Recently in Plasma physics Category

A Hall-effect thruster (HET) is a type of electric propulsion system that produces thrust by the formation of an electron current around a circular channel that interacts with a radial applied magnetic field to create a strong axial electric field. That electric field then accelerates propellant ions to very high speeds. (For an introduction to HETs, see the Quick Study by Mark Cappelli, Physics Today, April 2009, page 76.) HETs have been used on many near-Earth missions, but most deep-space travel requires extended thruster operation, typically for years, which raises a major challenge: Some of the ions smash into the ceramic channel and erode it over time, leaving critical thruster components exposed to high-energy ions. Such erosion is known to limit a thruster's lifetime. However, during recent testing of the commercial HET design shown here, the erosion surprisingly stopped after about 5600 hours of operation, and remained suppressed until the test ended after more than 10 000 hours. Now a team of scientists at NASA's Jet Propulsion Laboratory has developed a new simulation code for HETs that exposed the physics behind those test results. As the channel eroded, the magnetic field topology changed and induced an effective shield against ion bombardment. The scientists suggest that carefully designing the magnetic field in future HETs can reduce channel erosion by several orders of magnitude. (I. G. Mikellides et al., Phys. Plasmas 18, 033501, 2011.)—Stephen G. Benka

At the center of many an active galaxy lies an exceedingly powerful engine that, among other things, shoots out collimated jets of fast-moving plasma. Such jets can extend well beyond the galaxy's luminous boundary, ending in vast lobes that light up the intergalactic medium in the radio band. Closer to home, our sun's atmosphere has many a plasma-filled magnetic loop, the dynamics of which are somewhat mysterious. In February, at the joint meeting of the American Physical Society and the American Association of Physics Teachers, Paul Bellan (Caltech) reported on his group's recent experiments that shed light on both systems. The experimenters used the large currents and magnetic fields of spheromak technology to create plasma jets in a very large vacuum chamber, which ensured that the plasma configurations were unaffected by walls. With a preexisting magnetic field "frozen in," the physicists puffed some gas through an electrode, switched on a current, and watched as a plasma jet formed, self-collimated, underwent a kink instability, and then detached when the electric current was strong enough. In a different magnetic-field geometry, the figure shows counterpropagating collimated plasma jets—red hydrogen from the cathode and green nitrogen from the anode—colliding head-on within an arched magnetic loop, much like those seen in the Sun's corona. Bellan also developed a physical model for the self-collimation and a dusty-plasma dynamo mechanism suitable for generating actual astrophysical jets. (P. M. Bellan et al., invited APS/AAPT talk H3.2, 2010. Preprint available from the author.) —Stephen G. Benka

A tokamak traps plasma inside a helical magnetic field that winds around, and delimits, a bagel-shaped confinement vessel. For a range of reasons—some fundamental, some practical—the magnetic field can't be made strong enough to prohibit escape. Confinement is therefore imperfect: If the plasma becomes concentrated in eddies or other energetic instabilities, it can burst out of the confining field like a hernia. Now, a team from MIT's Alcator tokamak (shown here) has demonstrated a method to stabilize tokamak plasma. Rotation is the key. When the whole plasma is made to rotate in its vessel, instabilities at the edge of the plasma are suppressed. And if the rotation has shear, turbulent eddies within the plasma are broken up. Modest rotation has been observed before at various tokamaks as an unintended consequence of the methods used to heat plasma. The MIT team, led by Yijun Lin and John Rice, has found a way to boost rotation twofold using VHF radio waves. The waves resonate with the cyclotron gyration of helium-3 ions, which are spread throughout the tokamak's deuterium fuel as a minority component. It's not clear how the momentum is transferred from the waves to the ³He²⁺ ions and then to the D⁺ ions. Still, by adjusting the proportion of ³He, the MIT team found they could maximize the rotation gain. In principle, the MIT method can be applied at ITER, whose design is being finalized at its site in Cadarache, France. (Y. Lin et al., Phys. Plasmas 16, 056102, 2009.) — Charles Day

Ultrashort, ultraintense laser pulses undergo competing interactions: The nonlinear Kerr effect self-focuses the beam, while multiphoton ionization generates a plasma that defocuses the beam and prevents it from collapsing. The result is a self-channeled, nondiffracting beam with a tight core, termed a filament, consisting of the intense laser field and the generated plasma (see Physics Today, August 2001, page 17). Filaments emit broadband light in the forward direction and are self-healing, properties that yield a variety of applications, including remote atmospheric sensing and spectroscopy.
Recent work by Pavel Polynkin (University of Arizona), Demetrios Christodoulides (University of Central Florida), and colleagues has put a new twist on the filaments. Unlike earlier studies, which relied on Gaussian or other axially symmetric beam profiles, Polynkin and company used axially asymmetric beams: With a phase modulator, they shaped the transverse profile of their femtosecond pulses into the form of a two-dimensional Airy function. The resulting beams remained diffraction free, but their peak intensities followed a parabolic trajectory reminiscent of projectile motion. (Momentum was still conserved, however, thanks to the momentum of the other parts of the beam.) The figure shows the calculated plasma density that accompanies a 5-mJ Airy beam as its peak traces its parabolic path. The curvature could be controlled experimentally by changing the focal lengths of the lenses used. The forward emission from curved laser filaments could find use as a broadband, wide-angle illumination source for remote sensing and for laser-induced breakdown spectroscopy. (P. Polynkin et al., Science 324, 229, 2009.) — Richard J. Fitzgerald

