Fusion scientists gear up to learn how to harness plasma energy

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(PhysOrg.com) -- Researchers working on an advanced experimental fusion machine are readying experiments that will investigate a host of scientific puzzles, including how heat escapes as hot magnetized plasma, and what materials are best for handling intense plasma powers.

Scientists conducting research on the National Spherical Torus Experiment (NSTX) at the U.S. Department of Energy’s PrincetonPlasmaPhysics Laboratory (PPPL) have mapped out a list of experiments to start in July and run for eight months. The experimental machine is designed to deepen understanding of how plasmas can be mined for energy.

A major topic of investigation by scientists for the coming round of experiments will be the issue of transport– how plasma energy and particles escape the machine. Of particular interest is transport near the plasma edge where the temperature can reach several million degrees Centigrade within a few centimeters. The plasma edge region is very important, since it has a strong influence on the temperature in the even hotter plasma core. The edge is also the region where the plasma must be cooled to lower temperature so it doesn’t damage the walls that confine the experiment.

“We have made a lot of progress already on the machine,” said Jonathan Menard, a principal research physicist and program director for NSTX at the laboratory.“And these experiments will push us further in understanding these mechanisms more fully.”

Menard made the remarks at the close of the four-day NSTX Research Forum, held March 15 to 18 at PPPL, where 55 team members from 17 institutions gathered to focus on experimental research plans and priorities.

Masayuki Ono, a principal research physicist who heads the NSTX department at PPPL, characterized the resulting plan as“highly thoughtful and exciting.”“The planning process,” he added,“is particularly important this year.” He noted that the coming run will represent the research team’s last opportunity to conduct experiments for several years on the device before machine operation is paused to enable a major device upgrade between 2012 and 2014.

Among several other topics, researchers also will focus on the potential of lithium to blanket the hot ionized gas produced in fusion reactions. A major question in magnetic fusion is how to make the plasma and its surrounding walls work well together. For example, if the plasma is too hot when it touches the wall, it can damage it in ways that are difficult to control. One possible solution is to make the wall a liquid, and lithium is special because it liquefies at relatively low temperature, and it does not easily enter hot plasma. Further, lithium has the special property of absorbing particles that hit it so that much less cold gas enters the plasma. As a result, the edge temperature can be increased, turbulence is reduced, and the whole plasma stays hotter–three important characteristics for the creation of fusion energy. Experimenters are actively preparing to cover the floor of the machine with molybdenum tiles that will then be coated with lithium.“We are trying to get more aggressive with this approach of improving the walls that surround the plasma,” Menard said.

Experiments on the NSTX, the largest of the lab’s experiments, began in 1999. The plasmas in NSTX are, like most fusion experiments, confined using magnetic fields and walls designed to withstand the heat from plasmas with temperatures that exceed 100 million degrees Centigrade. Unlike other machines which confine plasmas in a doughnut-like shape, the plasmas in NSTX are spherical in shape with a hole through the center.

PPPL, in Plainsboro, N.J., is devoted both to creating new knowledge about the physics of plasmas– ultrahot, charged gases– and to developing practical solutions for the creation of fusion energy. Through the process of fusion, which is constantly occurring in the sun and other stars, energy is created when the nuclei of two lightweight atoms, such as those of hydrogen, combine in plasma at very high temperatures. When this happens, a burst of energy is released, which could theoretically be used to generate electricity.

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Simulating tomorrow's accelerators at near the speed of light

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(PhysOrg.com) -- As conventional accelerators like CERN's Large Hadron Collider grow ever more vast and expensive, the best hope for the high-energy machines of the future may lie in"tabletop"accelerators like BELLA (the Berkeley Lab Laser Accelerator), now being built by the LOASIS program at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). BELLA, a laser-plasma wakefield accelerator, is remarkably compact. In just one meter a single BELLA stage will accelerate an electron beam to 10 billion electron volts, a fifth the energy achieved by the two-mile long linear accelerator at the SLAC National Accelerator Laboratory.

