Van Allen Probes show how to accelerate electrons

NASA's Van Allen Probes orbit through two giant radiation belts surrounding Earth. 

Their observations help explain how particles in the belts can be sped up to nearly the speed of light. 

Credit: NASA

One of the great, unanswered questions for space weather scientists is just what creates two gigantic donuts of radiation surrounding Earth, called the Van Allen radiation belts.

Recent data from the Van Allen Probes, two nearly identical spacecraft that launched in 2012, address this question.

The inner Van Allen radiation belt is fairly stable, but the outer one changes shape, size and composition in ways that scientists don't yet perfectly understand.

Some of the particles within this belt zoom along at close to light speed, but just what accelerates these particles up to such velocities?

Recent data from the Van Allen Probes suggests that it is a two-fold process: One mechanism gives the particles an initial boost and then a kind of electromagnetic wave called Whistlers does the final job to kick them up to such intense speeds.

"It is important to understand how this process happens," said Forrest Mozer, a space scientist at the University of California in Berkeley and the first author of the paper on these results that appeared online in Physical Review Letters on July 15, 2014, in conjunction with the July 18 print edition.

"Not only do we think a similar process happens on the sun and around other planets, but these fast particles can damage the electronics in spacecraft and affect astronauts in space."

Over the last few decades, numerous theories about where these extremely energetic particles come from have been developed. They have largely fallen into two different possibilities.

The first theory is that the particles drift in from much further out, some 400,000 miles or more, gathering energy along the way.

The second theory is that some mechanism speeds up particles already inhabiting that area of space. After two years in space, the Van Allen Probes data has largely pointed to the latter.

Additionally, it has been shown that once particles attain reasonably large energies of 100 keV, they are moving at speeds in synch with giant electromagnetic waves that can speed the particles up even more, the same way a well-timed push on a swing can keep it moving higher and higher.

"This paper incorporates the Whistler waves theory previously embraced," said Shri Kanekal, the deputy mission scientist for the Van Allen Probes at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "But it provides a new explanation for how the particles get their initial push of energy."

This first mechanism is based on something called time domain structures, which Mozer and his colleagues have identified previously in the belts.

They are very short duration pulses of electric field that run parallel to the magnetic fields that thread through the radiation belts.

These magnetic field lines guide the movement of all the charged particles in the belts: The particles move along and gyrate around the lines as if they were tracing out the shape of a spring.

During this early phase, the electric pulses push the particles faster forward in the direction parallel to the magnetic fields.

This mechanism can increase the energies somewhat, though not as high as traditionally thought to be needed for the Whistler waves to have any effect.

However, Mozer and his team showed, through both data from the Van Allen Probes and from simulations, that Whistlers can indeed affect particles at these lower energies.

More Information: Direct Observation of Radiation-Belt Electron Acceleration from Electron-Volt Energies to Megavolts by Nonlinear Whistlers Phys. Rev. Lett. 113, 035001 – Published 14 July 2014 - F. S. Mozer, O. Agapitov, V. Krasnoselskikh, S. Lejosne, G. D. Reeves, and I. Roth

Solar Flares Solution to Vanishing Electrons

Researchers from the University of California, Los Angeles (UCLA) have solved the mystery behind vanishing electrons on Earth. The scientists conducted an experiment which suggested solar flares were the major reason behind the mystery.

During the experiment, the researchers installed three networks of orbiting spacecrafts, positioned at different distances from Earth to catch escaping electrons in the act. The data shows that a majority of the electrons were stripped away from the radiation belt by solar wind particles, when the solar flare reached Earth.

According to the researchers, when flares erupt on the Sun's atmosphere, parts of the magnetized outer layers of the Sun's atmosphere crash onto Earth's magnetic field, thereby triggering geomagnetic storms that are capable of damaging satellites and affecting electrons present on Earth.

