Plasma Crystals

January 11, 2016

Prior to the introduction of inexpensive digital display devices, such as seven-segment LED displays, scientific instrumentation contained nixie tube displays (see figure). These were vacuum tube displays, and they were not out of place in those instruments with their vacuum tube circuitry. As transistors replaced vacuum tubes in most instruments, nixie tubes were still the best display choice.

The decimal digits displayed on nixie tubes. (Modified Wikimedia Commons image by Hellbus.)

Nixie tubes operate by electrically-exciting a plasma in neon gas. As such, they are a slightly advanced version of neon lamps in which electrically-excited plasma produces light in a glow discharge. This useful phenomenon in the noble gases was discovered by William Ramsay (1852-1916), who received the 1904 Nobel Prize in Chemistry for his isolation of helium, neon, argon, krypton and xenon.

Noble gases glowing under inductive excitation. (Wikimedia Commons source images of helium, neon, argon, krypton, and xenon, by Jurii.)

Plasma is supposedly the "fourth state of matter," although I've always thought that this characterization was somewhat overblown. After all, we've recognized three states of matter, solids, liquids, and gases from antiquity. Should this plasma variant of a gas, produced under exotic conditions, really have such a special designation?

To form a plasma, you excite a gas by heating, by application of an electric field, or by application of an electromagnetic field to the extent that it separates into a sea of electrons and positive ions. There's quite a bit of natural plasma in the Solar System, since the Sun is a ball of hydrogen and helium plasma. There's no way that hydrogen and helium could escape complete ionization at corona temperatures of millions of degrees kelvin.

Plasma technology still had some life left after nixie tubes in the form of plasma displays. In such displays, an electrically-excited plasma produces ultraviolet light that's used to excite multi-colored phosphors. Plasma display was an important technology for larger area television screens until the present decade. Now, liquid-crystal displays are the dominant large screen display technology.

Just as dust bunnies make the undersides of furniture more interesting, dusty plasmas are hosts to a lot of new physics. The American Physical Society Division of Plasma Physics had several scheduled papers on dusty plasmas in its 57th Annual Meeting, November 16–20, 2015, in Savannah, Georgia. Dust particles will be dielectric, conductive, or magnetic, so they will interact with the charged plasma in interesting ways.

One interesting natural occurrence of dusty plasma is in Saturn's rings. In 1980, the Voyager spacecraft found radial spokes in Saturn's B ring. Such radial features cannot be explained by gravitation. While the cause of these apparently seasonal spokes is still unknown, it's suspected that they are microscopic dust particles that are closely synchronized with the rotation of Saturn's magnetosphere.

Spokes in Saturn's B ring, imaged by the Cassini spacecraft on September 28, 2008. (Still frame from a NASA/JPL/Space Science Institute video, via Wikimedia Commons.)

Papers presented at the plasma physics meeting include work by scientists at Auburn University, the University of Iowa, and the University of California, San Diego, on magnetized dusty plasmas in electric field gradients.[1-3] One interesting observed effect is the formation of an ordered lattice of these particles; that is, a crystal embedded in the plasma.[1-3]

While molecular dynamics simulations of such dusty plasmas were done, experiments were also performed on magnetic crystals embedded in plasma in the Magnetized Dusty Plasma Experiment (MDPX) hosted at Auburn University and funded by the US Department of Energy and the National Science Foundation.[1,4] The MDPX is the first experiment of this kind performed in the US.[3]

The MDPX device can produce a uniform magnetic field up to 3.0 tesla. In these experiments, the ordered structures were observed at magnetic fields above 1 tesla.[2] The dusty plasma structures are diminished as the neutral pressure is increased, and they align with the spatial structure of electric field imposed by a wire mesh placed in the plasma.[1,2] As a consequence, changing the symmetry of the mesh changes the lattice type (see figure).[3]

Choose your symmetry. hexagonal and square lattice arrays of magnetic particles in a dusty plasma. (Max Planck Institute image)

Plasmas processes are used in such things as semiconductor device fabrication and fusion power experiments. Control of nanometer-sized particles in plasmas could lead to interesting applications.[3]

Application of pressure destroys the lattice arrays of magnetized particles in a plasma. (Auburn University/Edward Thomas image.)

References:

1. Brian Bender and Edward Thomas, "Analysis of particle trajectories in a simulated, magnetized dusty plasma in a radially-increasing electric field,", Paper JP12.00034 of the 57th Annual Meeting of the APS Division of Plasma Physics (November 16-20, 2015, Savannah, Georgia).
2. Edward Thomas, Spencer LeBlanc, Brian Lynch, Uwe Konopka, Robert Merlino, and Marlene Rosenberg, "Imposed, ordered dust structures and other plasma features in a strongly magnetized plasma," Paper UP12.00060 of the 57th Annual Meeting of the APS Division of Plasma Physics (November 16-20, 2015, Savannah, Georgia).
3. Made to order: Researchers discover a new form of crystalline matter, American Physical Society Press Release, November 11, 2015.
4. Magnetized Dusty Plasma Experiment (MDPX) Web Site.
5. Edward Thomas Jr., Brian Lynch, Uwe Konopka, Robert L. Merlino, and Marlene Rosenberg, "Observations of imposed ordered structures in a dusty plasma at high magnetic field," Phys. Plasmas, vol. 22, no. 3 (March, 2015), Document No. 030701, doi:10.1063/1.4914089.