Non-neutral plasma

In physics a non-neutral plasma is a plasma whose net charge creates an electric field large enough to play 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. Positron plasmas are also used in atomic physics experiments that study the interaction of antimatter with neutral atoms and molecules. Cryogenic pure ion plasmas have been used in studies of strongly coupled plasmas[9] and 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, acting to hold back the more mobile species until the loss rates are the same. The electrostatic potential (as measured in electron-volts) required to produce this electric field depends on many variables but is often 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).[10] Confinement times of up to several hours have been achieved.[11] Using the "rotating wall" method,[12] 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.[13] The body-centered-cubic structure of these plasma crystals has been observed by Bragg scattering in experiments on laser-cooled pure beryllium plasmas.[9]

  1. ^ R. C. Davidson, "Physics of Non-neutral Plasmas", (Addison-Wesley, Redwood City, CA, 1990)
  2. ^ Malmberg, J. H.; deGrassie, J. S. (1975-09-01). "Properties of Nonneutral Plasma". Physical Review Letters. 35 (9). American Physical Society (APS): 577–580. doi:10.1103/physrevlett.35.577. ISSN 0031-9007.
  3. ^ Bollinger, J. J.; Wineland, D. J. (1984-07-23). "Strongly Coupled Nonneutral Ion Plasma". Physical Review Letters. 53 (4). American Physical Society (APS): 348–351. doi:10.1103/physrevlett.53.348. ISSN 0031-9007.
  4. ^ Danielson, J. R.; Dubin, D. H. E.; Greaves, R. G.; Surko, C. M. (2015-03-17). "Plasma and trap-based techniques for science with positrons". Reviews of Modern Physics. 87 (1). American Physical Society (APS): 247–306. doi:10.1103/revmodphys.87.247. ISSN 0034-6861.
  5. ^ Andresen, G. B.; Ashkezari, M. D.; Baquero-Ruiz, M.; Bertsche, W.; Bowe, P. D.; Butler, E.; Cesar, C. L.; Chapman, S.; Charlton, M.; Fajans, J.; Friesen, T.; Fujiwara, M. C.; Gill, D. R.; Hangst, J. S.; Hardy, W. N.; Hayano, R. S.; Hayden, M. E.; Humphries, A.; Hydomako, R.; Jonsell, S.; Kurchaninov, L.; Lambo, R.; Madsen, N.; Menary, S.; Nolan, P.; Olchanski, K.; Olin, A.; Povilus, A.; Pusa, P.; Robicheaux, F.; Sarid, E.; Silveira, D. M.; So, C.; Storey, J. W.; Thompson, R. I.; van der Werf, D. P.; Wilding, D.; Wurtele, J. S.; Yamazaki, Y. (2010-07-02). "Evaporative Cooling of Antiprotons to Cryogenic Temperatures". Physical Review Letters. 105 (1). American Physical Society (APS): 013003. arXiv:1009.4687. doi:10.1103/physrevlett.105.013003. ISSN 0031-9007.
  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. ^ Durkin, D.; Fajans, J. (2000). "Experiments on two-dimensional vortex patterns". Physics of Fluids. 12 (2). AIP Publishing: 289–293. doi:10.1063/1.870307. ISSN 1070-6631.
  8. ^ Anderegg, F.; Driscoll, C. F.; Dubin, D. H. E.; O’Neil, T. M. (2009-03-02). "Wave-Particle Interactions in Electron Acoustic Waves in Pure Ion Plasmas". Physical Review Letters. 102 (9). American Physical Society (APS): 095001. doi:10.1103/physrevlett.102.095001. ISSN 0031-9007.
  9. ^ a b Tan, Joseph N.; Bollinger, J. J.; Jelenkovic, B.; Wineland, D. J. (1995-12-04). "Long-Range Order in Laser-Cooled, Atomic-Ion Wigner Crystals Observed by Bragg Scattering". Physical Review Letters. 75 (23). American Physical Society (APS): 4198–4201. doi:10.1103/physrevlett.75.4198. ISSN 0031-9007.
  10. ^ Dubin, Daniel H. E.; O’Neil, T. M. (1999-01-01). "Trapped nonneutral plasmas, liquids, and crystals (the thermal equilibrium states)". Reviews of Modern Physics. 71 (1). American Physical Society (APS): 87–172. doi:10.1103/revmodphys.71.87. ISSN 0034-6861.
  11. ^ 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
  12. ^ Huang, X.-P.; Anderegg, F.; Hollmann, E. M.; Driscoll, C. F.; O'Neil, T. M. (1997-02-03). "Steady-State Confinement of Non-neutral Plasmas by Rotating Electric Fields". Physical Review Letters. 78 (5). American Physical Society (APS): 875–878. doi:10.1103/physrevlett.78.875. ISSN 0031-9007. S2CID 35799154.
  13. ^ Malmberg, J. H.; O'Neil, T. M. (1977-11-21). "Pure Electron Plasma, Liquid, and Crystal". Physical Review Letters. 39 (21). American Physical Society (APS): 1333–1336. doi:10.1103/physrevlett.39.1333. ISSN 0031-9007.