The fact that these electrons were missing was discovered back in the early 1960s. Initial hypothesis suggested they were lost to the Earth's atmosphere, while others said they were not permanently lost merely drained of energy (temporarily) so they appeared absent. The mystery has now been solved.

"During the onset of a geomagnetic storm, nearly all the electrons trapped within the radiation belt vanish, only to come back with a vengeance a few hours later," said Vassilis Angelopoulos, a Professor at the UCLA, "It's a puzzling effect."

"This is an important milestone in understanding Earth's space environment," said Drew Turner, an Assistant Researcher at UCLA, "We are one step closer towards understanding and predicting space weather phenomena."

A breakthrough in superlens development: Cheap, simple

This is an illustration of Durdu Guney's theoretical negative-index metamaterial, which would be the heart of a perfect lens. The colors show magnetic fields generated by plasmons. 

The black arrows show the direction of electrical current in metallic layers, and the numbers indicate current loops that contribute to negative refraction. Credit: Durdu Guney

A superlens would let you see a virus in a drop of blood and open the door to better and cheaper electronics. It might, says Durdu Guney, make ultra-high-resolution microscopes as commonplace as cameras in our cell phones.

No one has yet made a superlens, also known as a perfect lens, though people are trying.

Optical lenses are limited by the nature of light, the so-called diffraction limit, so even the best won't usually let us see objects smaller than 200 nanometers across, about the size of the smallest bacterium.

Scanning electron microscopes can capture objects that are much smaller, about a nanometer wide, but they are expensive, heavy, and, at the size of a large desk, not very portable.

To build a superlens, you need metamaterials: artificial materials with properties not seen in nature. Scientists are beginning to fabricate metamaterials in their quest to make real seemingly magical phenomena like invisibility cloaks, quantum levitation—and superlenses.

Now Guney, an assistant professor of electrical and computer engineering at Michigan Technological University, has taken a major step toward creating superlens that could use visible light to see objects as small as 100 nanometers across.

The secret lies in plasmons, charge oscillations near the surface of thin metal films that combine with special nanostructures. When excited by an electromagnetic field, they gather light waves from an object and refract it in a way not seen in nature called negative refraction.

This lets the lens overcomes the diffraction limit. And, in the case of Guney's model, it could allow us to see objects smaller than 1/1,000th the width of a human hair.

Other researchers have also been able to sidestep the diffraction limit, but not throughout the entire spectrum of visible light.

Guney's model showed how metamaterials might be "stretched" to refract light waves from the infrared all the way past visible light and into the ultraviolet spectrum.

Making these superlenses would be relatively inexpensive, which is why they might find their way into cell phones. But there would be other uses as well, says Guney.

"It could also be applied to lithography," the microfabrication process used in electronics manufacturing.

"The lens determines the feature size you can make, and by replacing an old lens with this superlens, you could make smaller features at a lower cost. You could make devices as small as you like."

Computer chips are made using UV lasers, which are expensive and difficult to build. "With this superlens, you could use a red laser, like the pointers everyone uses, and have simple, cheap machines, just by changing the lens."

What excites Guney the most, however, is that a cheap, accessible superlens could open our collective eyes to worlds previously known only to a very few.

"The public's access to high-powered microscopes is negligible," he says. "With superlenses, everybody could be a scientist. People could put their cells on Facebook. It might just inspire society's scientific soul."

Guney and graduate student Muhammad Aslam published an article on their work, "Surface Plasmon Diven Scalable Low-Loss Negative-Index Metamaterial in the visible spectrum," in Physical Review B, volume 84, issue 19.

Safer cancer treatments: Case Studies

A new piece of medical technology unveiled by NPL will help improve the success rates of radiotherapy cancer treatments.

The new clinical electron linear accelerator (linac), a £1.5 million government-funded investment, will help ensure patients are treated with accurate doses of radiation.

Radiotherapy is used to treat cancer by using ionising radiation such as high-energy X-rays or electron beams to destroy cancer cells.

