Thursday, October 30, 2008

Ultrafast lasers give researchers a snapshot of electrons in action

( -- In the quest to slow down and ultimately understand chemistry at the level of atoms and electrons, University of Colorado at Boulder and Canadian scientists have found a new way to peer into a molecule that allows them to see how its electrons rearrange as the molecule changes shape.

Understanding how electrons rearrange during chemical reactions could lead to breakthroughs in materials research and in fields like catalysis and alternative energy, according to CU-Boulder physics professors and JILA fellows Margaret Murnane and Henry Kapteyn, who led the research efforts with scientist Albert Stolow of the Canadian National Research Council's Steacie Institute for Molecular Sciences.

To be able to chart a chemical reaction, scientists need to be able to see how bonds are formed or broken between atoms in a molecule during chemical reactions. But only extremely limited tools are available to view the rapidly changing electron cloud that surrounds a molecule as the atoms move around, Murnane said. Changes in the electron cloud can happen on timescales of less than a femtosecond, or one quadrillionth of a second, representing some of the fastest processes in the natural world.

In a paper to appear in the Oct. 30 issue of Science Express, the online version of the journal Science, the CU team describes how they shot a molecule of dinitrogen tetraoxide, or N2O4, with a short burst of laser light to induce very large oscillations within the molecule. They then used a second laser to produce an X-ray, which was used to map the electron energy levels of the molecule, and most importantly, to understand how these electron energy levels rearrange as the molecule changes its shape, according to Kapteyn.

The researchers describe their process of stretching the N2O4 molecule as being similar to pulling on a Slinky toy and then letting it go and watching it vibrate. They used the N2O4 molecule because it vibrates more slowly compared to other molecules, allowing them to observe the physical processes under way.

In many ways, molecules are like tiny masses connected by tiny springs of differing strengths, Murnane said. These springs are the chemical bonds, made up of shared electrons, which hold all matter together. In this experiment they used ultrafast laser pulses to "twang" these springs, making the nanoscale molecular Slinkies vibrate. However, unlike real springs, when researchers vibrate the molecules their properties can change, she said.

Being able to watch and understand why the electrons did what they did is very useful in fields like alternative energy, according to the researchers.

Tuesday, October 28, 2008

Chirped fibre delivers short pulses

Reported from A photonic crystal fibre has for the first time been engineered to transmit sub-100 fs pulses over extended distances.

Researchers from Russia and Germany have fabricated a chirped photonic crystal fibre that guides ultrashort pulses with much less distortion than previous designs. The team says that applications requiring ultrashort pulses and flexible beam delivery such as photodynamic therapy and two-photon microscopy could benefit (Nature Photonics doi:10.1038/nphoton.2008.203).

"No fibre-based technology performs as well in terms of guiding sub-100 fs pulses of considerable energy (nanojoules)," Günter Steinmeyer, a researcher at the Max Born Institute in Germany, told "Our design effectively reduces distortion effects such as stretching of the pulses and pulse break-up, which decreases the peak power of the pulse."

The team fabricated photonic crystal fibres with core diameters of 22 and 53 microns and believes that fibre lengths of up to 1 km can be manufactured. The fibres operate around 800 nm range and exhibit transmission bandwidths of up to 120 nm.

Until now, photonic crystal fibres have been manufactured so that every cell in the design is of equal size. The team has instead introduced a radial chirp so that the cell size changes along the radius of the fibre.

"Chirping is a popular concept in ultra-fast optics and has already been successfully applied in one dimensional photonic structures such as chirped mirrors and chirped fibre Bragg gratings," commented Steinmeyer. "Chirping usually increases the dispersion of the device, but in chirped photonic fibre, dispersion effects are much weaker than an unchirped structure."

In the design, the fibre consists of a hollow core surrounded by five circular layers of glass tubes of different diameters. Each layer consists of 30 identical cells with radii ranging from 1.35 µm at the innermost layer to 2.6 µm at the outermost layer.

