AIP's Physics News Highlights: March 12, 2012

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TOPICS IN THIS ISSUE:1. Detecting Clouds from Both Sides Now: Researchers have developed a more precise method to detect the boundary between clouds and clear air, by exploiting the swinging motions of a weather balloon and its payload.2. Laser Lightning Rod: Guiding bursts of electricity with a flash of light: Lightning is a fascinating but dangerous atmospheric phenomenon. New research reveals that brief bursts of intense laser light can redirect these high-power electrical discharges. 3. Artificially Structured Metamaterials May Boost Wireless Power Transfer: Scientists calculate that a “perfect lens,” a slab of artificial material engineered to focus electromagnetic fields in ways that natural materials can’t, may increase the efficiency of some wireless power transfer systems. 4. Other Content: Upcoming Conferences of Interest; Physics Today: March Articles_______________________________________________________1. Detecting Clouds from Both Sides Now“Bows and flows of angel hair, and ice cream castles in the air;” we’ve looked at clouds that way. But the interface between clouds and clear air isn’t as well-defined as these imaginative shapes might lead us to believe. Detecting that hazy line can help scientists to better understand the processes that lead to cloud formation, which is important for good weather forecasts and climate modeling. Now atmospheric scientists from the University of Reading in the United Kingdom have designed a sunlight-measuring tool that uses the natural swinging and spinning of a rising weather balloon to distinguish clouds from clear air and may provide higher-resolution measurements of cloud boundaries than is currently possible. The researchers describe their device in a paper published in the American Institute of Physics’ journal Review of Scientific Instruments.

Traditional cloud detection using weather balloons relies on measurements of temperature and relative humidity. The Reading researchers reasoned that they could sense clouds optically, using a simple and inexpensive design: a light sensor carried on a weather balloon. This sensor responds to the intensity of light, producing a maximum reading when pointing directly at the Sun in clear air but reduced readings at oblique angles to the Sun. As the sensor swings beneath a moving balloon, its orientation to the Sun changes continually, resulting in large fluctuations in the sensor’s light intensity readings in cloudless conditions. But inside a cloud – where light intensity is roughly the same in all directions – the fluctuations become much smaller. The team showed that cloud edges could be detected by looking for an abrupt change in the size of these fluctuations.

Laboratory experiments demonstrated that the new instrument worked consistently over the wide range of temperatures that weather balloons encounter. In test flights, the optical technique was able to detect cloud boundaries with greater precision than traditional relative humidity measurements alone. Though this method works best to detect the upper boundaries of clouds, the researchers say that the new system could also be used to determine lower boundaries of clouds in broken cloud conditions or for high-level clouds.

Article: “Balloon-borne disposable radiometer for cloud detection” has been published in the Review of Scientific Instruments.

Authors: Keri A. Nicoll (1) and Giles Harrison (1).(1) Department of Meteorology, University of Reading, UK

2. Laser Lightning Rod: Guiding bursts of electricity with a flash of lightUsing an experimental apparatus reminiscent of a classic Frankenstein movie, French researchers have coaxed laboratory-generated lightning into striking the same place, not just twice, but over and over. This feat of electrical reorientation used femtosecond (one quadrillionth of a second) pulses of laser light to create a virtual lightning rod out of a column of ionized gas. This is the first time that these laser-induced atmospheric filaments were able to redirect an electrical discharge away from its intended target and guide it to a normally less-attractive electrode.

The experiment demonstrates the potential of using laser-based lightning rods for research and protection. “The laser lightning rod would be a valuable alternative to lightning rockets,” says Aurlien Houard, Ph.D., of the Laboratoire d’Optique Appliquée and co-author on a paper published in the American Institute of Physics’ journal AIP Advances.

Previous experiments confirmed that femtosecond laser could produce ultrashort filaments of ionized gas that act like electrical guide wires. Further studies revealed that these filaments could function over long distances, potentially greater than 50 meters.

In a series of new experiments, the French research team sent a laser beam skimming past a spherical electrode to an oppositely charged planar electrode. The laser stripped away the outer electrons from the atoms along its path, creating a plasma filament that channeled an electrical discharge from the planar electrode to the spherical one. To determine if the filament had the ability to redirect an electrical discharge from its normal path, the researchers added a longer, pointed electrode to their experiment. Since lightning tends to follow the path of least resistance, it would preferentially strike the nearest object; in nature, that would be the tallest object.