But realizing the promise of laser-plasma accelerators crucially depends on being able to simulate their operation in three-dimensional detail. Until now such simulations have challenged or exceeded even the capabilities of supercomputers.

A team of researchers led by Jean-Luc Vay of Berkeley Lab’s Accelerator and Fusion Research Division (AFRD) has borrowed a page from Einstein to perfect a revolutionary new method for calculating what happens when a laser pulse plows through a plasma in an accelerator like BELLA. Using their“boosted-frame” method, Vay’s team has achieved full 3-D simulations of a BELLA stage in just a few hours of supercomputer time, calculations that would have been beyond the state of the art just two years ago.

Not only are the recent BELLA calculations tens of thousands of times faster than conventional methods, they overcome problems that plagued previous attempts to achieve the full capacity of the boosted-frame method, such as violent numerical instabilities. Vay and his colleagues, Cameron Geddes of AFRD, Estelle Cormier-Michel of the Tech-X Corporation in Denver, and David Grote of Lawrence Livermore National Laboratory, publish their latest findings in the March, 2011 issue of the journal Physics of Plasma Letters.

Space, time, and complexity

The boosted-frame method, first proposed by Vay in 2007, exploits Einstein’s Theory of Special Relativity to overcome difficulties posed by the huge range of space and time scales in many accelerator systems. Vast discrepancies of scale are what made simulating these systems too costly.

“Most researchers assumed that since the laws of physics are invariable, the huge complexity of these systems must also be invariable,” says Vay.“But what are the appropriate units of complexity? It turns out to depend on how you make the measurements.”

Laser-plasma wakefield accelerators are particularly challenging: they send a very short laser pulse through a plasma measuring a few centimeters or more, many orders of magnitude longer than the pulse itself (or the even-shorter wavelength of its light). In its wake, like a speedboat on water, the laser pulse creates waves in the plasma. These alternating waves of positively and negatively charged particles set up intense electric fields. Bunches of free electrons, shorter than the laser pulse,“surf” the waves and are accelerated to high energies.

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A simulation of laser-plasma acceleration in the laboratory frame. The wavefronts of laser light are the red and blue disks at right; the fields in the wake are colored from pale blue (accelerating) to orange (decelerating). Electrons accelerated by the first of many buckets in the wake are shown in white at right. Both the laser and the wakefield buckets must be resolved over the entire domain of the plasma, requiring many cells and many time steps. Researchers often use a simulation window that moves with the pulse, but this is not a true transformation of frame and reduces only the multitude of cells, not the multitude of time steps.

“The most common way to model a laser-plasma wakefield accelerator in a computer is by representing the electromagnetic fields as values on a grid, and the plasma as particles that interact with the fields,” explains Geddes, a member of the BELLA science staff who has long worked on laser-plasma acceleration.“Since you have to resolve the finest structures– the laser wavelength, the electron bunch– over the relatively enormous length of the plasma, you need a grid with hundreds of millions of cells.”

The laser period must also be resolved in time, and calculated over millions of time steps. As a result, while much of the important physics of BELLA is three-dimensional, direct 3-D simulation was initially impractical. Just a one-dimensional simulation of BELLA required 5,000 hours of supercomputer processor time at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC).

Choosing the right frame

The key to reducing complexity and cost lies in choosing the right point of view, or“reference frame.” When Albert Einstein was 16 years old he imagined riding along in a frame moving with a beam of light– a thought experiment that, 10 years later, led to his Theory of Special Relativity, which establishes that there is no privileged reference frame. Observers moving at different velocities may experience space and time differently and even see things happening in a different order, but calculations from any point of view can recover the same physical result.

Among the consequences are that the speed of light in a vacuum is always the same; compared to a stationary observer’s experience, time moves more slowly while space contracts for an observer traveling near light speed. These different points of view are called Lorentz frames, and changing one for another is called a Lorentz transformation. The“boosted frame” of the laser pulse is the key to enabling calculations of laser-plasma wakefield accelerators that would otherwise be inaccessible.

