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Plasma Physics Meeting This Week One Of The Largest

How can a little bit of geometry improve microwave cooking? What encouraging news has the world's largest unclassified supercomputer provided on fusion energy? Could the hotter-than-expected temperatures on the sun's surface be caused by the most common wave in outer space?

These and many other questions will be addressed at one of the world's largest physics meetings this year: the 43rd annual meeting of the American Physical Society Division of Plasma Physics (APS-DPP), to be held this week from October 29 - November 21 in Long Beach, California. Almost 1,600 papers are scheduled to be delivered at this meeting.

Plasmas are gases of electrically charged particles such as electrons and protons. Plasmas make up astrophysical objects such as stars and supernovas, dying stars that collapse under their own weight and then explode. On Earth, they exist naturally as lightning bolts and the bath of charged particles in our upper atmosphere. In high-tech electronics factories, beams of artificially created plasmas engrave the sophisticated patterns in computer chips.

In attempts to provide the world with an abundant source of energy, many physicists are working hard to make artificial suns -- plasmas so hot and so dense that their particles fuse to release energy. This pursuit of nuclear fusion as a practical energy source is a major branch of plasma physics research.

Meeting Highlights

Here are some highlights from among the many papers being given at the meeting. Full abstracts of the papers mentioned below can be viewed in the web press release. Additional meeting topics are described at this URL.

X-Pinch Flash Illuminates Flies

Researchers at Cornell University have used the brilliant burst of x-rays emitted by vaporizing wires to create striking images of tiny subjects, including houseflies and fruit flies.

The radiographs (x-ray photographs) help to demonstrate the characteristics of the flash that erupts when 100,000 amps of current are rammed through the crossed wires of an X-pinch machine.

When a current courses through X-pinch wires, they vaporize into plasma. The plasma continues to guide the current, which in turn generates a magnetic field that confines the plasma.

As the current increases, the magnetic field grows and the plasma implodes, typically resulting in one or two dense plasma points less than a thousandth of an inch across with temperatures as high as 10 million degrees centigrade.

The unstable plasma points emit bursts of x-rays that last less than a billionth of a second and then explode. Bright, point-source x-ray bursts generated by the X-pinch machine are ideal illumination for x-ray radiographs of thin objects.

Details on the order of a few millionths of a meter, such as the hairs on a fly's wing, would be impossible to discern with larger x-ray sources, but are clearly visible in images created with X-pinch flashes.

Sergei Pikuz will discuss X-pinch photography in session RP1.101, while papers by T. A. Shelkovenko (UI2.001) and D. B. Sinars (RP1.104) will address detailed studies of the X-pinch plasma itself. (Contact: David Hammer.)

Encouraging News For Commercial-Scale Fusion Reactors

Using the new IBM SP, the world's fastest non-classified supercomputer, located at the National Energy Research Scientific Computing Center (NERSC) at California's Lawrence Berkeley National Laboratory, researchers have discovered that future large-scale fusion reactors may be able to trap or "confine" hot plasma fuel more efficiently than previously projected.

The deleterious effects of heat loss resulting from the turbulence within the plasma seem to be reduced as one scales up from present day experimental devices to a bigger, commercial-reactor-scale machine.

Better confinement means that it would be cheaper to operate such a reactor, since less energy would have to be expended to maintain the requisite high plasma temperatures. Alternately, better confinement could enable researchers to build a somewhat smaller fusion device to achieve the same conditions envisioned for a large-scale machine.

In these new studies, Zhihong Lin of the Princeton Plasma Physics Laboratory and his colleagues have calculated the ever-changing "dynamical" interactions of a billion (10^9) particles to simulate the turbulence in a large reactor-scale version of a tokamak, a widely studied, donut-shaped fusion device that uses magnetic fields to trap the hot plasma.

Access to the newly configured capability at NERSC has enabled large-scale simulations of future tokamaks that are about 6 meters in radius, as compared to the 3-meter radius of the largest existing machines.

The new simulations explore some of the key consequences of scaling up from present day experimental devices to those of reactor dimensions. In particular, the influence of the fluctuating electric fields ("electrostatic turbulence") in the plasma presently believed to play a major role in reducing the effectiveness of confinement in current experiments, is found to be reduced as one moves up to reactor scales.

Results indicate that the presence of turbulent eddies allow heat and particles to escape more easily from the central region of the plasma.

For the plasma conditions in present-day experiments, the relative level of turbulent heat loss increases with device size while the size of these eddies remains small. This trend is consistent with recent observations in the DIII-D tokamak at General Atomics in San Diego.

However, for the larger sized reactor-scale plasmas, the simulations indicate that the relative level of turbulent heat loss from electrostatic turbulence does not increase with size. This is a positive outcome provided there are no other significant deleterious processes present. (Papers TR1, UI1.002 at meeting.)