Every hospital needs to ensure that its radiotherapy equipment is stable and accurate because delivering correct radiation doses is critical.

If the dose is too low, the cancer may continue to grow. If they are too high, the patient may be endangered by healthy tissue being damaged.

NPL's new clinical linac’s ability to provide highly stable beams and accurate doses will enable calibrations with smaller uncertainties.

The new technology allows it to calibrate the full range of beam qualities currently in therapeutic use in the UK in a very short period of time. This will allow hospitals to deliver more accurate and effective radiation doses to cancer patients.

The new facility helps the UK respond to a recent report from the National Radiotherapy Advisory Group which states that the UK has a huge gap between the number of people treated with radiotherapy and optimal treatment levels.

For further information, please contact James Manning

Find out more about NPL's research in Ionising Radiation

Discoverers of graphene bring graphene-based electronics a step closer

The researchers who unveiled graphene in 2004 and who were awarded the Nobel Prize in 2010 for “groundbreaking experiments regarding the two-dimensional material" have led new research that reveals more about the electronic properties of the wonder material.

The team says their findings promise to accelerate research looking at ways to build graphene-based devices such as touch-screens, ultrafast transistors and photodetectors, and will potentially open up countless more electronic opportunities.

Discoverers of graphene bring graphene-based electronics a step closer - Image 1 of 1

Graphene, Space, Chess boards and Electron Spin

Physicists at UCLA set out to design a better transistor and ended up discovering a new way to think about the structure of space.

 

Space is usually considered infinitely divisible — given any two positions, there is always a position halfway between.

 

But in a recent study aimed at developing ultra-fast transistors using graphene, researchers from the UCLA Department of Physics and Astronomy and the California NanoSystems Institute show that dividing space into discrete locations, like a chessboard, may explain how point-like electrons, which have no finite radius, manage to carry their intrinsic angular momentum, or “spin.”

 

While studying graphene’s electronic properties, professor Chris Regan and graduate student Matthew Mecklenburg found that a particle can acquire spin by living in a space with two types of positions — dark tiles and light tiles. The particle seems to spin if the tiles are so close together that their separation cannot be detected.

 

“An electron’s spin might arise because space at very small distances is not smooth, but rather segmented, like a chessboard,” Regan said.

 

Their findings are published in the March 18 edition of the journal Physical Review Letters.

 

In quantum mechanics, “spin up” and “spin down” refer to the two types of states that can be assigned to an electron. That the electron’s spin can have only two values — not one, three or an infinite number — helps explain the stability of matter, the nature of the chemical bond and many other fundamental phenomena.

 

However, it is not clear how the electron manages the rotational motion implied by its spin. If the electron had a radius, the implied surface would have to be moving faster than the speed of light, violating the theory of relativity. And experiments show that the electron does not have a radius; it is thought to be a pure point particle with no surface or substructure that could possibly spin.

 

In 1928, British physicist Paul Dirac showed that the spin of the electron is intimately related to the structure of space-time. His elegant argument combined quantum mechanics with special relativity, Einstein’s theory of space-time (famously represented by the equation E=mc²).

 

Dirac’s equation, far from merely accommodating spin, actually demands it. But while showing that relativistic quantum mechanics requires spin, the equation does not give a mechanical picture explaining how a point particle manages to carry angular momentum, nor why this spin is two-valued.

 

Unveiling a concept that is at once novel and deceptively simple, Regan and Mecklenburg found that electrons’ two-valued spin can arise from having two types of tiles — light and dark — in a chessboard-like space. And they developed this quantum mechanical model while working on the surprisingly practical problem of how to make better transistors out of a new material called graphene.

 

Graphene, a single sheet of graphite, is an atomically-thin layer of carbon atoms arranged in a honeycomb structure. First isolated in 2004 by Andre Geim and Kostya Novoselov, graphene has a wealth of extraordinary electronic properties, such as high electron mobility and current capacity.