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A simulation of the same system in the boosted frame of the wake, moving at near lightspeed. Space has contracted, so that the laser wavelengths and the buckets in the wake have similar proportions. Time has stretched, so that first the laser interacts with the plasma, then the buckets are formed one at a time, and finally the electron beam is accelerated. Because of the separation of events in time, the laser has already left the plasma (in yellow rectangle) when the bucket accelerates the beam. Relatively few time steps are needed to model the events, which means less computer time.

A laser pulse pushing through a tenuous plasma moves only a little slower than light through a vacuum. An observer in the stationary laboratory frame sees it as a rapid oscillation of electromagnetic fields moving through a very long plasma, whose simulation requires high resolution and many time steps. But for an observer moving with the pulse, time slows, and the frequency of the oscillations is greatly reduced; meanwhile space contracts, and the plasma becomes much shorter. Thus relatively few time steps are needed to model the interaction between the laser pulse, the plasma waves formed in its wake, and the bunches of electrons riding the wakefield through the plasma. Fewer steps mean less computer time.

Eliminating instability

Early attempts to apply the boosted-frame method to laser-plasma wakefield simulations encountered numerical instabilities that limited how much the calculation frame could be boosted. Calculations could still be speeded up tens or even hundreds of times, but the full promise of the method could not be realized.

Vay’s team showed that using a particular boosted frame, that of the wakefield itself– in which the laser pulse is almost stationary– realizes near-optimal speedup of the calculation. And it fundamentally modifies the appearance of the laser in the plasma. In the laboratory frame the observer sees many oscillations of the electromagnetic field in the laser pulse; in the frame of the wake, the observer sees just a few at a time.

Not only is speedup possible because of the coarser resolution, but at the same time numerical instabilities due to short wavelengths can be suppressed without affecting the laser pulse. Combined with special techniques for interpreting the data between frames, this allows the full potential of the boosted-frame principle to be reached.

“We produced the first full multidimensional simulation of the 10 billion-electron-volt design for BELLA,” says Vay.“We even ran simulations all the way up to a trillion electron volts, which establishes our ability to model the behavior of laser-plasma wakefieldacceleratorstages at varying energies. With this calculation we achieved the theoretical maximum speedup of the boosted-frame method for such systems– a million times faster than similar calculations in the laboratory frame.”

Simulations will still be challenging, especially those needed to tailor applications of high-energy laser-plasma wakefield accelerators to such uses as free-electron lasers for materials and biological sciences, or for homeland security or other research. But the speedup achieves what might otherwise have been virtually impossible: it puts the essential high-resolution simulations within reach of new supercomputers.

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Image: Pretty in pink

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(PhysOrg.com) -- Inside the Plasma Spray-Physical Vapor Deposition, or PS-PVD, ceramic powder is introduced into the plasma flame, which vaporizes it and then condenses it to form the ceramic coating.

The PS-PVD rig at NASA's Glenn Research Center uses new technology to create super thin ceramic coatings, which are being developed to protect high efficiency engines. The coatings created in the PS-PVD rig are thinner and more complex than those previously available.

The PS-PVD rig uses a system of vacuum pumps and a blower to remove air from the chamber, reducing the pressure inside to fraction of normal atmospheric pressure. The plasma flame is extremely hot and reaches 10,000 degrees Celsius.Ceramic powderis introduced from the torch into the plasma flame. The plasma vaporizes the ceramic powder, which then condenses 5 feet away from the torch onto the component to form the ceramic coating.

Plasma--not a gas, liquid or solid--is the fourth state of matter and often behaves like a gas, except that it conductselectricityand is affected by magnetic fields. On an astronomical scale, plasma is common. The sun is composed of plasma, fire is plasma, fluorescent and neon lights contain plasma.NASA’s PS-PVD rig is one of only two such facilities in the country and one of four in the world.

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NRL scientists focus on light ions for fast ignition of fusion fuels

Scientists at the Naval Research Laboratory Plasma Physics Division demonstrate significant progress in the efficiency and cost effectiveness of light ions in the fast ignition of fusion targets. Light ions such as lithium or carbon are easier to produce technologically and the ion beam properties can be manipulated and tailored best to suit the necessary requirements for fast ignition.

