@article{RuedigerSchultzHollerbach2021,
  author    = {R{\"u}diger, G{\"u}nther and Schultz, Manfred and Hollerbach, Rainer},
  title     = {Destabilization of super-rotating Taylor-Couette flows by current-free helical magnetic fields},
  series = {Journal of plasma physics},
  volume    = {87},
  journal   = {Journal of plasma physics},
  number    = {2},
  publisher = {Cambridge University Press},
  address   = {London},
  issn      = {1469-7807},
  doi       = {10.1017/S0022377821000295},
  pages     = {19},
  year      = {2021},
  abstract  = {In an earlier paper we showed that the combination of azimuthal magnetic fields and super-rotation in Taylor-Couette flows of conducting fluids can be unstable against non-axisymmetric perturbations if the magnetic Prandtl number of the fluid is Pm not equal 1. Here we demonstrate that the addition of a weak axial field component allows axisymmetric perturbation patterns for Pm of order unity depending on the boundary conditions. The axisymmetric modes only occur for magnetic Mach numbers (of the azimuthal field) of order unity, while higher values are necessary for the non-axisymmetric modes. The typical growth time of the instability and the characteristic time scale of the axial migration of the axisymmetric mode are long compared with the rotation period, but short compared with the magnetic diffusion time. The modes travel in the positive or negative z direction along the rotation axis depending on the sign of B phi Bz. We also demonstrate that the azimuthal components of flow and field perturbations travel in phase if vertical bar B phi vertical bar >> vertical bar B-z vertical bar, independent of the form of the rotation law. Within a short-wave approximation for thin gaps it is also shown (in an appendix) that for ideal fluids the considered helical magnetorotational instability only exists for rotation laws with negative shear.},
  language  = {en}
}
@article{RuedigerSchultz2019,
  author    = {R{\"u}diger, G{\"u}nther and Schultz, M.},
  title     = {Non-diffusive angular momentum transport in rotating z-pinches},
  series = {Journal of plasma physics},
  volume    = {85},
  journal   = {Journal of plasma physics},
  number    = {6},
  publisher = {Cambridge Univ. Press},
  address   = {New York},
  issn      = {0022-3778},
  doi       = {10.1017/S0022377819000606},
  pages     = {16},
  year      = {2019},
  abstract  = {The stability of conducting Taylor-Couette flows under the presence of toroidal magnetic background fields is considered. For strong enough magnetic amplitudes such magnetohydrodynamic flows are unstable against non-axisymmetric perturbations which may also transport angular momentum. In accordance with the often used diffusion approximation, one expects the angular momentum transport to be vanishing for rigid rotation. In the sense of a non-diffusive Lambda effect, however, even for rigidly rotating z-pinches, an axisymmetric angular momentum flux appears which is directed outward (inward) for large (small) magnetic Mach numbers. The internal rotation in a magnetized rotating tank can thus never be uniform. Those particular rotation laws are used to estimate the value of the instability-induced eddy viscosity for which the non-diffusive Lambda effect and the diffusive shear-induced transport compensate each other. The results provide the Shakura \& Sunyaev viscosity ansatz leading to numerical values linearly growing with the applied magnetic field.},
  language  = {en}
}