Steinmeyer and colleagues speculate that the weaker dispersion is due to a process called amorphization.

"Depending on the wavelength, light experiences reflection in different resonant sections of the chirped cladding, effectively localizing reflection to a particular layer of the structure," explained Steinmeyer. "We think that distributing the cell resonances over a wide wavelength range lessens their negative effect on the dispersion."

One drawback of the group's design is that it is no match for the extremely low linear guiding losses demonstrated by single-cell hollow core fibres. The group hopes to reduce these losses and fabricate fibres with even wider transmission bands by modifying the geometry of the fibre.

Thursday, October 23, 2008

Powerful x-rays made from sticky tape

Peeling ordinary sticky tape can generate bursts of X-rays intense enough to produce an image of the bones in your fingers.

Seth Putterman and colleagues from the University of California, Los Angeles used a motor to unwind a roll of sticky tape and recorded the electromagnetic emissions. Ripping the tape from its roll at 3 centimetres per second generated X-ray bursts of 15 kiloelectronvolts – each lasting one-billionth of a second, and containing over a million photons.

Putterman admits he is not sure exactly what is going on. "My attitude is to marvel at the phenomenon – all we are doing is peeling tape, and nature sets up a process that gives you nanosecond X-ray bursts."
Charged mystery

Exactly what drives this process is still a mystery, but it is well known that if two surfaces rub over one another, one becomes positively charged and one negatively charged.

In this case, the sticky adhesive becomes positive, and the polyethylene roll negative. This charge difference builds up until an electron jumps from the adhesive to the roll, with enough energy to produce X-rays when it hits the tape.

The strength of the X-rays means that they could be a useful source for X-ray photography.
Sticky tape fusion

Putterman has even loftier ambitions. "The energy in the X-rays is enough to generate nuclear fusion, if it is given to the molecules rather than the electrons," he says. "It's a matter of engineering design, not physics."

Tom Todd, chief engineer of UKAEA Culham Division says, "It is true that the emitted X-ray energies are broadly representative of the electron energies – and that, if you could produce copious quantities of deuterium and tritium [the heavy hydrogen atoms needed for fusion] ions at around 15 keV, in sufficiently high density, they would produce fusion reactions."

However, it is unlikely that all these conditions will be met at the same time, so any power produced from the fused nuclei would be tiny, compared to the power required to unwind the sticky tape.

"It's not unphysical, just uneconomical by a great many orders of magnitude," concludes Todd.

Journal reference: Nature, DOI: 10.1038/nature07378

Friday, October 10, 2008

Brilliantly bright light source is one step closer to reality

The European X-ray Laser Project (XFEL) will harness a high energy short-wave laser light that is one billion times more brilliant than most modern x-rays to provide immensely detailed images of molecules and atoms.

Scientists believe a greater understanding of atoms and molecules could be used to develop better drugs to treat diseases or more environmentally efficient technologies for cleansing chemical effluents including carbon dioxide from the atmosphere.

Scientists will be able to carry out a range of experiments that were previously impossible before. For instance, researchers will be able to film atoms as they undergo chemical reactions, or see molecules that were once too small for conventional technology, and analyze gas plasma, the stuff of which stars are made, in microscopic detail.

To see these images, electrons are shot down a 3.3 km long tube at very high speeds and are stimulated to emit x-ray light. These can analyze molecules and atoms in unprecedented detail because the x-ray light emitted is at extremely short wavelengths, between six and one tenth of a nanometer, which enables very high resolution images to be taken of microscopic surfaces.

Countries participating in the XFEL project include Denmark, France, Germany, Greece, Hungary, Italy, Poland, Russia, Slovakia, Spain, Sweden, Switzerland, China and the UK.

Thursday, October 09, 2008

Europe moves forward with laser-fusion plans

I have blogged a report about this HiPER project last year, here I cited a report from physics world website.

Physicists and politicians from across Europe and beyond gathered at London's Science Museum on Monday to mark the beginning of a three-year "preparatory phase" of a new €1bn project known as the European High Power Laser Energy Research Facility (HiPER). So why do we need another fusion energy project? looks for the answers.