The fast ignition concept has been conceived as an alternative to other approaches fornuclear fusionenergy. In the fast ignitor scenario a high-energy particle beam, driven by an ultrashort pulse laser, is deposited into a pre-compressed deuterium-tritium (DT) fuel capsule and creates a 'hot spot' with temperature and density parameters suitable for ignition, approximately 10 kiloelectron volts (keV).

Initially, the easiest path for ignition was taken using electrons, but it was soon recognized that numerous problems such as instabilities exists. The next logical step was to use ions, more specifically, protons. Subsequent experiments demonstrated that protons could be accelerated to relevant energies with conversion efficiencies of 5 to 10 percent and they were proposed as an alternative to relativistic electrons. However, the number of protons required for fast ignition is in order of magnitude two times greater than that of light ions that have aconversion efficiencyof laser energy into ions of up to 25 percent.

"Presently, all efforts in the direction of fast ignition focus entirely on protons, but this continues to be plagued by problems,"said Dr. Jack Davis."Our research strongly indicates that the use of light ions, heavier than protons, in the lithium to aluminum range is a path in the right direction for ignition."

For ions of the appropriate range, thebeam energycan be deposited directly in the fuel, with high efficiency. In general, ion beams offer the advantage of more localized energy deposition, improved beam focusing, straight line trajectory while traversing the DT fuel, maximum energy deposition at the end of their range and suppression of the various kinds of instabilities.

The Ion stopping power— the gradual energy loss of fast particles as they pass through matter— results in a quadratic increase in the required ion kinetic energy relative to atomic number, but a decreasing number of these ions is needed to deliver the fast ignition hot spot energy, translating into a decreased irradiated spot size on the coupling target. The ionization density (number of ions per unit of path length) produced by a fast charged particle along its track increases as the particle slows down. It eventually reaches a maximum called the Bragg peak close to the end of its trajectory. After that, the ionization density dwindles quickly to insignificance.

Other considerations such as tailoring the ion energy and angular distribution, which are responsible for ion beam focusing andenergydensity deposition in time and space, may turn out to be more important for the practical realization of the fast ignition.

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Plasma nanoscience needed for green energy revolution

A step change in research relating to plasma nanoscience is needed for the world to overcome the challenge of sufficient energy creation and storage, says a leading scientist from CSIRO Materials Science and Engineering and the University of Sydney, Australia.

Professor Kostya (Ken) Ostrikov of thePlasmaNanoscienceCentre Australia, CSIRO Materials Science and Engineering, has highlighted, in IOP Publishing'sJournal of Physics D: Applied Physics, the unique potential of plasma nanoscience to control energy and matter at fundamental levels to produce cost-effective, environmentally and human health friendlynanoscale materialsfor applications in virtually any area of human activity.

Professor Ostrikov is a pioneer in the field of plasma nanoscience, and was awarded the Australian Future Fellowship (2011) of the Australian Research Council, Walter Boas Medal of the Australian Institute of Physics (2010), Pawsey Medal of the Australian Academy of Sciences (2008), and CEO Science Leader Fellowship and Award of CSIRO (2008) on top of gaining seven other prestigious fellowships and eight honorary and visiting professorships in six different countries.

He said:"We can find the best, most suitable plasmas and processes for virtually any application-specific nanomaterials using plasma nanoscience knowledge.

"The terms 'best' and 'most-suitable' have many dimensions including quality, yield, cost, environment and human friendliness, and most recently,energy efficiency."

Plasma nanoscience involves the use of plasma– an ionised gas at temperatures from just a few to tens of thousands Kelvin– as a tool to create and process very small (nano) materials for use in energy conversion, electronics, IT, health care, and numerous other applications that are critical for a sustainable future.

In particular, Ostrikov points out the ability of plasma to synthesise carbon nanotubes– one of the most exciting materials in modern physics, with extraordinary properties arising from their size, dimension, and structure, capable of revolutionising the way energy is produced, transferred and stored.

