Plasma Window 'Force Field' Featured at 2004 DPP Meeting

New wakefield acceleration techniques for electrons and X-ray movies of Z-pinch explosions were among the research highlights at the 46th annual meeting of the APS Division of Plasma Physics, held November 15-19, 2004 in Savannah, GA. The DPP meeting is the world's largest yearly gathering of plasma physicists, with more than 1500 attendees presenting 1425 papers covering the latest advances in plasma-based research and technology.

In addition to the technical program, the conference included a free Plasma Sciences Expo on November 18, open to teachers, students, and the general public.

The objective was to introduce the local community to the excitement of plasmas and the benefits of plasma research. Scientists from around the country and the world were there, ready to engage participants in lively hands-on demonstrations and explorations. Those attending were able to create arcs of lightning, observe their fluctuating body temperature on a special monitor, manipulate a glowing plasma with magnets, watch an electromagnetic wave demonstration, and confine a plasma in a tokamak video game.

Compact Particle Beams for Science and Medicine. New techniques for accelerating electrons are producing tightly focused, energetically uniform beams in compact devices that will be ideal for numerous scientific and medical applications. The accelerators, known as laser wakefield devices, are only meters in length and could replace accelerators that are currently miles long. Because of their compact size, laser wakefield accelerators are likely to find applications in laboratories that lack space for conventional accelerators. In laser wakefield machines, electrons in a plasma are accelerated when they ride the wake of an intense laser pulse, much like dolphins riding the wake of a ship on the ocean. Typically, the laser pulses in such machines spread out as they pass through a plasma, leading to diffuse beams with few energetic electrons.

Researchers at the Lawrence Berkeley Laboratory have improved the quality of laser wakefield beams by injecting preliminary pulses into a gas to create a plasma channel that guides a subsequent, accelerating laser pulse. The result is a nearly uniform, 100 million electron volt bunch of electrons only 10 femtoseconds long. The devices should fulfill applications in research and medicine that rely on accelerators to produce pulses of x-ray and infrared radiation, including high resolution imaging and treatments for certain types of cancer.

Plasma Window Leads to New Welding Technique. Electron Beam Welding (EBW), which relies on beams of electrons to melt and join metal pieces, provides the highest quality welds currently achievable. However, the technique requires parts to be kept under vacuum during welding because the electron guns that produce the beams cannot function in normal atmospheric conditions. EBW, therefore, has not typically been an option for welding of large structures such as cars, airplanes, or ships. Researchers with Brookhaven National Laboratory and Acceleron Inc. have developed a novel plasma window that separates the vacuum of EBW beam sources from ambient pressures while allowing electron beams to pass through.

The plasma window is formed of electric and magnetic fields, effectively leading to something resembling "force fields" trapping a plasma that separates an evacuated electron beam source from the atmosphere.

Taming Plasma Bursts. Creating a fusion "sun" on Earth, in plasma fusion machines such as tokamaks, will provide a critically needed, environmentally acceptable long-term source of energy. However, the task is complicated by the bursts from the 100-million-degree plasma that reach and threaten the life of the chamber surrounding the man-made sun. International teams of scientists at the PPPL National Spherical Torus Experiment (NSTX) and the General Atomics DIII-D National Fusion Facility carried out a series of investigations of these bursts, their varieties, and their dependence on the plasma conditions.

A new type of burst is identified to be particularly interesting, with much higher frequency and lower energy, and therefore delivers much weaker punches than the more studied varieties. Multiple ultra-fast high-resolution cameras (up to one million frames per second), infrared cameras, spectrometers, edge probes, fast gas puffs, and modern computing and modeling codes helped reveal the detailed nature and conditions of these bursts. An advanced diagnostic using atomic lithium beams has been developed to provide information on our understanding of when these bursts arise. Maintaining the proper fusion plasma conditions now holds the potential of taming these "astrophysical" bursts to ensure the fusion chamber survival.

Progress in Direct-Drive Inertial Fusion Research. Significant advances on the route to inertial confinement fusion have been achieved by researchers at the University of Rochester's Laboratory for Laser Energetics (LLE). Laser inertial confinement fusion consists of heating and compressing fuel in millimeter- sized capsules irradiated with powerful laser beams. In a series of papers presented at the meeting, LLE researchers reported on tests at the OMEGA, 60-beam laser facility that are helping to set the stage for the National Ignition Facility—the nation's premier fusion laser facility scheduled to be completed later in the decade.

X-Ray Vision for the Z-Pinch. X-ray movies of wire-array z-pinch implosions on Sandia National Laboratories' Z-machine have been made for the first time, revealing a rich array of implosion phenomena. Wire-array z-pinches at Sandia National Laboratories' "Z-machine" are the world's most powerful laboratory x-ray sources, producing 1-2 million Joules of x-rays in 100-200 TW bursts.

Researchers presenting at the APS meeting successfully took x-ray pictures of z-pinch plasmas on the Z facility using a special crystal imaging diagnostic.

Now, for the first time researchers are able to study the growth and evolution of plasma instabilities during the z-pinch implosion. Z pinches begin as a cylindrical array of wires, each thinner than a human hair, that are vaporized into plasma by 20 million amperes of current. This plasma is then "pinched" to the axis of the array where it rapidly heats up and radiates soft x rays. Until now, very little information existed for the earliest stages of the z-pinch implosion. Each stage of this process has now been imaged, providing quantitative information about the mass distribution of the plasma that is being used to constrain existing physical models and simulations of z-pinch implosions.

New Measurements in Plasma Heating. In plasmas that include multiple species of ions, like those expected in potential fusion devices, the long wavelength, penetrating radio waves used to heat the plasma can spontaneously convert into short wavelength waves. It's important to identify where and how these waves convert to understand heating in machines such as tokamaks, which are likely to lead to the first practical fusion energy sources. Researchers at MIT have now succeeded in simultaneously measuring both the short wavelength and long wavelength waves in a tokamak for the first time on the Alcator C-Mod tokamak. The experimental results are consistent with theoretical predictions, bolstering physicists' confidence that they are on the right track in developing models for the complex interactions in plasma fusion machines.

—James Riordon, Ben Stein and Phil Schewe contributed to this story.