It's one of the great natural mysteries: How do the Sun's corona and wind become thousands of times hotter than the Sun's surface? Somehow, energy makes its way up into the corona against a temperature gradient and is converted to heat. A new analysis of data collected by NASA's Wind spacecraft doesn't solve the mystery, but it is consistent with a popular explanation. The analysis was done by Justin Kasper of the Harvard-Smithsonian Center for Astrophysics, Alan Lazarus of MIT, and Peter Gary of Los Alamos National Laboratory. They looked at 14 years of in situ observations of particles and fields made as Wind flew in and out of the solar wind. The team focused on the two most abundant ion species in the solar wind, H⁺ and He²⁺. Because He²⁺ is four times heavier than H⁺ and carries twice the charge, the two species' kinematics can discriminate among various models for transport and heating. Kasper, Lazarus, and Gary found strong evidence for one picture of coronal heating: Ions are carried upward by magnetohydrodynamic disturbances known as Alfvén waves; heating occurs when ions entrained in the waves fall into resonance at their respective cyclotron frequencies. Confirming or refuting the resonance model will require a spacecraft to enter and explore the corona. That's the aim of two missions planned for the next decade: NASA's Solar Probe and ESA's Solar Orbiter. (J. C. Kasper, A. J. Lazarus, S. P. Gary, Phys. Rev. Lett., in press.) — Charles Day

The melting transition has long fascinated physicists, both for its ubiquity in nature and industry and for the sophisticated physics of the phase transition in general. Two-dimensional systems can mimic surfaces, which melt differently from bulk matter. One such system is a 2D dusty plasma: Background gas in a vacuum chamber is ionized when RF power is applied to an electrode. With sufficient care, one can levitate a single layer of charged “dust” microspheres above the electrode; electrostatic repulsion spreads the particles apart, usually in a stable 2D crystalline pattern. At Ohio Northern University, Terrence Sheridan came up with a new way to heat only the layer of dust. He modulated the RF power at a resonance frequency so as to jiggle the dust up and down; some of that motional energy coupled to an in-plane acoustic instability, increasing the dusty plasma's effective temperature. The panels show the dust distributions for different modulation amplitude levels. At 1.0%, the entire system oscillates vertically as a crystalline rigid body. As the hexagonal crystal is “heated,” the coupling becomes evident in the central region at 1.6%. The crystal begins to melt at 2.2% and enters a hexatic liquid-crystal phase; it fully melts at 2.8%. For more on dusty plasmas, see PHYSICS TODAY, July 2004, page 32. (T. E. Sheridan, Phys. Plasmas, in press.) —Stephen G. Benka

In plasma physics as in fluid dynamics, turbulence remains one of the most challenging fundamental problems to understand. The nonlinear processes that lead to characteristic turbulence power spectra observed in the solar wind—the plasma that flows from the Sun out through the solar system—are poorly understood. So too are the dissipation mechanisms by which plasma turbulence transfers its energy to plasma electrons and ions. Most research in plasma turbulence has assumed that dissipation is weak and the plasma may be approximated as a fluid. But interest has increased in the so-called short-wavelength regime, in which dissipation plays out. Recently, an international team performed the first kinetic, particle-in-cell simulations of decaying short-wavelength whistler-mode turbulence in a collisionless plasma, using parameters similar to those of the solar wind near Earth. (Whistlers acquired their name from World War I radio operators who frequently heard what they thought were outgoing artillery shells, brief whistling sounds that decreased in frequency.) Limited not by any approximations but only by computing resources, the researchers found steep power-law magnetic fluctuation spectra consistent with those observed in space. In addition, they found and analyzed anisotropies in the turbulence whereby stronger initial fluctuations generated more magnetic energy perpendicular to the background magnetic field than along it. Because of the anisotropies, the whistlers were found to be more compressible than expected. The physicists also demonstrated the first simulation results of whistler turbulence dissipation by showing signatures of two well-known types of wave–particle interactions. (S. Saito et al., Phys. Plasmas, in press.) —Stephen G. Benka