Until recently, the unpredictable nature of plasma caused some scientists to question its ability to control energy and matter in order to construct nanomaterials, however Ostrikov draws on existing research to provide evidence that it can be controlled down to fundamental levels leading to cost-effective and environmentally friendly processes.

Compared to existing methods of nanomaterials production, Ostrikov states that plasma can offer a simple, cheaper, faster, and moreenergyefficient way of moving"from controlled complexity to practical simplicity"and has encouraged researchers to grasp the opportunities that present themselves in this field.

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Imaging of Alfven waves and fast ions in a fusion plasma

Fusion plasmas in the laboratory typically reach 100 million degrees. These high temperatures are required to ignite the hydrogen plasma and maintain the fusion burn by the production of high-energy alpha particles. One challenge for a fusion reactor is how to contain the alpha particles in the vessel long enough for the particles to efficiently heat the hydrogen plasma. One way that these alpha particles can escape the fusion chamber prematurely is by exciting high frequency Alfven waves and riding these waves to the vessel walls, like a surfer rides a wave to the beach.

While it is easy to sit on the seashore and watch surfers riding waves to the beach, it is far more challenging to see thealpha particlesriding Alfvén waves to the walls of a fusion reactor. Recently, researchers have provided the first 2-D visualization of the elegant 3-D spiral pattern of these Alfvén waves together with the observation of the energetic particles that ride these waves to the walls of the reactor. The breakthrough allowing the measurement of these Alfvén waves is the development of a highly sensitive camera designed to measure minute temperature fluctuations inside theplasmathat indicate the presence of these Alfvén waves. These results will be presented at the American Physical Society Division of Plasma Physics 52nd annual meeting, November 8󈝸, in Chicago, Illinois by researchers from DIII-D National Fusion Facility and the ASDEX Upgrade tokamak.

In the experiments on the DIII-D tokamak, beams of high-energy particles are injected into the plasma to simulate the alpha particles expected in a fusion reactor. These particles then excite Alfvén waves similar to what's expected in a reactor and under the right conditions they can ride these waves to the wall. By studying the behavior of the energetic particles and Alfvén waves, we can learn a great deal about what to expect in a fusion reactor.

Unprecedented images of these Alfvén waves have recently been obtained by recording the variation in the plasma temperature using a special camera. These cameras are basically heat detectors, much like IR cameras used to image thermal objects at night. However the camera developed on DIII-D is optimized for resolving tiny variations in the plasma temperature by measuring the"heat"radiated in the form of microwaves, much like the radiation emitted by a microwave oven.

The images show that the fusion plasma has a torus shape and the plasma waves spiral around the torus. Many features of the theoretical prediction of these waves are observed in the images, such as the location of the waves in the plasma, the wavelength of the wave and the twisting spiral pattern of the wave.

In addition to these remarkable images of the Alfvén waves, new measurements have been obtained of the particles that excite these waves and ride the waves to the walls.

Recently, a technique has been developed to directly measure ions that strike the wall after riding the Alfvén waves out of the plasma. A phosphor screen is used that lights up when struck by these escaping particles and a camera is used to image the phosphor. The pattern of the light on the screen provides specific information on the energy and direction of the particles arriving at the wall. Fast images of the phosphor show bunches of beam particles arriving to the wall synchronized with the arrival of the Alfvén waves. This is similar to watching multiple surfers riding a single wave, where these surfers all arrive at the beach together.

Thanks to the focused effort of a large international collaboration, modeling efforts to simulate these recent experiments are now able to reproduce many of the features of both the Alfvén wave structure and the particle losses. These same codes are presently being used to predict the presence of Alfvén eigenmodes in ITER and initial results show that modes similar to those observed in DIII-D and ASDEX-Upgrade will be present. A key challenge for the future is to find ways of suppressing these Alfvén waves in afusion reactoror at least minimizing their effect on the alpha particles.

