Omnigeneity

A flux surface of Wendelstein 7-X (yellow), a magnetic field line on that flux surface (green), and the coils needed to generate the magnetic field (blue). Wendelstein 7-X is designed to be nearly omnigenous.

Omnigeneity (sometimes also called omnigenity) is a property of a magnetic field inside a magnetic confinement fusion reactor. Such a magnetic field is called omnigenous if the path a single particle takes does not drift radially inwards or outwards on average.[1] A particle is then confined to stay on a flux surface. All tokamaks are exactly omnigenous by virtue of their axisymmetry,[2] and conversely an unoptimized stellarator is generally not omnigenous.

Because an exactly omnigenous reactor has no neoclassical transport (in the collisionless limit),[3] stellarators are usually optimized in a way such that this criterion is met. One way to achieve this is by making the magnetic field quasi-symmetric,[4] and the Helically Symmetric eXperiment takes this approach. One can also achieve this property without quasi-symmetry, and Wendelstein 7-X is an example of a device which is close to omnigeneity without being quasi-symmetric.[5]

  1. ^ Cary, John R.; Shasharina, Svetlana G. (September 1997). "Omnigenity and quasihelicity in helical plasma confinement systems". Physics of Plasmas. 4 (9): 3323–3333. Bibcode:1997PhPl....4.3323C. doi:10.1063/1.872473. ISSN 1070-664X.
  2. ^ Landreman, Matt (2019). "Quasisymmetry: A hidden symmetry of magnetic fields" (PDF).
  3. ^ Beidler, C.D.; Allmaier, K.; Isaev, M.Yu.; Kasilov, S.V.; Kernbichler, W.; Leitold, G.O.; Maaßberg, H.; Mikkelsen, D.R.; Murakami, S.; Schmidt, M.; Spong, D.A. (2011-07-01). "Benchmarking of the mono-energetic transport coefficients—results from the International Collaboration on Neoclassical Transport in Stellarators (ICNTS)". Nuclear Fusion. 51 (7): 076001. Bibcode:2011NucFu..51g6001B. doi:10.1088/0029-5515/51/7/076001. hdl:11858/00-001M-0000-0026-E9C1-C. ISSN 0029-5515. S2CID 18084812.
  4. ^ Rodríguez, E.; Helander, P.; Bhattacharjee, A. (June 2020). "Necessary and sufficient conditions for quasisymmetry". Physics of Plasmas. 27 (6): 062501. arXiv:2004.11431. Bibcode:2020PhPl...27f2501R. doi:10.1063/5.0008551. ISSN 1070-664X. S2CID 216144539.
  5. ^ Nührenberg, Jürgen (2010-12-01). "Development of quasi-isodynamic stellarators". Plasma Physics and Controlled Fusion. 52 (12): 124003. Bibcode:2010PPCF...52l4003N. doi:10.1088/0741-3335/52/12/124003. ISSN 0741-3335. S2CID 54572939.