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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