Plasma antenna

The International Thermonuclear Experimental Reactor (ITER) currently under construction is the next big step in achieving fusion energy. As such, it is fueling many new advances in plasma physics research that are relevant to magnetic confinement, including methods to suppress plasma instabilities and periodic bursts, as well as control losses due to leakage. Such advances were among the highlights of the 49th annual meeting of the APS Division of Plasma Physics (DPP), held November 12-16 in Orlando, Florida. With more than 1500 physicists in attendance, it is one of the largest divisional meetings of the Society.

This year, the DPP reprised its popular Plasma Sciences Expo on November 15-16, an outreach program designed to engage the local Orlando community with plasma physicists via lively hands-on demonstrations. Participants created arcs of lightning, observed their fluctuating body temperature on a special monitor, manipulated a glowing plasma with magnets, and learned how to confine a plasma in a fusion device by playing a video game. Local teachers attended morning and afternoon workshops about plasma science on Tuesday, November 13, gleaning tips on how to bring the study of plasma into their classrooms. Throughout the week, plasma scientists also visited local schools.

Other technical highlights included the latest research on plasma wakefield accelerators, plasma-based antennas, new models for how magnetic reconnection may drive the solar wind, and using plasmas as an amplifying mechanism in lasers. In addition to the plasma education program and the official DPP banquet, the annual DPP soccer match took place mid-day on Wednesday, November 14, pitting teams from the East and the West against each other.

Controlling Plasma Bursts. Recent experiments at the DIII-D tokamak fusion research lab at General Atomics in San Diego, California, used controlled chaotic magnetic fields to demonstrate that the theory of magnetic island overlap can be used as the basis for designing specialized magnetic coils that will eliminate large periodic bursts of plasma in future fusion reactors. Such bursts can cause significant erosion of the material surfaces in tokamak power reactors, so eliminating them is a critical step toward making fusion power a reality. Described as “small archipelagos in a chaotic magnetic sea,” these magnetic islands essentially create an escape route that relieves plasma pressure gradually, preventing plasma eruptions.

In related work, new computer simulations of experiments at the Alcator C-Mod tokamak at MIT are shedding light on why filling the chamber with a dense cloud of gas (such as neon, argon, or krypton) as a disruption begins can remove the plasma’s heat and protect the device’s walls from damage. The new simulations show that injected neon gas does not penetrate deeply into the plasma, and yet the cooling of the surface leads to instabilities that destroy good magnetic confinement in the plasma center. As a result, the plasma’s heat is conducted rapidly from the center to the surface, where it is absorbed and then radiated by the neon. More detailed simulations should determine whether the gas injection technique can also prevent the formation of intense beams of high-energy electrons that are sometimes generated during disruptions.

Plasma in a Bottle. Fusion researchers build magnetic bottles to keep their 100 million C plasmas hot and dense by keeping them away from contact with the cold surrounding walls. A tokamak confinement device (donut-shaped magnetic bottle) is particularly effective, but all such devices experience leakage, degrading fusion performance. Plasma physicists at MIT are studying the physical mechanisms that drive such losses, and have found that fusion plasmas tend to build up pressure in their boundary to a critical value, spilling material sporadically outside their magnetic container–a dynamical behavior akin to avalanches, which occur when snow piles up on a mountainside. This provides strong evidence that electromagnetic turbulence plays the key role in regulating the plasma’s leakage through the surface of the bottle.

Catching a Plasma Wave. Plasma wakefield accelerators (PWFAs) can double electron energy in just one meter, compared to full-scale accelerators such as the one at Stanford Linear Accelerator Center (SLAC), which requires about two miles to achieve similar energy levels. That’s the latest finding of a collaboration of scientists from SLAC, the University of California, Los Angeles (UCLA), and the University of Southern California (USC). They also discovered that electrons from the plasma can be trapped in the wake and exit the plasma in a bunch, resulting in very high energies and brightness.

The SLAC/UCLA/USC collaboration is now preparing its next experiments, which will focus on demonstrating the acceleration of an electron bunch with a narrow energy spread. The researchers also hope to demonstrate the acceleration of positrons to high energies in plasma. Taken together, these advances could one day contribute to the miniaturization of future linear colliders.

Plasma-Amplified Lasers. Researchers from Princeton University, Princeton Plasma Physics Laboratory, and the University of California, Berkeley, have experimentally demonstrated an ultrashort pulse laser system using a plasma as the amplifying medium, which can support much stronger electric fields and is less vulnerable to optical damage than the standard chirped-pulse-amplification method. The plasma-based laser achieved an unprecedented pulse intensity amplification of 20,000 times in a plasma length of just 2 millimeters, accompanied by very effective pulse compression: from 500 to 90 femtoseconds in a single pass, reduced further to 50-60 femtoseconds in a two-pass version of the experiment. Further improvements to the energy transfer efficiency are currently underway, bringing this compact, tabletop plasma laser system close to becoming a practical device.

A Mighty Solar Wind. Scientists believe magnetic reconnection is the primary mode by which the solar wind couples to the terrestrial magnetopshere, driving phenomena such as magnetic storms and aurorae. The theory of 2D reconnection is well-developed and has been successfully applied in lab-based plasma experiments and fusion devices, but is not suitable for application to systems like the Earth’s magnetosphere. The 3D theory of magnetic reconnection is less well developed. John Dorelli of the University of New Hampshire described his latest magnetosphere MDH simulation results. He has used this approach to identify two qualitatively distinct types of reconnection phenomena: steady separator reconnection involving plasma flow across magnetic separatrix boundaries, and time-dependent reconnection involving a global change in the topology of the magnetic field.

“Smart” Plasma Antennas. Igor Alexeff of the University of Tennessee maintains that plasma antennas are just as effective as metal antennas, and in addition can transmit, receive and reflect lower frequency signals while being transparent to higher frequency signals. When de-energized, they electrically “disappear.” New technologies include a novel technique to reduce noise, and a method of opening a plasma window in a plasma microwave barrier on a much smaller time scale (microseconds, compared to milliseconds). Alexeff reports testing an intelligent plasma antenna that is garnering strong commercial interest. The method involves finding a radio transmitter by creating an azimuthally-rotating plasma “window” in a circular plasma barrier surrounding an antenna. When located, a computer locks onto the transmitter. Once the transmitter is de-energized, the plasma window begins scanning again.

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