 

In fact, these properties hold such promise for revolutionary advances that Geim and Novoselov were awarded the 2010 Nobel Prize a mere six years after their achievement.

 

Regan and Mecklenburg are part of a UCLA effort to develop extremely fast transistors using this new material.

 

“We wanted to calculate the amplification of a graphene transistor,” Mecklenburg said. “Our collaboration was building them and needed to know how well they were going to work.”

 

This calculation involved understanding how light interacts with the electrons in graphene.

 

The electrons in graphene move by hopping from carbon atom to carbon atom, as if hopping on a chessboard. The graphene chessboard tiles are triangular, with the dark tiles pointing “up” and light ones pointing “down.”  When an electron in graphene absorbs a photon, it hops from light tiles to dark ones. 

 

Mecklenburg and Regan showed that this transition is equivalent to flipping a spin from “up” to “down.”

In other words, confining the electrons in graphene to specific, discrete positions in space gives them spin. 

 

This spin, which derives from the special geometry of graphene’s honeycomb lattice, is in addition to and distinct from the usual spin carried by the electron. In graphene the additional spin reflects the unresolved chessboard-like structure to the space that the electron occupies.

 

“My adviser [Regan] spent his Ph.D. studying the structure of the electron,” Mecklenburg said. “So he was very excited to see that spin can emerge from a lattice. It makes you wonder if the usual electron spin could be generated in the same way.”

 

“It’s not yet clear if this work will be more useful in particle or condensed matter physics,” Regan said, “but it would be odd if graphene’s honeycomb structure was the only lattice capable of generating spin.”

 

The California NanoSystems Institute at UCLA is an integrated research facility located at UCLA and UC Santa Barbara.

Researchers Create a new light source

Scientists have created a completely new light source by getting light particles to act like atoms.

By cooling photons, the light particles condensed so they could behave like a single entity.

Researchers at the University of Bonn have shown that super particles can be made with light.

Albert Einstein and Satyendra Nath Bose thought this was possible back in 1925.

The Bonn researchers proved Einstein and Bose had the right instincts all along.

Why should you care? Well, the discovery could one day shrink electronic devices.

Until now, this behavior has only been seen in atoms. The applications of this physics breakthrough could one day be used to build more powerful computer chips and make lasers that work in the X-ray range.

Nobody has ever created a Bose-Einstein condensate (BEC) with photons before. BECs are usually made with cold atoms of gas.

In the 1920s, it was thought this strange quantum phase of matter existed. If the atoms were cooled to close to absolute zero, then the atoms would be pushed into the same quantum state and they’d act as one.

Zeeya Merali wrote in Nature:
In 1995, two experimental groups independently produced the first examples of BECs with rubidium and sodium atoms. In theory, physicists knew that it should also be possible to form a BEC using particles of light, or photons. But in practice it seemed near impossible because, unlike atoms, the number of photons in an experiment is not conserved.

LHC Towards higher intensities - CERN Bulletin

Towards higher intensities - CERN Bulletin

Over the past 2 weeks, commissioning of the machine protection system has advanced significantly, opening up the possibility of higher intensity collisions at 3.5 TeV. The intensity has been increased from 2 bunches of 1010 protons to 6 bunches of 2x1010 protons. Luminosities of 6x1028 cm-2s-1 have been achieved at the start of fills, a factor of 60 higher than those provided for the first collisions on 30 March.

The recent increase in LHC luminosity as recorded by the experiments.

(Graph courtesy of the experiments and M. Ferro-Luzzi)

To increase the luminosity further, the commissioning crews are now trying to push up the intensity of the individual proton bunches.

After the successful injection of nominal intensity bunches containing 1.1x1011 protons, collisions were subsequently achieved at 450 GeV with these intensities.

However, half-way through the first ramping of these nominal intensity bunches to 3.5 TeV on 15 May, a beam instability was observed, leading to partial beam loss.

Work is now under way to understand, control and cure this instability by using longer bunches and by powering the special correction magnets (octupoles).