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A non-neutral plasma is a plasma for which the charge is sufficiently different from zero, so that the electric field created by the un-neutralized charge plays an important or even dominant role in the plasma dynamics.[1] The simplest non-neutral plasmas are plasmas consisting of a single charge species. Examples of single species non-neutral plasmas that have been created in laboratory experiments are plasmas consisting entirely of electrons,[2] pure ion plasmas,[3] positron plasmas,[4] and antiproton plasmas. [5]

Non-neutral plasmas are used for research into basic plasma phenomena such as cross-magnetic field transport[6] , nonlinear vortex interactions [7], and plasma waves and instabilities [8]. They have also been used to create cold neutral antimatter, by carefully mixing and recombining cryogenic pure positron and pure antiproton plasmas. Cryogenic pure ion plasmas have been used in studies of quantum entanglement. More prosaically, pure electron plasmas are used to produce the microwaves in microwave ovens, via the magnetron instability.

Neutral plasmas in contact with a solid surface (that is, most laboratory plasmas) are typically non-neutral in their edge regions. Due to unequal loss rates to the surface for electrons and ions, an electric field (the "ambipolar field" ) builds up until the loss rates are the same, acting to hold back the more mobile species. The electrostatic potential (as measured in electron-volts) required to produce this electric field is typically on the order of the electron temperature.

Non-neutral plasmas for which all species have the same sign of charge have exceptional confinement properties compared to neutral plasmas. They can be confined in a thermal equilibrium state using only static electric and magnetic fields, in a Penning trap configuration (see Fig. 1).[9] Confinement times of up to several hours have been achieved.[10] Using the “rotating –wall” technique,[11] the plasma confinement time can be increased arbitrarily.

Such non-neutral plasmas can also access novel states of matter. For instance, they can be cooled to cryogenic temperatures without recombination (since there is no oppositely-charged species with which to recombine). If the temperature is sufficiently low (typically on the order of 10 mK), the plasma can become a non-neutral liquid or a crystal.[12] The body-centered-cubic structure of these plasma crystals has been observed by Bragg scattering in experiments on laser-cooled pure Beryllium plasmas.[13]

Diagram of a nonneutral plasma in a Penning trap.
Fig. 1. Diagram of a non-neutral plasma confined in a Penning trap.
  1. ^ R. C. Davidson, "Physics of Non-neutral Plasmas", (Addison-Wesley, Redwood City, CA, 1990)
  2. ^ J. H. Malmberg and J. S. DeGrassie, Properties of a Non-neutral Plasma, Phys. Rev. Lett. 35, 577 (1975)
  3. ^ J. J. Bollinger and D. J. Wineland, Strongly Coupled Non-neutral Ion Plasma, Phys. Rev. Lett. 53, 348 (1984)
  4. ^ R. G. Greaves, M. D. Tinkle, and C. M. Surko, "Creation and uses of positron plasmas", Physics of Plasmas 1 (1994)
  5. ^ G. B. Andresen et al., "Evaporative Cooling of Trapped Antiprotons to Cryogenic Temperatures", Phys. Rev. Lett. 105, 013003 (2010).
  6. ^ F. Anderegg, "Internal Transport in Non-Neutral Plasmas," presented at Winter School on Physics with Trapped Charged Particles; to appear, Imperial College Press (2013) http://nnp.ucsd.edu/pdf_files/Anderegg_transport_leshouches_2012.pdf
  7. ^ D. Durkin and J. Fajans, "Experiments on Two-Dimensional Vortex Patterns", Phys. Fluids, 12:289, 2000
  8. ^ F. Anderegg, C.F. Driscoll, D.H.E. Dubin, and T.M. O'Neil "Wave-Particle Interactions in Electron Acoustic Waves in Pure Ion Plasmas," Phys. Rev. Lett. 102, 095001 (2009)
  9. ^ Daniel H. E. Dubin and T. M. O’Neil, “Trapped Non-neutral Plasmas, Liquids and Crystals (the thermal equilibrium states), Rev. Mod. Phys. 71, 87 (1999)
  10. ^ J. H. Malmberg et al., "The Cryogenic Pure Electron Plasma", Proceedings of the 1984 Sendai Symposium on Plasma Nonlinear Phenomena" http://nnp.ucsd.edu/pdf_files/Proc_84_Sendai_1X.pdf
  11. ^ X.-P. Huang, F. Anderegg, E.M. Hollmann, C.F. Driscoll and T.M. O'Neil "Steady-State Confinement of Non-neutral Plasma by Rotating Electric Fields,"Phys. Rev. Lett. 78, 875 (1997)
  12. ^ J. H. Malmberg and T. M. O’Neil, “The pure electron plasma, liquid and crystal, Phys. Rev. Lett. 39, 1333 (1977)
  13. ^ J. N. Tan et al., "Observation of Long-Range Order in Trapped Ion Plasmas by Bragg Scattering", Phys. Rev. Lett. 75, 4198 (1995)
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