Synopsis: Helicons in a Lab Plasma
Plasmas support a wide variety of complex waves, which differ in how their ions and electrons respond to, and generate, electric and magnetic fields. Low-frequency modes called helicons, for example, have helical wave fronts and exist in a constant background magnetic field like that in the Earth’s magnetosphere. They were first noticed decades ago when lightning strikes created “whistlers” whose electrical signals fall in the audible kilohertz range.
In contrast to open space environment, studying helicons in laboratory plasmas has required confining them to narrow waveguides. Now, Reiner Stenzel and Manuel Urrutia of the University of California, Los Angeles, have measured helicons far from metal walls in a meter-scale apparatus. In a uniform magnetic field of a few gauss, the researchers created discharges to repeatedly generate identical plasmas and used an antenna to synchronously launch helicons in the plasma. Stenzel and Urrutia then moved a separate detection antenna around the apparatus during subsequent pulses to build up a detailed three-dimensional map of the fields.
By adjusting the electrical signals applied to the launching antenna and its geometry, the team controllably created a variety of helicon modes. These included both right- and left-handed helices, in contrast to previous generation schemes that produced only one handedness. With these detailed maps, researchers will have a more realistic picture of how these non-plane-wave modes appear, propagate, and spread in unbounded space plasmas.
This research is published in Physical Review Letters.