtosecond light sources because all the energy of the electromagnetic field is confined into a tiny temporal window. Thus, also the strength of the electromagnetic forces increases.

It is precisely these powerful and specially tailored electromagnetic forces which are necessary to control electrons in atoms and molecules, as they are similar in strength to the forces occurring in such microscopic systems. However, to steer electron motion on a microscopic scale, strength is not the only prerequisite because precision is also needed. This level of desired precision is provided by the well-controlled wave forms of the synthesized light pulses.

Electronics which can be controlled with light waves comes close

Thanks to these latest results, the scientists have accomplished a major step towards the control of the microcosm. “These newly developed tools allow us to initiate, control and therefore further understand inner-atomic processes. With these devices, we can master the fine structuring of ultrashort light fields and reliably measure the newly formed light”, explains Dr. Adrian Wirth, Postdoctoral Fellow in the research team of Eleftherios Goulielmakis, leader of the ERC-research group “Attoelectronics”.

As a matter of fact, the physicists have already applied this novel technique in an experiment. By shining the newly designed light pulses onto krypton atoms, the outermost electron was ripped away within less than 700 attoseconds, marking the fastest electronic process which has been initiated by optically visible light. Similar processes can certainly be achieved in more complex systems such as molecules, solids and nano-particles.

This new technology may very well lead the way towards light-based electronics in the future. Light fields are expected to drive electrons not only in isolated systems such as atoms or molecules, but even on microscopic circuits so as to perform logic operations at unprecedented speeds” said Goulielmakis, whose group is exploring the principles of electronics on these extreme time scales. “We are progressively increasing our understanding of the principles in the microcosm and learning how to control it”, adds Ferenc Krausz.

More information: Adrian Wirth, Mohammed Th. Hassan, Ivanka Grguraš, Justin Gagnon, Antoine Moulet, Tran T. Luu, Stefan Pabst, Robin Santra, Zeyad A. Alahmed, Abdallah M. Azzeer, Vladislav S. Yakovlev, Volodymyr Pervak, Ferenc Krausz & Eleftherios Goulielmakis, Synthesized Light Transients
Science Express September 6, 2011