Virtual Pressroom

Intricate details emerge in this radiograph of a fruit fly produced with the flash emitted by an X-pinch plasma.

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 paper RP1.101, while talks 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, 607-255-3916)

Author Synopsis

Reporters seeking additional information should contact Ben Stein, 301-209-3091 or James Riordon, 301-209-3238.

X-ray pinhole camera image of an imploded cryogenic target core.

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, 716-275-4973; D. Meyerhofer, 716-275-0255)

Author Synopsis

Reporters seeking additional information should contact Ben Stein, 301-209-3091 or James Riordon, 301-209-3238.

Twelve-element array of radio-frequency sources used to launch compressional Alfven waves at the NSTX laboratory.

In an Earth-bound laboratory, researchers have recreated Alfvén 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 a hot, dense plasma environment, a magnetic field can be imagined as a stiff spring. An Alfvén wave is a disturbance to this spring. Squeezing the spring along its length produces a "compressional" Alfvén wave. Masayuki Ono of Princeton University (609- 243-2105, mono@pppl.gov) and his colleagues generated compressional Alfvén 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 injection of energetic particles in the plasma to excite the waves. The antenna array launched waves with 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 Alfvén waves transferred a significant amount of energy to the plasma electrons.

Applying 3.4 million watts of Alfvén 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 Alfvén 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 Alfvén waves in NSTX by energetic particles suggest that Alfvén waves may supply a powerful acceleration mechanism for the ions in the solar wind that streams from the sun. This in turn 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)

Application of compressional Alfven wave heating produced very high central electron temperature and pressure in NSTX, where electrons were heated from nearly 2 million degrees Kelvin to nearly 40 million degrees Kelvin.

Author Synopsis

Reporters seeking additional information should contact Ben Stein, 301-209-3091 or James Riordon, 301-209-3238.

Section of a "photonic band-gap structure," a specially arranged group of 102 metal rods designed to reflect specific microwave frequencies.

Using metal rods arranged in a specific geometric pattern, MIT physicists (Michael Shapiro, 617-253-8656) 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)

Author Synopsis

Reporters seeking additional information should contact Ben Stein, 301-209-3091 or James Riordon, 301-209-3238

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, 608-265-3574; Stewart Prager, University of Wisconsin, 608-262-7768)

Author Synopsis

Reporters seeking additional information should contact Ben Stein, 301-209-3091 or James Riordon, 301-209-3238.

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 creates 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, 609-243-3243 and Paper C01.008, Jim Terry, MIT, 617-253-8637).

Images of the edge of the Alcator C-Mod tokamak showing the space and time evolution of the edge turbulence. The white line shows the location of the boundary between the "closed" and "open" magnetic field lines. The white arrow shows the outward (toward the vessel wall) and upward movement of one "blob." The black arrow indicates the movement of another. Both arrows remain in the same position frame to frame. This predominantly outward movement is most likely responsible for at least some of the outward plasma transport across the magnetic field. For movies of the "blob" motion, click here.

Author Synopsis of the Alcator C-Mod work

Researchers with the Center for Energy Research at the UC San Diego Jacobs School of Engineering (Jose Boedo, 858-455-2832) have also found new clues on how particles and energy are lost at the plasma edge. In experiments at the Department of Energy's DIII-D tokamak operated by General Atomics, the researchers used multiple sensors inserted in the plasma to identify and quantify 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 (G. McKee 858-455 2419) 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 reducing the heat and particle loads on the divertor. (Paper F01.009)

Author Synopsis of the DIII-D work

Shown are two sequential time frames viewing the plasma near the scrape-off layer, the gap between the confined plasma and the walls of a fusion device. The red zone is a localized region of increased density, which the researchers call an "intermittent plasma object" (IPO) that has moved both radially outward and vertically in the 6 microseconds (millionths of a second) between frames. Such IPOs caused undesired losses of particles and heat from the plasma.

Researchers have produced a stunning movie of plasma turbulence at the DIII-D tokamak in San Diego. 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 the lowest.

Credit: G. McKee (U-Wisconsin) and J. Boedo (UCSD)

Reporters seeking additional information should contact Ben Stein, 301-209-3091 or James Riordon, 301-209-3238.