Promising Inertial Fusion Tests

Frozen fusion fuel pellets tested at the University of Rochester's OMEGA laser facility have performed exceptionally well in experiments that will help lay the foundation for future inertial confinement fusion (ICF) research.

The pellets are tiny spherical shells less than a millimeter in diameter containing an inner layer of frozen deuterium, which serves as fuel in ICF experiments. To ignite ICF reactions, numerous laser beams directed at a pellet's surface vaporize the shell, compressing and heating the deuterium to the extreme conditions necessary for fusion to begin.

In the recent tests, researchers aimed the sixty beams of the OMEGA laser system at pellets similar to those that will be imploded by the 192-beam National Ignition Facility (NIF) currently under construction in Livermore, California. Although the NIF will ultimately provide 75 times more energy to target pellets than is available at OMEGA, the results of the comparatively modest tests are in line with expectations and should help refine theoretical models predicting the outcome of future ICF experiments. (Paper KO2.007; R. L. McCrory; D. Meyerhofer.)

Alfven Waves May Illuminate Solar Mysteris

In an Earth-bound laboratory, researchers have recreated Alfven waves, the most common kind of plasma disturbance in space, in an environment similar to that of the sun. Their work has provided potential new explanations for several enduring mysteries on the sun. For example, the new results may shed light on how the sun's outer layer, known as the corona, manages to reach significantly higher temperatures than the sun's core.

The sun is a hot, dense environment of plasma particles immersed in powerful magnetic fields. In such an environment, a magnetic field can be imagined as a stiff spring. An Alfven wave is a disturbance to this spring. Squeezing the spring along its length produces a "compressional" Alfven wave.

Masayuki Ono of Princeton University and his colleagues generated compressional Alfven waves in the National Spherical Torus Experiment (NSTX), a magnetic fusion device at Princeton's Plasma Physics Laboratory.

This was done in two ways -- by using an array of twelve radio antennas and by injecting energetic particles in the plasma to excite the waves.

The antenna array launched waves with the desired velocity by adjusting the antenna elements' relative phase, or the relative positions of peaks and troughs in the radio waves.

Injection of energetic ions with velocities much faster than the wave velocity also excited a rich variety of waves. For plasma inside the NSTX device, the environment is similar to the sun, in that the outward pressure of the plasma at the center of the device nearly equals the inward pressure of the magnetic fields, which trap the plasma. In this environment, the Alfven waves transferred a significant amount of energy to the plasma electrons.

Applying 3.4 million watts of Alfven wave power increased the plasma electron temperature from about 2 million degrees Kelvin to 40 million degrees Kelvin. In addition to providing encouraging signs of effective plasma heating in the spherical torus for fusion research, these Alfven wave processes might provide insights into the enhanced electron heating that gives the solar corona such a high temperature.

Recent theoretical work together with observations of the excitation of Alfven waves in NSTX by energetic particles suggest that Alfven waves may supply a powerful acceleration mechanism for the ions in the solar wind that streams from the sun. This may help explain recent observations by the NASA TRACE satellite, which detected unusually energetic ion populations. (Papers BI1.003, LI1.002, GO1.001, GP1.010, LI1.003, FI1.006)

Photonic Crystal Produces Powerful High-Frequency Microwaves

Using metal rods arranged in a specific geometric pattern, MIT physicists (contact Michael Shapiro) have designed a gyrotron, a device that generates powerful microwaves at very high frequencies.

Such microwaves could provide more effective long-range telecommunications, and improve microwave cooking, as higher-frequency ovens on airplanes could more effectively prepare food.

Traditional microwave sources employ a metal cavity (a tiny space consisting, for example, of a pair of microwave reflecting walls) whose size diminishes with increase in operating frequency to generate microwaves. The small size of the cavity makes it unsuitable for producing high-power microwaves. Cavities with larger dimensions produce microwaves at other unwanted frequencies.

The metal cavity in the new device is formed of a "photonic band gap" (PBG) structure consisting of 102 metal rods geometrically arranged in such a way that it lets some microwave frequencies pass through the cavity while a particular frequency is trapped inside the cavity.

The PBG structure helps in building larger cavities without generating microwaves at unwanted frequencies. In the gyrotron, the PBG structure keeps microwaves trapped at a particular frequency, which builds up their strength just as in a laser.