Without the laser, the discharge obeyed this rule and always struck the taller, pointed electrode. With the laser, however, the discharge was redirected, following the filaments and striking the spherical electrode instead. This occurred even after the initial path of the discharge began to form.

Article: “Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament” has been published in AIP Advances.

Authors: Benjamin Forestier (1), Aurlien Houard (1), Ivan Revel (2), Magali Durand (1), Yves-Bernard André (1), Bernard Prade (1), Amelie Jarnac (1), Jerome Carbonnel (1), Marc Le Nevé, (3), Jean-Claude De Miscault (3), Bruno Esmiller (4), Denis Chapios (3), and Andre Mysyrowicz (1).(1) Laboratoire d’Optique Appliquée, ENSTA, Ecole Polytechnique, CNRS, Palaiseau, France(2) EADS France, Innovations Works, France(3) CILAS, Laser Sources Development and Industrialization, Orleans, France(4) ASTRIUM, Space Transportation, Les Mureaux, France

3. Artificially Structured Metamaterials May Boost Wireless Power TransferMore than one hundred years after the pioneering inventor Nikola Tesla first became fascinated with wireless energy transfer, the spread of mobile electronic devices has sparked renewed interest in the ability to power up without plugging in. Now researchers from Duke University in Durham, N.C., and the Mitsubishi Electric Research Laboratories in Cambridge, Mass., have proposed a way to enhance the efficiency of wireless power transfer systems by incorporating a lens made from a new class of artificial materials.

When a changing electric current flows through a wire it generates a magnetic field, which in turn can induce a voltage across a physically separate second wire. Called inductive coupling, this electromagnetic phenomenon is already used commercially to recharge devices such as cordless electric toothbrushes and mobile phones, as well as in more recently developed experimental systems that can, for example, wirelessly power a light bulb across a distance of more than two meters. Finding a way to increase the inductive coupling in such systems could improve the power transfer efficiency. The research team from Duke and Mitsubishi hypothesized that a superlens, which can only be made from artificially-structured metamaterials, might be able to do the trick.

A superlens has a property call negative permeability. This means it can refocus a magnetic field from a source on one side of the lens to a receiving device on the other side. By running numerical calculations, the team determined that the addition of a superlens should increase system performance, even when a fraction of the energy was lost by passing through the lens.

When the researchers first began studying how a superlens might affect wireless energy transfer, they focused on lenses made from metamaterials that exhibited uniform properties in all directions. In their new study, accepted for publication in the American Institute of Physics’ Journal of Applied Physics, the team also considered materials with magnetic anisotropy, meaning the magnetic properties are directionally dependent. Their results suggest that strong magnetic anisotropy of the superlens can offer further improvements to the system, such as reduction of the lens thickness and width.

Article: “Magnetic superlens-enchanced inductive coupling for wireless power transfer” is accepted for publication in the Journal of Applied Physics.

Authors: Da Huang (1), Yaroslav Urzhumov (1), David R. Smith (1), Koon Hoo Teo (2), and Jinyun Zhang (2).(1) Center for Metamaterials and Integrated Plasmonics, Duke University, Durham, N.C.(2) Mitsubishi Electric Research Laboratories, Cambridge, Mass._____________________________________________________Upcoming Conferences of Interest• The Optical Society’s Conference on Lasers and Electro-Optics (CLEO) will be held May 6 – 11, 2012, in San Jose, Calif.http://www.cleoconference.org/• The Acoustical Society of America’s Acoustics 2012 Hong Kong meeting will be held May 13 – 18, 2012, in Hong Kong, China.http://acousticalsociety.org/

Physics Today: March Articleshttp://www.physicstoday.org

1. Predicting and managing extreme weather events: Earth’s climate is warming, and destructive weather is growing more prevalent. Coping with the changes will require collaborative science, forward-thinking policy, and an informed public.

2. Water in Earth’s mantle: In the form of ionic impurities in rocks and minerals, water lubricates tectonic plates, influences rock viscosities and melting processes, and slows down seismic waves.

3. The many uses of electron antineutrinos: They have become tools for understanding Earth’s internal heat engine and for surveillance of nuclear reactors.To receive Physics News Highlights by email, please contact Catherine Meyers at [email protected].

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