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3D model of the ionosphere F-region developed by NRL scientists

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The first global simulation study of equatorial spread F (ESF) bubble evolution using a comprehensive 3D ionosphere model, SAMI3, has been demonstrated. The model self-consistently solves for the neutral wind driven dynamo electric field and the gravity driven electric field associated with plasma bubbles.

Developed by Dr. Joseph Huba and Dr. Glenn Joyce at the NRL Plasma Physics Division, SAMI3 is a fully three-dimensional model of the low- to mid-latitude ionosphere. SAMI3 has been modified recently to use a sun-fixed coordinate system to eliminate rotation of the dawn-dusk line and a high-resolution longitudinal grid to capture the evolution of equatorial plasma bubbles in the pre- to post-sunset sector.

The new modeling capability with SAMI3 has found that ESF can be triggered by pre-sunset ionospheric density perturbations and that an existing ESF plasma bubble can trigger a new bubble.

"Understanding and modeling ESF is important because of its impact onspace weather,"said Dr. Joseph Huba, head of the Space Plasma Physics Section of the Beam Physics Branch."ESF anomalies can cause radio wave scintillation that degrades communication and navigation systems and serves as the primary focus of the Air Force Communications/ Navigation Outage Forecast System.

Post-sunset ionospheric irregularities in the equatorial F-region were first observed in 1938 byterrestrial magnetismresearchers, H.G. Booker and H.W. Wells at the Carnegie Institution of Washington. During that time, analysis of the scattering ofradio wavesby the F-region of the ionosphere at an equatorial location (Huancayo, Peru) revealed ESF is fundamentally a nighttime event, with greatest frequency of occurrence in the period from four hours before midnight to four hours after midnight.

"The ionosphere builds up after sunrise and reaches a maximumelectron densityin mid-afternoon, said Huba."Subsequently, the ionosphere can be lifted to higher altitudes just after sunset because of the pre-reversal enhancement of the eastwardelectric field. During this time the ionosphere can become unstable."

The F-region of the ionosphere is home to the F-layer, or Appleton layer, and is the densest part of the ionosphere as it extends from about 200 km to more than 500 km above the surface of Earth. Beyond this layer is the topside ionosphere. Here extreme ultraviolet solar radiation ionizes atomic oxygen. The F-layer consists of one layer at night, but during the day, a deformation often forms creating layers labeled F₁and F₂. The F-region is the region of theionospherethat is very important for high-frequency (HF) radio wave propagation facilitating HF radio communications over long distances.

The upgraded version of SAMI3 represents a unique resource to investigate the physics of equatorial spread F, particularly the processes that control the day-to-day variability of ESFs. Future improvements to the current model include: modification to the geomagnetic field to have a tilt allowing the inclusion of longitudinal effects; coupling SAMI3 with a physics-based model of the thermosphere; and replacement of the full donor cell algorithm, currently being used for crossfield transport, with a high-order flux transport algorithm allowing for the capture of complex bubble evolution involving bifurcation.

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NRL begins field tests of laser acoustic propagation

An NRL research team led by physicist, Dr. Ted Jones, Plasma Physics Division, performed the first successful long distance acoustic propagation and shock generation demonstration of their novel underwater photo-ionization laser acoustic source. These tests, performed at the Lake Glendora Test Facility of Naval Surface Warfare Center-Crane, expanded on their earlier laboratory research on pulsed laser propagation through the atmosphere.

Using a pulsed Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) 532 nanometer wavelengthlaserhoused in a floating platform, pulses were directed by steering mirrors down through a focusing lens and into the water surface. Each laser pulse produced an acoustic pulse with a sound pressure level of approximately 190 decibels (dBs), which was detected and measured by boat-mounted hydrophones at distances up to 140 meters, roughly the length and a half of a football field. Prior laboratory acoustic propagation distances were limited to about three meters.

"The goal of this laser acoustic source development is to enable efficient remote acoustic generation from compact airborne and ship-borne lasers, without the need for any source hardware in the water,"said Jones."This new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications."

The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. A properly tailored laser pulse has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, a highly useful and efficient tool for undersea communications from aircraft.

