
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

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.,
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.
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,