What is HiPER?

HiPER is designed to show that laser-driven fusion can provide the world with energy in the future. The idea is to direct a series of extremely powerful laser beams onto a small capsule of deuterium-tritium fuel, heating up the outer surface of the capsule and forcing it to expand outwards, which, by Newton's third law, causes the centre of the capsule to implode.

Another ultra high-power laser heats this high-density core to around one hundred million degrees Kelvin. This energizes the deuterium and tritium nuclei sufficiently so that they overcome their mutual repulsion and fuse, releasing excess energy in the form of neutrons, which can be used to produce electricity.

Aren't physicists already studying inertial confinement?

They are, but of a different sort. Scientists know that inertial-confinement works, since it is this that generates the fusion reactions inside hydrogen bombs. These bombs use an initial fission explosion to rapidly compress a deuterium-tritium mixture, with shock waves created inside the mixture heating it to the point of ignition. This "central ignition" process is being reproduced in a controlled way at billion-dollar military facilities — the National Ignition Facility (NIF) at the Lawrence Livermore Laboratory in the US and the Mégajoule laboratory in France — where a single set of lasers both compresses and heats the fuel. HiPER, on the other hand, will use a separate laser pulse to do the heating, a process known as "fast ignition" because the second laser must heat the fuel within 10-11s of the implosion.

What are the advantages of fast ignition?

It is more efficient than central ignition. Setting up shock waves requires the fuel to be compressed to enormous densities, which needs very high laser energy per unit mass of fuel. Since fast ignition requires only intermediate densities, it can in principle be used to ignite a larger mass of fuel for a given input energy. And more mass equals more output energy, which means higher efficiencies. In fact, proponents of fast ignition reckon that it is some two to three times more efficient than central ignition. In addition, fast ignition does not require the same degree of precision in the uniformity of the compressing laser pulses and the shape of the fuel pellet.

So where does HiPER fit in?

HiPER is being designed to show that fast ignition, once proven in principle, can then be used as an energy source. This means demonstrating that the fusion process can be repeated at high frequencies. Inertial confinement is a pulsed technique — similar in principle to the repeated cycles of chemical combustion in the engine of a car — and at NIF the laser system fires perhaps once a month, whereas a commercial power plant would need to fire about five times a second to provide the 2 gigawatts typical of a large power station. HiPER will trial the fully-robotic process needed to achieve such a frequency.

What happens next?

The six countries that have officially backed HiPER - the UK, France, Spain, the Czech Republic, Italy and Greece - marked the formal start of a three-year "preparatory phase" for the project on Monday. This phase, which will involve detailed studies of short-pulsed lasers and fuel pellets, as well as decisions on costs, location etc., is being funded with €13m of cash and €50m of work in kind.

Some two to three years down the line another €100m will be needed to develop prototypes, and a few years after that the remainder of the roughly ?1bn construction costs will be needed to actually build the thing. Operating costs over the facility's roughly 20-year life time will also be about €1bn. If all goes well, the facility should start up by around the end of the next decade. As to where it will be built, this depends ultimately on who is prepared to commit the cash, but the UK, which is coordinating the project, is certainly in the running.

When might a commercial fusion plant start operating?

The billion-dollar question. The quest to derive energy from nuclear fusion has been plagued by wildly optimistic expectations in the past, and critics have quipped that fusion is always 40 years from commercialization. Fusion advocates, however, are confident that it could happen by about 2050. Indeed, Dunne thinks this estimates holds good for both magnetic and inertial confinement. He concedes that magnetic fusion is "a generation ahead" of its laser equivalent, but believes that fast ignition could potentially close the gap quickly.

David Meyerhofer of Rochester University believes that fusion reactors could ultimately replace all large power plants and be used to extract hydrogen from water for transport. "Thus," he says "it is possible that fusion could eventually produce more than 50% of the world's energy needs." However, he adds that this estimate is "very speculative".