The researchers generated 140 gigahertz (GHz) microwaves peaking at 25 kilowatts of power. The researchers' design also has the potential of producing microwaves in the range of terahertz, or trillions of cycles per second. Microwaves at such ultra-high frequencies could perform new tasks such as high-resolution medical imaging, high-resolution radar and high-speed communication. (Paper K12.006)

Calming Chaos Leads To Hotter, Longer-Lived Plasmas

Researchers at the Madison campus of the University of Wisconsin have reduced the magnetic chaos in a plasma confinement machine by a factor of two, significantly diminishing particle and energy loss.

The improvements to the Madison Symmetric Torus (MST) double the peak plasma temperature to 8 million K and increase the energy confinement time ten-fold to about 10 milliseconds.

The MST is one of a class of toroidal plasma confinement machines known as Reversed Field Pinch (RFP) devices. Typically, RFPs include toroidal electric fields that drive plasma currents in the long direction around the torus.

The key to the recent MST achievements is the introduction of an electric field that wraps around the donut-shaped machine in the shorter poloidal direction. Current flow due to the additional field helps calm the chaotic ripples in magnetic fields that confine heated plasma.

Magnetic fluctuation is the dominant mechanism that allows plasma particles and energy to escape to the chamber walls in RFPs. Reducing magnetic chaos could help improve the performance of RFPs and related machines to the point that they move beyond their status as interesting research tools and perhaps become promising candidates for magnetic confinement fusion. (Paper KL1.003; Brett Chapman, University of Wisconsin; Stewart Prager, University of Wisconsin.)

Plasma Discoveries At The Edge

In a magnetic fusion device, or tokamak, one of the most crucial regions for reducing turbulence is at the plasma region's edge, where magnetic fields make a transition from being "nested" surfaces which close in on themselves to "open" magnetic fields that intersect the walls of the plasma device.

Particles crossing this boundary become lost to the fusion plasma, and carry energy with them. Most tokamaks use an arrangement of magnetic fields called a "divertor" to handle the large particle and heat loads imposed on the walls of the machines and which create a gap, known as a scrape-off layer, between the hot, confined plasma and the walls of the fusion device.

The divertor has significantly improved the ability to confine and heat plasmas produced in the last 15 years. However, particle and heat losses at the edge are still larger than expected as researchers push tokamaks to higher levels of performance.

Using an ultra-high speed CCD camera, researchers have captured movies of this poorly understood "edge turbulence" at MIT's Alcator C-Mod tokamak. Taking snapshots every 4 microseconds, they found that a typical whirlpool or eddy of turbulence formed, grew, and died away extremely quickly: in about 10 millionths of a second (10 microseconds).

Using a puff of neutral deuterium gas to illuminate the plasma, they routinely observed "blobs" of high-density plasma which spontaneously formed and traveled outward, away from the region of closed magnetic surfaces. These blobs presumably caused at least part of the turbulent transport of plasma across the magnetic field.

With further studies, the researchers hope to understand the physics of edge turbulence and minimize its occurrence. (Paper U11.004; contact Stewart Zweben, Princeton, and Paper C01.008, Jim Terry, MIT.)

In separate experiments at the DIII-D tokamak in San Diego, researchers (contact Jose Boedo, General Atomics UCSD), have found new clues on how particles and energy are lost at the plasma edge.

The researchers, using multiple sensors inserted in the plasma, have identified and quantified rapid-traveling (1000 m/s) "intermittent plasma objects" (IPO's) as carrying away approximately half of the energy and particles that are lost in the edge region of their fusion device.

It is likely that the Alcator C-Mod "blobs" and the DIII-D IPO's are different names for the same underlying phenomenon. The IPOs move from the core and across the scrape-off layer by means of the heat-transfer process of convection carrying heat and particles, analogously to how hot water rises and cold water sinks in a heated pot.

This process can be much faster than diffusion or conduction, which occur when heat moves from the hot to the cold end of an object or ink particles spread in a still glass of water.

The results obtained were verified by high time resolution (1 microsecond) 2-D images obtained by University of Wisconsin researchers (contact G. McKee) showing turbulent structures or eddies swirling in the plasma edge, forming, merging and traveling across the edge and towards the walls of the DIII-D device.

Understanding and controlling this convection could significantly reduce losses in the edge region or help reduce the heat and particle loads on the divertor. (Paper F01.009)

A stunning movie of plasma turbulence at the DIII-D tokamak is at this URL. The frames are 1 microsecond apart and the area of the frame is roughly 4x5 cm. To the authors' knowledge, this is the first plasma turbulence movie with 1 microsecond resolution. Additionally, it is a quantitative measurement, with the colors representing plasma density values. Red is high density and blue is low, black is lowest. The movie was prepared by G. McKee (U-Wisconsin) and J. Boedo (UCSD).

(Editor's Note: This story is based on a press release by Ben Stein and James Riordon, with only minor editing.)

[Contact: Ben Stein, James Riordon]






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