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Plasma as a fast optical switch

Laser uses relativistic effects to turn otherwise opaque plasma transparent, creating an ultra-fast optical switch useful in next-generation particle accelerators.

Just like an electrical switch allows the flow of electricity into electrical circuits, relativistic transparency in plasma can act like a fast optical switch allowing the flow of light through otherwise opaque plasma. Modern day lasers, such as the Tridentlaserin Los Alamos National Laboratory delivers a 200 terawatt power pulse (roughly 400 times the average electrical consumption of the United States) in half a trillionth of a second (picosecond) time. When the laser power reaches a threshold limit, relativistic transparency in plasma turns the initially opaque plasma transparent in less than a tenth of a picosecond.

Powerful lasers are used to drive plasmas in next-generation particle accelerators and x-ray beams. One shortcoming of these beams is that they typically have a range of energy, caused by the gradual rise of laser power from zero to its maximum level. Using an optical switch, this ramp up time can be reduced to less than a tenth of a picosecond, delivering peaklaser powerto the plasma on a faster time scale.

So, how does this relativistic transparency happen inside plasma? When alaser beamis incident on (or strikes) plasma, electrons in the plasma react to the laser to cancel its presence inside the plasma. But when the laser is powerful enough to accelerate electrons close to the speed of light, the mass of the electrons increases, making them"heavier."These heavier electrons cannot react quickly enough; hence the laser beam propagates through the plasma.

Now, for the first time, scientists at Los Alamos National Laboratory and Ludwig-Maximilian Universität (LMU) in Germany have been able to make a direct observation of relativistic transparency in thin plasmas using a Frequency-Resolved Optical Gating (FROG) device. The discovery was made possible by two key capabilities: the ability to fabricate carbon foils a few nanometers thick to produce thin plasma, and the elimination of optical noise preceding the Trident laser pulse on a few picosecond timescale.

Initially, the researchers observed pulse shortening due to relativistic transparency and consistent spectral broadening. Later, they also measured the shape of the laser pulses incident on and transmitted through theplasmato directly observe the transparency. The transmitted laser pulse is roughly half the duration of the incident laser pulse, with a transparency turn on time around a fifth of a picosecond. The experimental results are well consistent with that of computer simulation, except the loss of fast turn-on time due to propagation effects arising from diffraction. Efforts are currently underway to eliminate diffraction limitations to observe the true turn-on time.

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Wave power could contain fusion plasma

Researchers at the University of Warwick’s Centre for Fusion Space and Astrophysics and the UK Atomic Energy Authority’s Culham Centre for Fusion Energy may have found a way to channel the flux and fury of a nuclear fusion plasma into a means to help sustain the electric current needed to contain that very same fusion plasma.

The researchers used large scale computer simulations to confirm a longstanding prediction by US researchers that highenergyalpha particles born in fusion reactions will be key to generating fusion power in the next planned generation of tokamaks.

The Warwick and Culham researchers were modelling the interaction of particular types of waves with alpha particles in afusion plasmawhen they found that an expected type of wave was forming naturally within the plasma and that it was quickly growing in strength. As the simulation progressed the wave began to transfer energy from alpha particles to make an electric current which could help confine the plasma.

This particular type of waves, LH (lower hybrid) waves, are in fact often used by fusion researchers to generate theelectric currentrequired to confine and control the plasma– but these waves are usually generated externally to the plasma and channelled into it to create the current. The Warwick researchers’ model suggests that in fact thesewaveswill occur naturally in the plasmas of fusion reactors and in doing so may be able to help exploit the energy of alpha particles. This would open up far more efficient methods of creating and sustaining the current needed to confine the plasma and could provide a mechanism that would confirm earlier predictions by US researchers, that the energy ofAlpha particleswould be key to the development of fusion energy.

This work was only possible using the recently commissioned large scale computing facilities at the University of Warwick supported by EPSRC, in particular for theoretical work supporting fusion energy generation.

University of Warwick researcher Professor Sandra Chapman said:

“These large scale computer simulations capture the plasma dynamics in unprecedented detail and have opened up an exciting new area.”

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